1.1 Discovery of the electron and radioactivity.

The birth of ideas about the complex structure of the atom

The discrete nature of the electric current is reflected in Faraday's work on electrolysis - the same current leads to the release of different amounts of substance on the electrodes, depending on what substance is dissolved. When one mole of a monovalent substance is released, a charge of 96,500 C passes through the electrolyte, and with a divalent substance, the charge doubles. After being defined at the end of the 19th century. Avogadro's number made it possible to estimate the magnitude of the elementary electric charge. Since 6.02 10 23 atoms transfer a charge of 96,500 C, then the share of one is 1.2-10 -19 C. Therefore, this is the smallest portion of electricity or an “atom of electricity.” Georg Stoney suggested calling this “atom of electricity” an electron.

Working with currents in gases is complicated by the difficulties of obtaining a rarefied gaseous environment. The German glassblower G. Geisler made tubes for entertainment with rarefied gas that glowed when an electric current was passed through it. In them, V. Gitthoff discovered radiation from the cathode that caused fluorescence of the tube walls, which was called cathode rays. As the English physicist W. Crookes established, these rays propagated in a straight line, were deflected by a magnetic field and had a mechanical effect.

The French physicist J. Perrin placed a metal cylinder with a hole opposite the cathode inside the tube in front of the cathode and discovered that the cylinder was negatively charged. When the rays were deflected by the magnetic field and did not enter the cylinder, it turned out to be uncharged. Two years later, J. Thomson placed the cylinder not in front of the cathode, but on the side: a brought magnet bent the cathode rays so that they entered the cylinder and charged it negatively, but the fluorescent spot on the glass shifted. This means that the rays are negatively charged particles. Such a measuring device is called a high-vacuum cathode ray tube. Under the influence of the Lorentz force caused by the magnetic field turned on in the area of ​​the capacitor, the luminous trace of the beam incidence on the screen shifts. So in 1895 a new science was born - electronics.

Acting simultaneously with electric and magnetic fields and changing their magnitude, Thomson selected them so that they were compensated, the cathode rays did not deviate, and the spot on the glass did not move. He obtained the ratio of electric charge to particle mass e/t = 1.3 10 -7 C/g. Independently of Thomson, this value was measured for cathode rays by V. Kaufman and obtained a similar value. Thomson named this particle corpuscle, and an electron is only its charge, but then the particle of cathode rays itself was called an electron (from the Greek. elektron - amber).

The discovery of the electron and the study of its unique properties stimulated research into the structure of the atom. The processes of absorption and emission of energy by matter became clear; similarities and differences of chemical elements, their chemical activity and inertness; the internal meaning of D.I. Mendeleev’s Periodic Table of Chemical Elements, the nature of chemical bonds and the mechanisms of chemical reactions; Completely new devices have appeared in which the movement of electrons plays a decisive role. Views on the nature of matter changed. The discovery of the electron (1897) began the age of atomic physics.

From numerous experiments with the transmission of electrons through matter, J. Thomson concluded that the number of electrons in an atom is related to the size of the atomic mass. But in the normal state, the atom must be electrically neutral, and therefore in each atom the numbers of charges of different signs are equal. Since the mass of an electron is approximately 1/2000 the mass of a hydrogen atom, the mass of the positive charge must be 2000 times the mass of the electron. For example, hydrogen has almost all its mass associated with a positive charge. With the discovery of the electron, new problems immediately appeared. An atom is neutral, which means there must be other particles with a positive charge in it. They haven't been opened yet.

The French physicist A. Becquerel, while studying luminescence, discovered (1896) the phenomenon of radioactivity. He was interested in the relationship between the fluorescence from the cathode rays on the walls of the tube and the X-rays emitted from this part of the tube. By irradiating various substances, he tried to find out whether x-rays could be emitted by phosphorescent bodies irradiated with sunlight. Soon the Curies took up the matter and discovered a more active element, which they named polonium in honor of Poland, the birthplace of Marie Curie. Measuring the magnitude of the effect, Sklodowska-Curie discovered a new element - radium, and called the radiation effect itself radioactivity(from lat. radio- I emit rays). The radiation intensity of radium is hundreds of thousands of times greater than that of uranium. Then the third radioactive element was discovered - actinium. And there was a certain “boom” in the study of radioactivity.

By the end of 1899, J. Thomson's collaborator E. Rutherford concluded: “... experiments show that the radiation of uranium is complex and consists of at least two different types: one, quickly absorbed, let's call it a-radiation; another, more penetrating one, let's call it

-radiation." Three years later, P. Villar found another component of radiation that was not deflected by a magnetic field; it was called -rays. Radioactivity quickly found application in natural science and medicine.

The atom was no longer considered indivisible. The idea of ​​the structure of all atoms from hydrogen atoms was expressed back in 1815 by the English physician W. Prout. Doubts about the indivisibility of atoms gave rise to the discovery of spectral analysis and the Periodic Table of Chemical Elements. It turned out that the atom itself is a complex structure with internal movements of its constituent parts responsible for the characteristic spectra. Models of its structure began to appear.

The model of an atom - a positive charge is distributed in a positively charged fairly large region (possibly spherical in shape), and electrons are interspersed into it, like “raisins in a pudding” - was proposed by Kelvin in 1902. J. Thomson developed his idea: an atom is a drop of pudding of positively charged matter, inside which electrons are distributed, which are in a state of vibration. Because of these vibrations, atoms emit electromagnetic energy; This way he was able to explain the dispersion of light, but many questions arose. To explain the Periodic Table of chemical elements, he studied different configurations of electrons, suggesting that stable configurations correspond to the structure of inactive elements such as noble gases, and unstable ones correspond to more active ones. Based on the wavelengths of light emitted by atoms, Thomson estimated the area occupied by such an atom to be about 10 -10 m. He made a lot of assumptions, being carried away by calculating the characteristics of radiation according to Maxwell's theory, since he believed that only electromagnetic forces act inside the atom. In 1903, Thomson obtained that electrons should emit elliptical waves when moving, in 1904 - that when the number of electrons is more than 8, they should be arranged in rings and their number in each ring should decrease with decreasing radius of the ring. The number of electrons does not allow radioactive atoms to be stable; they emit alpha particles, and a new atomic structure is established. The experiment of E. Rutherford, one of Thomson's students, led to the nuclear model of the structure of the atom.

Discoveries at the end of the 19th century. - X-rays (1895), natural radioactivity (Becquerel, 1896), electron (J. Thomson, 1897), radium (Pierre and Marie Curie, 1898), the quantum nature of radiation (Planck, 1900) were the beginning of a revolution in science.

1.2 Planetary model of the structure of the atom. Modern science and Bohr's postulates

The planetary model of the structure of the atom was first proposed by J. Perrin, trying to explain the observed properties by the orbital motion of electrons. But V. Vin considered it untenable. Firstly, when an electron rotates, according to classical electrodynamics, it must continuously radiate energy and, ultimately, fall onto the nucleus. Secondly, due to the continuous loss of energy, the radiation of an atom should have a continuous spectrum, but a line spectrum is observed.

Experiments on the passage of α-particles through thin plates of gold and other metals were carried out by E. Rutherford's employees, E. Marsden and H. Geiger (1908). They found that almost all particles pass through the plate freely, and only 1/10,000 of them experience strong deflection - up to 150°. Thomson's model could not explain this, but Rutherford, his former assistant, made estimates of the fraction of deviations and came to the planetary model: the positive charge is concentrated in a volume of the order of 10 - 15 with a significant mass.

Considering the orbits of electrons in an atom to be fixed, Thomson in 1913 also came to a planetary model of the structure of the atom. But, solving the problem of the stability of such an atom using Coulomb’s law, he found a stable orbit for only one electron. Neither Thomson nor Rutherford could explain the emission of alpha particles during radioactive decay - it turned out that there must be electrons in the center of the atom?! His assistant G. Moseley measured the frequency of the spectral lines of a number of atoms of the Periodic Table and found that “an atom has a certain characteristic value that regularly increases when moving from atom to atom. This quantity cannot be anything other than the charge of the inner core.”

The construction of a theory of atomic structure based on the planetary model encountered a lot of contradictions.

At first, the Danish physicist N. Bohr tried to apply classical mechanics and electrodynamics to the problem of deceleration of charged particles when moving through matter, but for a given value of the electron energy, it became possible to assign arbitrary orbital parameters (or frequencies) to it, which led to paradoxes.

Bohr agreed on the theory of atomic structure with the problem of the origin of spectra. He supplemented Rutherford's model with postulates that ensured the stability of the atom and the line spectrum of its radiation. Bohr abandoned the ideas of classical mechanics and turned to Planck's quantum hypothesis: a certain relationship between the kinetic energy in the ring and the period of revolution is a transfer of the relationship E= hv , expressing the relationship between energy and frequency of the oscillator for a system undergoing periodic motion. The spectral formulas of Balmer, Rydberg and Ritz made it possible to formulate the requirements for ensuring the stability of the atom and the line nature of the spectrum of the hydrogen atom: in the atom there are several stationary states (or electron orbits in the planetary model) in which the atom does not emit energy; When an electron moves from one stationary orbit to another, the atom emits or absorbs a portion of energy proportional to the frequency, consistent with the Rydberg-Ritz frequency rule.

April 30, 1897 is officially considered the birthday of the first elementary particle - the electron. On this day, the head of the Cavedish Laboratory and a member of the Royal Society of London, Joseph John Thomson, made a historical announcement " Cathode rays"at the Royal Institution of Great Britain, in which he announced that his many years of research into electrical discharge in gases at low pressure resulted in the clarification of the nature of cathode rays. By placing a gas-discharge tube in crossed magnetic and electric fields, he, by observing the compensating effect of these fields, reliably determined the specific charge of the particles, the flow of which was cathode rays.

The idea of ​​the discrete nature of electric charge was firmly established in science thanks to previous studies of electrical phenomena. Even Michael Faraday (1791-1867) in the first half of the 1830s, while studying the passage of current through electrolytes, established that in order to release one gram equivalent of any substance on the electrode, the same amount of electricity must be passed through the solution, which became known as the Faraday number .

In his work he wrote: “The atoms of bodies... contain equal amounts of electricity naturally associated with them.” But still he did not conclude about the existence of a minimum elementary charge.

Irish physicist Stoney Stoney (1826-1911) came to this conclusion from the laws of electrolysis in 1874, and then in 1891 he postulated the existence of a charge in an atom, calling it an electron. But these predictions implied, of course, that the carrier of negative electricity would be a particle of a substance such as ions in an electrolyte deposited on the positive electrode.

However, the result obtained by J. J. Thomson turned out to be completely unexpected and even paradoxical for his contemporaries. First of all, a series of experiments carried out showed that the results of measurements with cathode rays were completely independent of the type of gas in which the discharge took place. In addition, the measured ratio e/m (specific charge) turned out to be anomalously large: it turned out to be almost 2 thousand times greater than the ratio of the value of the elementary electric charge to the mass of the lightest hydrogen atom. He also emphasized that the particles he discovered are part of the atoms of any gas. Let us quote here the words of J. J. Thomson on this subject: “The result of this, obviously, is a value of charge independent of the nature of the gas, since the charge carriers are the same for any gas. Thus, cathode rays represent a new state of matter, a state in which the division of matter goes much further than in the case of the ordinary gaseous state, ... this matter represents the substance from which all chemical elements are built."

Even before the discovery of the electron, J. J. Thomson reliably proved the corpuscular nature of cathode rays, which were taken by many prominent scientists (Heinrich Hertz, Philip Lenard, etc.) to be electromagnetic waves. I. Pulyuy did the same.

Later (1903), J. J. Thomson put forward a model of the atom in which electrons were included in the form of point-like individual particles floating in the continuous positively charged environment of the atom. One should be aware of how difficult it was then to imagine an atom in the form of a void, in which positive charges were concentrated in a small volume of the central nucleus. (Yet a similar planetary model was proposed even earlier by the French scientist Jean Perrin in 1901 and then in 1904 by the Japanese physicist Hantaro Nagaoka, who compared the electrons in an atom to the rings of the planet Saturn). J. J. Thomson in 1904 also introduced the idea that electrons in atoms are divided into separate groups and thereby determine the periodicity of the properties of chemical elements. The small value of the electron's mass was taken as a measure of the inertia inherent in the electric field of the particle itself. At the beginning of his scientific career (1881), J. J. Thomson showed that an electrically charged sphere increases its inertial mass by a certain amount, which depended on the magnitude of the charge and the radius of the sphere, and thereby he introduced the concept of electromagnetic mass. The relation he obtained was used to estimate the size of an electron under the assumption that all its mass is of an electromagnetic nature. This classical approach showed that the size of an electron is hundreds of thousands of times smaller than the size of an atom.

It is interesting that the discovery of the electron preceded the discovery of the proton, which was led by studies of channel rays in a Crookes tube. These rays were discovered in 1886 by the German physicist Eugen Holstein (1850-1930) from the glow formed in a channel made in the cathode.

In 1895, J. Perrin established the positive charge carried by channel particles. The German physicist Wilhelm Wien (1864-1928) in 1902, using measurements in crossed magnetic and electric fields, determined the specific charge of particles, which, when filling the tube with hydrogen, corresponded to the weight of the positive ion of the hydrogen atom.

The discovery of the electron immediately influenced the entire further development of physics. In 1898, several scientists (K. Rikke, P. Drude, and J. Thomson) independently put forward the concept of free electrons in metals. This concept was later used as the basis for the Drude-Lorentz theory. A. Poincaré entitled his fundamental work on the theory of relativity “On the dynamics of the electron.” But all this was not only the beginning of the rapid development of electron physics, but also the beginning of a revolutionary transformation of the basic physical principles. With the discovery of the electron, the idea of ​​the indivisibility of the atom collapsed, and after this, the initial ideas of a completely non-classical theory of the behavior of electrons in atoms began to form.

Over the past century, the importance of the discovery of the electron has continuously increased.

His works are devoted to the study of the passage of electric current through rarefied gases, the study of cathode and X-rays, and atomic physics. He also developed the theory of electron motion in magnetic and electric fields. And in 1907, he proposed the principle of operation of a mass spectrometer. For his work on cathode rays and the discovery of the electron, he was awarded the Nobel Prize in 1906.

?Ministry of Education and Science of the Russian Federation
Federal State Budgetary Educational Institution
higher professional education
"Sterlitamak State Pedagogical Academy
them. Zainab Biisheva"
Faculty of Mathematics and Natural Sciences

Department of General Physics

Essay
History of the discovery of electrons

Completed by: student of group FM-52
Saifetdinov Arthur
Checked by: Ph.D., Associate Professor Korkeshko O.I.

Sterlitamak 2011Introduction
Chapter I. Background of the discovery
Chapter II. Discovery of the electron

3.1. Thomson's experiment
3.2. Rutherford's experience
3.3. Millikan method
3.3.1. Short biography:

3.3.3. Description of installation



Conclusion
Literature

Introduction

ELECTRON - the first elementary particle to be discovered; the material carrier of the smallest mass and the smallest electric charge in nature; component of an atom.
The electron charge is 1.6021892. 10-19 Grades
- 4.803242. 10-10 units SGSE.
The mass of the electron is 9.109534. 10-31 kg.
Specific charge e/me 1.7588047. 1011 Cl. kg -1.
The electron spin is equal to 1/2 (in units of h) and has two projections ±1/2; electrons obey Fermi-Dirac statistics, fermions. They are subject to the Pauli exclusion principle.
The magnetic moment of an electron is equal to - 1.00116 mb, where mb is the Bohr magneton.
The electron is a stable particle. According to experimental data, the lifetime is te > 2. 1022 years old.
Does not participate in the strong interaction, lepton. Modern physics considers the electron as a truly elementary particle that does not have structure or size. If the latter are nonzero, then the electron radius re< 10 -18 м.

Chapter I. Background of the discovery

The discovery of the electron was the result of numerous experiments. By the beginning of the 20th century. the existence of the electron was established in a number of independent experiments. But, despite the colossal experimental material accumulated by entire national schools, the electron remained a hypothetical particle, because experience had not yet answered a number of fundamental questions. In reality, the “discovery” of the electron took more than half a century and did not end in 1897; Many scientists and inventors took part in it.
First of all, there has not been a single experiment involving individual electrons. The elementary charge was calculated based on measurements of the microscopic charge, assuming the validity of a number of hypotheses.
There was uncertainty at a fundamentally important point. The electron first appeared as a result of an atomic interpretation of the laws of electrolysis, then it was discovered in a gas discharge. It was not clear whether physics was actually dealing with the same object. A large group of skeptical natural scientists believed that the elementary charge is a statistical average of charges of the most varied sizes. Moreover, none of the experiments measuring the electron charge gave strictly repeatable values.
There were skeptics who generally ignored the discovery of the electron. Academician A.F. Ioffe in his memories of his teacher V.K. Roentgene wrote: “Until 1906 - 1907. the word electron should not have been uttered at the Physics Institute of the University of Munich. Roentgen considered it an unproven hypothesis, often used without sufficient grounds and needlessly.”
The question of the mass of the electron has not been resolved, and it has not been proven that the charges on both conductors and dielectrics consist of electrons. The concept of “electron” did not have an unambiguous interpretation, because the experiment had not yet revealed the structure of the atom (Rutherford’s planetary model appeared in 1911, and Bohr’s theory in 1913).
The electron has not yet entered into theoretical constructions. Lorentz's electronic theory featured a continuously distributed charge density. The theory of metallic conductivity, developed by Drude, dealt with discrete charges, but these were arbitrary charges, on the value of which no restrictions were imposed.
The electron has not yet left the framework of “pure” science. Let us recall that the first electron tube appeared only in 1907. To move from faith to conviction, it was necessary, first of all, to isolate the electron and invent a method for direct and accurate measurement of the elementary charge.
The solution to this problem was not long in coming. In 1752, the idea of ​​discreteness of electric charge was first expressed by B. Franklin. Experimentally, the discreteness of charges was justified by the laws of electrolysis, discovered by M. Faraday in 1834. The numerical value of the elementary charge (the smallest electrical charge found in nature) was theoretically calculated based on the laws of electrolysis using Avogadro's number. Direct experimental measurement of the elementary charge was carried out by R. Millikan in classical experiments performed in 1908 - 1916. These experiments also provided irrefutable proof of the atomism of electricity. According to the basic concepts of electronic theory, the charge of a body arises as a result of a change in the number of electrons contained in it (or positive ions, the charge value of which is a multiple of the charge of the electron). Therefore, the charge of any body must change abruptly and in such portions that contain an integer number of electron charges. Having experimentally established the discrete nature of the change in electric charge, R. Millikan was able to obtain confirmation of the existence of electrons and determine the value of the charge of one electron (elementary charge) using the oil drop method. The method is based on the study of the movement of charged oil droplets in a uniform electric field of known strength E.

Chapter II. Discovery of the electron

If we ignore what preceded the discovery of the first elementary particle - the electron, and what accompanied this outstanding event, we can say briefly: in 1897, the famous English physicist THOMSON Joseph John (1856-1940) measured the specific charge q/m cathode ray particles - “corpuscles,” as he called them, based on the deflection of cathode rays *) in electric and magnetic fields.
By comparing the obtained number with the specific charge of the monovalent hydrogen ion known at that time, through indirect reasoning, he came to the conclusion that the mass of these particles, which later received the name “electrons,” is significantly less (more than a thousand times) than the mass of the lightest hydrogen ion.
In the same year, 1897, he hypothesized that electrons are an integral part of atoms, and cathode rays are not atoms or electromagnetic radiation, as some researchers of the properties of rays believed. Thomson wrote: "Thus the cathode rays represent a new state of matter, essentially different from the ordinary gaseous state...; in this new state matter is the substance from which all the elements are constructed."
Since 1897, the corpuscular model of cathode rays began to gain general acceptance, although there were a wide variety of opinions about the nature of electricity. Thus, the German physicist E. Wichert believed that “electricity is something imaginary, existing really only in thoughts,” and the famous English physicist Lord Kelvin in the same year, 1897, wrote about electricity as a kind of “continuous fluid.”
Thomson's idea of ​​cathode ray corpuscles as the basic components of the atom was not met with much enthusiasm. Some of his colleagues thought that he had mystified them when he suggested that cathode ray particles should be considered as possible components of the atom. The true role of Thomson corpuscles in the structure of the atom could be understood in combination with the results of other studies, in particular with the results of the analysis of spectra and the study of radioactivity.
On April 29, 1897, Thomson made his famous message at a meeting of the Royal Society of London. The exact time of discovery of the electron - day and hour - cannot be named due to its uniqueness. This event was the result of many years of work by Thomson and his employees. Neither Thomson nor anyone else had ever actually observed an electron, nor had anyone been able to isolate a single particle from a beam of cathode rays and measure its specific charge. The author of the discovery is J.J. Thomson because his ideas about the electron were close to modern ones. In 1903, he proposed one of the first models of the atom - “raisin pudding”, and in 1904 he proposed that the electrons in an atom are divided into groups, forming different configurations that determine the periodicity of chemical elements.
The location of the discovery is precisely known - the Cavendish Laboratory (Cambridge, UK). Created in 1870 by J.C. Maxwell, over the next hundred years it became the “cradle” of a whole chain of brilliant discoveries in various fields of physics, especially in atomic and nuclear physics. Its directors were: Maxwell J.K. - from 1871 to 1879, Lord Rayleigh - from 1879 to 1884, Thomson J.J. - from 1884 to 1919, Rutherford E. - from 1919 to 1937, Bragg L. - from 1938 to 1953; Deputy Director 1923-1935 - Chadwick J.
Scientific experimental research was carried out by one scientist or a small group in an atmosphere of creative exploration. Lawrence Bragg later recalled his work in 1913 with his father, Henry Bragg: “It was a wonderful time when new exciting results were obtained almost every week, like the discovery of new gold-bearing areas where nuggets can be picked up directly from the ground. This continued until the beginning of the war*), which stopped our joint work."

Chapter III. Methods for discovering the electron

3.1. Thomson's experiment

Joseph John Thomson Joseph John Thomson, 1856–1940 English physicist, better known simply as J. J. Thomson. Born in Cheetham Hill, a suburb of Manchester, in the family of a second-hand antique dealer. In 1876 he won a scholarship to Cambridge. In 1884-1919, he was a professor at the Department of Experimental Physics at the University of Cambridge and, concurrently, the head of the Cavendish Laboratory, which, through Thomson’s efforts, became one of the most famous research centers in the world. At the same time, in 1905-1918, he was a professor at the Royal Institute in London. Winner of the Nobel Prize in Physics in 1906 with the wording “for his studies of the passage of electricity through gases,” which, naturally, includes the discovery of the electron. Thomson's son George Paget Thomson (1892-1975) also eventually became a Nobel laureate in physics - in 1937 for the experimental discovery of electron diffraction by crystals.
In 1897, the young English physicist J. J. Thomson became famous throughout the centuries as the discoverer of the electron. In his experiment, Thomson used an improved cathode ray tube, the design of which was supplemented by electric coils that created (according to Ampere's law) a magnetic field inside the tube, and a set of parallel electric capacitor plates that created an electric field inside the tube. Thanks to this, it became possible to study the behavior of cathode rays under the influence of both magnetic and electric fields.
Using a new tube design, Thomson showed successively that: (1) cathode rays are deflected in a magnetic field in the absence of an electric one; (2) cathode rays are deflected in an electric field in the absence of a magnetic field; and (3) under the simultaneous action of electric and magnetic fields of balanced intensity, oriented in directions that separately cause deviations in opposite directions, the cathode rays propagate rectilinearly, that is, the action of the two fields is mutually balanced.
Thomson found that the relationship between the electric and magnetic fields at which their effects are balanced depends on the speed at which the particles move. After conducting a series of measurements, Thomson was able to determine the speed of movement of the cathode rays. It turned out that they move much slower than the speed of light, which meant that cathode rays could only be particles, since any electromagnetic radiation, including light itself, travels at the speed of light (see Spectrum of electromagnetic radiation). These unknown particles. Thomson called them “corpuscles,” but they soon became known as “electrons.”
It immediately became clear that electrons must exist as part of atoms - otherwise, where would they come from? April 30, 1897 - the date of Thomson's report of his results at a meeting of the Royal Society of London - is considered the birthday of the electron. And on this day the idea of ​​the “indivisibility” of atoms became a thing of the past (see Atomic theory of the structure of matter). Together with the discovery of the atomic nucleus that followed a little over ten years later (see Rutherford's experiment), the discovery of the electron laid the foundation for the modern model of the atom.
The “cathode” tubes described above, or more precisely, cathode ray tubes, became the simplest predecessors of modern television picture tubes and computer monitors, in which strictly controlled quantities of electrons are knocked out from the surface of a hot cathode, under the influence of alternating magnetic fields they are deflected at strictly specified angles and bombard the phosphorescent cells of the screens , forming on them a clear image resulting from the photoelectric effect, the discovery of which would also be impossible without our knowledge of the true nature of the cathode rays.

3.2. Rutherford's experience

Ernest Rutherford, First Baron Rutherford of Nelson, 1871–1937 New Zealand physicist. Born in Nelson, the son of an artisan farmer. Won a scholarship to study at the University of Cambridge in England. After graduating, he was appointed to the Canadian McGill University, where, together with Frederick Soddy (1877–1966), he established the basic laws of the phenomenon of radioactivity, for which he was awarded the Nobel Prize in Chemistry in 1908. Soon the scientist moved to the University of Manchester, where, under his leadership, Hans Geiger (1882–1945) invented his famous Geiger counter, began researching the structure of the atom, and in 1911 discovered the existence of the atomic nucleus. During the First World War, he was involved in the development of sonars (acoustic radars) to detect enemy submarines. In 1919 he was appointed professor of physics and director of the Cavendish Laboratory at the University of Cambridge and in the same year discovered nuclear decay as a result of bombardment by high-energy heavy particles. Rutherford remained in this position until the end of his life, at the same time being for many years president of the Royal Scientific Society. He was buried in Westminster Abbey next to Newton, Darwin and Faraday.
Ernest Rutherford is a unique scientist in the sense that he made his main discoveries after receiving the Nobel Prize. In 1911, he succeeded in an experiment that not only allowed scientists to peer deep into the atom and gain insight into its structure, but also became a model of grace and depth of design.

Using a natural source of radioactive radiation, Rutherford built a cannon that produced a directed and focused stream of particles. The gun was a lead box with a narrow slot, inside of which radioactive material was placed. Due to this, particles (in this case alpha particles, consisting of two protons and two neutrons) emitted by the radioactive substance in all directions except one were absorbed by the lead screen, and only a directed beam of alpha particles was released through the slot.
Further along the path of the beam there were several more lead screens with narrow slits that cut off particles deviating from a strictly specified direction. As a result, a perfectly focused beam of alpha particles flew towards the target, and the target itself was a thin sheet of gold foil. It was the alpha ray that hit her. After colliding with the foil atoms, the alpha particles continued their path and hit a luminescent screen installed behind the target, on which flashes were recorded when alpha particles hit it. From them, the experimenter could judge in what quantity and how much alpha particles deviate from the direction of rectilinear motion as a result of collisions with foil atoms.
Rutherford, however, noted that none of his predecessors had even tried to test experimentally whether some alpha particles were deflected at very large angles. The raisin grid model simply did not allow for the existence of structural elements in the atom so dense and heavy that they could deflect fast alpha particles at significant angles, so no one bothered to test this possibility. Rutherford asked one of his students to re-equip the installation in such a way that it was possible to observe the scattering of alpha particles at large deflection angles - just to clear his conscience, to finally exclude this possibility. The detector was a screen coated with sodium sulfide, a material that produces a fluorescent flash when an alpha particle hits it. Imagine the surprise not only of the student who directly carried out the experiment, but also of Rutherford himself when it turned out that some particles were deflected at angles up to 180°!
The picture of the atom drawn by Rutherford based on the results of his experiment is well known to us today. An atom consists of a super-dense, compact nucleus that carries a positive charge, and negatively charged light electrons around it. Later, scientists provided a reliable theoretical basis for this picture (see Bohr's Atom), but it all started with a simple experiment with a small sample of radioactive material and a piece of gold foil.

3.3. Millikan method

3.3.1. Short biography:

Robert Milliken was born in 1868 in Illinois into a poor priest's family. He spent his childhood in the provincial town of Maquoketa, where a lot of attention was paid to sports and poor teaching. A high school principal who taught physics said, for example, to his young students: “How is it possible to make sound out of waves? Nonsense, boys, it’s all nonsense!”
Oberdeen College was no better, but Milliken, who had no financial support, had to teach high school physics himself. In America at that time there were only two textbooks on physics, translated from French, and the talented young man had no difficulty in studying them and teaching them successfully. In 1893 he entered Columbia University, then went to study in Germany.
Milliken was 28 years old when he received an offer from A. Michelson to take an assistant position at the University of Chicago. At first, he was engaged here almost exclusively in pedagogical work, and only at the age of forty began scientific research, which brought him world fame.

3.3.2. First experiences and solutions to problems

The first experiments boiled down to the following. Between the plates of a flat capacitor, to which a voltage of 4000 V was applied, a cloud was created, consisting of droplets of water deposited on the ions. First, the cloud top was observed to fall in the absence of an electric field. Then a cloud was created while the voltage was turned on. The fall of the cloud occurred under the influence of gravity and electrical force.
The ratio of the force acting on a drop in a cloud to the speed it acquires is the same in the first and second cases. In the first case, the force is equal to mg, in the second mg + qE, where q is the charge of the drop, E is the electric field strength. If the speed in the first case is?1 in the second?2, then

Knowing the dependence of the speed of cloud fall? from the air viscosity, we can calculate the required charge q. However, this method did not provide the desired accuracy because it contained hypothetical assumptions beyond the control of the experimenter.
To increase the accuracy of the measurements, it was necessary first of all to find a way to take into account the evaporation of the cloud, which inevitably occurred during the measurement process.
Reflecting on this problem, Millikan came up with the classical drop method, which opened up a number of unexpected possibilities. We’ll let the author himself tell the story of the invention:
“Realizing that the rate of evaporation of the droplets remained unknown, I tried to come up with a method that would completely eliminate this uncertain value. My plan was as follows. In previous experiments, the electric field could only slightly increase or decrease the speed of the cloud top falling under the influence of gravity. Now I wanted to strengthen this field so much that the upper surface of the cloud remained at a constant height. In this case, it became possible to accurately determine the rate of cloud evaporation and take it into account in calculations.”
To implement this idea, Millikan designed a small-sized rechargeable battery that produced a voltage of up to 104 V (for that time this was an outstanding achievement by an experimenter). It had to create a field strong enough to keep the cloud suspended, like the “coffin of Mohammed.” “When I had everything ready,” says Milliken, and when the cloud formed, I turned the switch, and the cloud was in an electric field. And at that moment it melted before my eyes, in other words, not even a small piece remained of the whole cloud that could be observed with the help of a control optical instrument, as Wilson did and I was going to do. As it seemed to me at first, the disappearance of the cloud without a trace in the electric field between the upper and lower plates meant that the experiment ended without results...” However, as often happened in the history of science, failure gave rise to a new idea. It led to the famous drop method. “Repeated experiments,” writes Millikan, “showed that after the cloud dissipated in a powerful electric field, several individual water drops could be distinguished in its place” (emphasis added by me - V.D.). The “unsuccessful” experiment led to the discovery of the possibility of keeping individual droplets in equilibrium and observing them for quite a long time.
But during the observation, the mass of a drop of water changed significantly as a result of evaporation, and Millikan, after many days of searching, moved on to experiments with drops of oil.
The experimental procedure turned out to be simple. Adiabatic expansion forms a cloud between the capacitor plates. It consists of droplets with charges of different magnitude and sign. When the electric field is turned on, drops with charges identical to the charge of the upper plate of the capacitor quickly fall, and drops with the opposite charge are attracted by the upper plate. But a certain number of drops have such a charge that the force of gravity is balanced by the electrical force.
After 7 or 8 minutes. the cloud dissipates, and a small number of drops remain in the field of view, the charge of which corresponds to the indicated balance of forces.
Millikan observed these drops as distinct bright dots. “The history of these drops usually goes like this,” he writes. “In the case of a slight predominance of gravity over the field force, they begin to fall slowly, but since they gradually evaporate, their downward movement soon stops, and they become motionless for quite a long time.” . Then the field begins to dominate and the drops begin to slowly rise. At the end of their life in the space between the plates, this upward movement becomes very strongly accelerated, and they are attracted with great speed to the upper plate.”

3.3.3. Description of installation

A diagram of Millikan's installation, with which decisive results were obtained in 1909, is shown in Figure 17.
A flat capacitor made of round brass plates M and N with a diameter of 22 cm (the distance between them was 1.6 cm) was placed in chamber C. A small hole p was made in the center of the top plate, through which drops of oil passed. The latter were formed by injecting a stream of oil using a sprayer. The air was previously cleared of dust by passing it through a pipe with glass wool. The oil droplets had a diameter of about 10-4 cm.
A voltage of 104 V was supplied from battery B to the plates of the capacitor. Using a switch, it was possible to short-circuit the plates and this would destroy the electric field.
Drops of oil falling between the plates M and N were illuminated by a strong source. The behavior of droplets was observed perpendicular to the direction of the rays through the telescope.
The ions necessary for droplet condensation were created by radiation from a piece of radium weighing 200 mg, located at a distance of 3 to 10 cm to the side of the plates.
Using a special device, lowering the piston expanded the gas. 1 - 2 s after expansion, the radium was removed or obscured by a lead screen. Then the electric field was turned on and the observation of drops into the telescope began. The pipe had a scale on which it was possible to count the path traveled by the drop over a certain period of time. Time was recorded using an accurate clock with a lock.
During his observations, Millikan discovered a phenomenon that served as the key to the entire series of subsequent precise measurements of individual elementary charges.
“While working on suspended drops,” writes Millikan, “I forgot several times to shield them from the radium rays. Then I happened to notice that from time to time one of the drops suddenly changed its charge and began to move along the field or against it, apparently capturing in the first case a positive, and in the second case a negative ion. This opened up the possibility of reliably measuring not only the charges of individual drops, as I had done until then, but also the charge of an individual atmospheric ion.
Indeed, by measuring the velocity of the same drop twice, once before and once after the capture of the ion, I could obviously completely exclude the properties of the drop and the properties of the medium and operate with a value proportional only to the charge of the captured ion.”

3.3.4. Elementary Charge Calculation

The elementary charge was calculated by Millikan based on the following considerations. The speed of movement of a drop is proportional to the force acting on it and does not depend on the charge of the drop.
If a drop fell between the plates of a capacitor under the influence of gravity alone with a speed?, then
?1=kmg (1)

When a field directed against gravity is turned on, the acting force will be the difference qE - mg, where q is the charge of the drop, E is the modulus of the field strength.
The speed of the drop will be equal to:
?2 =k(qE-mg) (2)

If we divide equality (1) by (2), we get

From here
(3)

Let the drop capture an ion and its charge become equal to q", and the speed of movement? 2. Let us denote the charge of this captured ion by e.
Then e= q"- q.
Using (3), we get

The value is constant for a given drop.

3.3.5. Conclusions from the Millikan method

Consequently, any charge captured by a drop will be proportional to the difference in speed (?2 - ?2), in other words, proportional to the change in the speed of the drop due to the capture of an ion! So, the measurement of the elementary charge was reduced to measuring the path traveled by the drop and the time during which this the path was passed. Numerous observations showed the validity of formula (4). It turned out that the value of e can only change abruptly! Charges e, 2e, 3e, 4e, etc. are always observed.
“In many cases,” writes Millikan, “the drop was observed for five or six hours, and during this time it captured not eight or ten ions, but hundreds of them. In total I have observed the capture of many thousands of ions in this way, and in all cases the charge captured... was either exactly equal to the smallest of all the charges captured, or it was equal to a small integer multiple of this value. This is direct and irrefutable proof that the electron is not a “statistical average,” but that all the electrical charges on the ions are either exactly equal to the charge of the electron or represent small integer multiples of that charge.”
So, the atomicity, discreteness or, in modern language, quantization of the electric charge has become an experimental fact. Now it was important to show that the electron is, so to speak, omnipresent. Any electric charge in a body of any nature is the sum of the same elementary charges.
Millikan's method made it possible to unambiguously answer this question. In the first experiments, charges were created by ionization of neutral gas molecules by a stream of radioactive radiation. The charge of ions captured by the droplets was measured.
When a liquid is sprayed with a spray bottle, the droplets become electrified due to friction. This was well known back in the 19th century. Are these charges also quantized, like the ion charges? Millikan "weighs" the droplets after spraying and measures the charges in the manner described above. Experience reveals the same discreteness of electric charge.
Further, the identity of electric charges on bodies of different physical nature was shown.
Sprinkling drops of oil (dielectric), glycerin (semiconductor), mercury (conductor), Millikan proves that charges on bodies of any physical nature consist in all cases, without exception, of individual elementary portions of strictly constant magnitude. In 1913, Millikan summarized the results of numerous experiments and gave the following value for the elementary charge: e = 4.774.10-10 units. SGSE charge. This was how one of the most important constants of modern physics was established. Determining electric charge became a simple arithmetic problem.

3.4. Compton imaging method

The discovery of C.T.R. played a major role in strengthening the idea of ​​the reality of the electron. Wilson, the effect of condensation of water vapor on ions, which led to the possibility of photographing particle tracks.
They say that A. Compton during a lecture could not convince a skeptical listener of the reality of the existence of microparticles. He insisted that he would believe only after seeing them with his own eyes.
Then Compton showed a photograph of a particle track, next to which was a fingerprint. “Do you know what this is?” - asked Compton. “Finger,” answered the listener. “In that case,” Compton said solemnly, “this luminous stripe is the particle.”
Photographs of electron tracks not only testified to the reality of electrons. They confirmed the assumption of the small size of electrons and made it possible to compare the results of theoretical calculations, which included the electron radius, with experiment. Experiments, which began with Lenard's study of the penetrating power of cathode rays, showed that very fast electrons emitted by radioactive substances produce tracks in the gas in the form of straight lines. The track length is proportional to the electron energy. Photographs of tracks of high-energy particles show that the tracks consist of a large number of points. Each dot is a water droplet that appears on an ion, which is formed as a result of the collision of an electron with an atom. Knowing the dimensions of an atom and their concentration, we can calculate the number of atoms through which a particle must pass at a given distance. A simple calculation shows that the?-particle
etc.................

The hypothesis about the existence of atoms, those indivisible particles, the various configurations of which in the void form the objective world around us, is as old as our civilization:

“Nature decomposes everything into basic bodies.”

Newton's solid, massy, ​​and indivisible atoms; atoms in kinetic theory, the average kinetic energy of which is identified with the temperature of the body; atoms in chemistry, harmonious combinations of which are found in chemical reactions; the hydrogen atom, from various combinations of which Prout composed all the elements. The concept of the atom has been around for at least 25 centuries, although it has often been relegated to the background or suppressed.

But what is an atom? And what meaning should be put into this question? By the end of the nineteenth century, when the creation of the classical theory was completed and new technical means appeared, all

The old question began to sound more insistently: what is the nature of the atom? This theme and its variations became the leitmotif of twentieth-century physics.

At the end of the nineteenth century, many experiments were carried out to study electric discharge in rarefied gases. The discharge was excited (by means of an induction coil or electrostatic machine, creating large potential differences) between a negative electrode, called the cathode, and a positive electrode, called the anode, both electrodes being sealed inside a glass tube from which the air was evacuated. When the air in the tube became sufficiently rarefied, the dark region around the cathode, known as the dark Crookes spot, gradually expanded until it reached the opposite end of the tube, which then began to glow, the color of the glow depending on the type of glass from which the tube was made.

If various screens are inserted into the tube, for example, as in FIG. 62, then a small spot located at the end of the tube will glow, as if something were passing through the holes in the screen and, reaching the glass, causing it to glow. This something was called cathode rays.

At the end of the nineteenth century there was lively debate about the nature of these rays. Some believed that rays, like light, owe their origin to processes in the ether; others believed that they consisted of electrically charged particles. In 1895, Jean Perrin managed to collect these rays in an isolated vessel and prove that they carry a negative charge. Shortly thereafter, J. J. Thomson carried out his classic experiment, in which he first identified cathode rays with particles later called electrons. He wrote:

“The experiments described in this article were carried out for the purpose of obtaining some information about the nature of cathode rays. There are completely opposite points of view regarding these rays; according to the almost unanimous opinion of German physicists, they are caused by some kind of processes in the ether, which - due to the fact that their path in a uniform magnetic field is not rectilinear, but circular - has no analogue in any of the previously observed phenomena; according to

Another opinion is that these rays are far from being of ethereal origin, but of material origin and are simply a stream of particles of matter charged with negative electricity.”

Fig. 63. Thomson installation diagram (taken from).

By creating an electric field between the plates indicated in FIG. 63 letters and or a magnetic field directed perpendicular to the direction of propagation of the rays, Thomson observed the displacement of a luminous spot at the end of the tube; The stronger the electric or magnetic fields, the more the spot shifted. Having made sure that this phenomenon does not depend on what kind of gas is in the tube, Thomson wrote:

“Since the cathode rays carry a negative charge, are deflected by electrostatic force as if they were negatively charged, and react to the magnetic force in the same way as negatively charged bodies moving along the line of propagation of the rays would react to it, I cannot help come to the conclusion that cathode rays are charges of negative electricity carried by particles of matter. Then the question arises: what are these particles? Are they atoms, molecules, or matter in a finer state of separation? In order to shed some light on this question, I carried out a number of measurements of the ratio of the mass of these particles to the amount of charge carried by them.”

At the same time, the force acting on a charged particle from the magnetic field B, perpendicular to the direction of its movement:

If, for example, the particle is negatively charged and the electric field is directed away from to, then the electric force will deflect the particle downward. The magnetic force acting on a particle moving in a magnetic field directed as shown in Fig. 64, will deflect the particle upward: Therefore, by selecting the strengths of the electric and magnetic fields so that the luminous spot remains undisplaced, Thomson thereby equalizes the forces acting on the particles from the electric and magnetic fields:

From here he learned the speed of hypothetical particles. Then, by turning off the electric field and varying the strength of the magnetic field, he could change the amount of deflection of the particles at the end of the tube. Knowing the time during which the particles were in the magnetic field (since he knew their speed), Thomson could thereby calculate the effect of this field on them. From here, from the measured deviation, he was able to determine the ratio of the charge of the particles to their mass.

He eventually arrived at the following mass-to-charge ratio for his hypothetical particles:

Thomson concluded:

“From these measurements it is clear that the value does not depend on the nature of the gas, and its value is very small compared to the value that is the smallest previously known value for this ratio and relates to the hydrogen ions that participate in electrolysis.

Thus, the magnitude of the ratios for electrical carriers in cathode rays is significantly less than the corresponding value in electrolysis. Smallness is explained either by smallness or by large significance, or by both at the same time.”

This carrier of electricity, the active constituent particle of cathode rays, was eventually called the electron, which was the first elementary particle of the twentieth century.

Thomson later wrote:

“My first attempt to deflect a beam of cathode rays was to pass them between two parallel metal plates mounted inside a discharge tube and excite an electric field between these plates. I was unable to obtain a regular deflection in this way... The lack of deflection was explained by the presence of gas in the tube (the pressure remained too high), so it was necessary to obtain a higher vacuum. But this was easier said than done. The technique of obtaining high vacuum in those days was in its infancy."

Not for the first time, the implementation of a decisive experiment encountered not the difficulties of its ideological concept, but the lack of necessary technical means.

After Thomson's measurements, it was extremely important to determine either the magnitude of the charge or the mass of these particles separately. The charge of the gaseous ions, previously measured in Thomson's laboratory, was approximately Assuming that the charge of these ions is the same as the charge carried by the cathode particle, it is not difficult to show that the mass of these particles is extremely small:

In those years Thomson called the cathode particles "corpuscles," or primordial atoms; the word "electron" was used by him to denote the amount of charge carried by the "corpuscle". However, over time, the particle itself began to be called an electron. Much later (in 1909), Millikan, measuring the amount of charge on oil droplets, established that the elementary charge (it was assumed that its value was the same as the charge of the electron) is approximately equal to Let us give the modern values ​​of the charge and mass of the electron:

There is complete disagreement on this matter. Some historians of science associate the discovery of the electron with the names of G. Lorentz and P. Zeeman, others attribute it to E. Wiechert, others - to other researchers, while the majority insist on the priority of Joseph John Thomson, or the great GG, as he is also called in scientific world.

Even the most prominent authorities who are closely involved in the problems of atomic physics are completely at a loss: who owns the honor of the discoverer? The outstanding theoretical physicist N. Bohr is convinced of the priority of F.E.A. Lenard, and the unsurpassed experimental physicist E. Rutherford is convinced of F. Kaufman.

In time, the controversial period of the actual discovery of the electron extends for 28 years: from 1871 to 1899. Who stood at the origins of this significant discovery, which gave rise to such long scientific battles, when spears were seriously broken? Moreover, in a situation where some of the disputants have already managed to make too much trouble. Some of them were busy with scientific research, and some with scientific intrigues. Just like in discussions to clarify the nature of light.

At first, in 1894, the prominent German naturalist Hermann Ludwig Helmholtz and his scientific opponent, the Irishman George Stoney, fought among themselves. Each of them attributed the priority of discovering the electron to himself. Stoney, in front of all the honest people, accused Helmholtz of obvious plagiarism, publishing the facts incriminating him in the article “On the Electron or Atom of Electricity,” which appeared in one of the issues of the Philosophics Magazine (1894, vo1.38, R.418). How true was this accusation?

Twelve years before this publication in the same journal (1882, vol. 11, R. 361), Stoney published a work in which he outlined his views on the existence of the electron, arguing that “for every broken chemical bond in an electrolyte there is a certain, identical in all cases, the amount of electricity."

Less than two months had passed when an article by Helmholtz appeared in the journal published by the Chemical Society, announcing his discovery of the electron. It said: “If the idea of ​​the atomic structure of simple substances is considered correct, then one cannot avoid the conclusion that electricity, both negative and positive, is divided into elementary portions, which are held together like atoms of electricity.”

Did Helmholtz know about Stoney's work when he wrote these lines? Apparently, he couldn’t help but know. It is also beyond explanation why, speculating on his authority, he literally crushed Stoney at every opportunity, constantly passing off his priority as his own? For the sake of increasing fame? But Helmholtz already bathed in its rays quite often. Stoney, due to his immersion in the “electronic” idea, which he continued to develop, simply did not have enough time to neutralize the irritant in the person of Helmholtz.

Its development absorbed him so much that he not only managed to give a quantitative assessment of the smallest electric charge, insisting on its inclusion in the number of fundamental natural constants, but also came up with a stable name for a negatively charged elementary particle - “electron”.

Apparently, hidden envy of the hard worker Stoney’s breakthrough into the future of science forced Helmholtz to first attack his colleague everywhere, and then wisely remain silent. It is difficult to predict whether active action, counteraction or inaction will best defeat the enemy. So he temporarily fell silent.

However, if we turn the clock back a little more, there was no point in starting a fight for scientific leadership at all, since after a meticulous study of the history of the issue, two more names surfaced. It turns out that in 1878 before Stoney, one of the pillars of physical science, the Dutchman Hendrik Lorentz, had already drawn the attention of scientists to the idea of ​​discreteness of electric charges, and seven years before Lorentz, the German physicist Wilhelm Eduard Weber spoke about the electron, anticipating the research of the Irishman, and all others their followers. Weber, for example, asserted with amazing insight: “... with the universal spread of electricity, it is permissible to perceive that an electric atom is associated with every atom of a substance.” Maybe he should have received honorary laurels?

Unlikely. After all, it is one thing to express a valuable idea, another thing to contribute in every possible way to its development. And therefore, without a twinge of conscience, priority in the theoretical substantiation of the existence of the electron, in fact in the prediction of a negatively charged elementary particle, can be safely given to the Irishman Stoney, whose name, unfortunately, is not mentioned anywhere: neither in reference books, nor in encyclopedias.

By the way, not only theorists, but also experimenters fought for the priority right to discover the electron, finding out who discovered the negatively charged particle experimentally? Today, every schoolchild knows the name of J. J. Thomson, who, according to most chroniclers of science, is the true “parent” of the electron. It was for this stunning discovery that he was awarded the Nobel Prize in 1906.

The priority is considered indisputable, although in fact the historical reality contradicts it. To be convinced of this, it is enough to pick up the journal of the University of Königsberg for January 1897, where the latest research in the field of chemistry and physics was published. In the January volume 38, on page 12 of this periodical, an article by the German physicist Emil Wichert was published, unambiguously asserting the priority in the experimental discovery of the electron behind it.

Thomson reported the same discovery to the scientific council of the Royal Institution of England two months later - on April 30, 1897, and his first publication detailing this issue appeared only in May. Scientists were introduced to it by the magazine "Electricity" (1897, ou1.39, R.104).

Thus, Wichert was five months ahead of the great GG. But who was interested in the chronology of events when it came to the work of an unquestioned authority in the scientific world? Here we return to the question of what should be taken as the starting point in the distribution of intellectual property: the idea itself, its development and justification, or the pioneering printed work that includes both?

It seems that, in any case, the chronological order of the entry of a discovery or invention into power cannot be ignored. Even provided that initially there was a hypothesis that needed to “settle” in time and minds. Therefore, to the same, if not greater extent, than Stoney, Weber and the famous Thomson, the little-known Wichert was involved in the discovery of the electron.

But only in a few special reference books can one read that, independently of J. J. Thomson, this physicist discovered the electron and determined its relative charge. In this example, we are convinced of the real power in science that the power of authority has.