The discovery of the electron



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Therefore, because of this experiment, it became known that something was proceeding from the surface of the cathode – the negative electrode – traveling in a straight line down the tube producing a glow on the glass of the tube at the point where it strike. An experiment where this was proved was “by placing some opaque object such as a Maltese Cross in the path of the flying entities and causing a shadow of this object with well-defined outlines to be formed on the wall at the end of the tube.

“6 Therefore, because of this shadow of the Maltese Cross at the end of the tube, when the discharge was past, the green glow no longer extends all over the end of the tube, as when it did when the cross was not there, but instead it glowed only around the outline of the shadow of the cross. However, some German physicists held that the cathode rays are a sort of wave of the same essential nature of light or other ether vibrations. Therefore, it remained for J. J. to explain their true nature. So at the Cavendish Laboratory, J.

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J. had been using tubes of various designs for the study of cathode rays. One form in which he used to most effect was where “the cathode was a flat plate set at right angles to the axis of the tube, the anode, the positive electrode, was a cylinder of metal completely blocking the tube except for a small opening, opening a few tenths of a millimeter in diameter. “7 It was through this opening that a very fine stream of cathode rays could pass down the tube to the right causing a small glow where it struck at the end of the tube.

Also in this tube were two parallel metal disks that were set at a given distance apart which could be charged to a known potential difference, producing an electrostatic field. An electrostatic field is created between these two plates because “the upper is charged positively and the lower negatively and hence the region between them contains electrostatic energy, a kind of potential energy. “8 So as a result, the medium between the plates are in a state of tension, or as they say it, “it is the seat of electric force. “9 Therefore, instead of an electric field, a magnetic field may be used to deflect the particles.

By inserting two coils on either side of the discharge tube in the region between the two parallel metal disks, a magnetic field is produced perpendicular to the previous electric field and to the direction of the cathode rays. “Thomson determined the velocity of the cathode rays by applying the electric and magnetic fields simultaneously and adjusting their relative magnitudes so that the deflections they produced were equal and opposite. ” 10 By providing a magnetic field of strength and direction suitable to restore the beam to its original undeflected course, “Thomson was able to compute the velocity of the postulated charged particles.

The measurement of the deflection suffered by the beam under the influence of the electric field alone then served to give sufficient information to compute the specific electric charge-to-mass ratio, e/m, of the particles. “11 This charge on cathode rays was also exactly the same as that on the hydrogen ion in electrolysis. This is so because the values of e/m for the cathode ray particle and that for the hydrogen ion both have the same order of magnitude. Meaning that “the carriers of the charge must be both light atoms and it is very improbable that one should have a charge nearly 2000 times that of the other.

“12 The difference found was very intriguing because it was indicated that the two charges are about the same and very exact, which was later confirmed that they were exactly the same. Therefore, it was later shown that the charge-to-mass ratio for cathode rays turned out to be over one thousand times smaller than that of a charged hydrogen atom. So, “either the cathode rays carried an enormous charge (as compared with a charged atom), or else they were amazingly light relative to their charge. “13 But if this is true, “it follows that the mass of the cathode ray particle must be very much less than the mass of the hydrogen atom.

Therefore, it follows that we have in these discharge tubes separate particles whose mass is only about 1/1840 of that of the hydrogen atom, the lightest known atom. “14 Moreover, since these particles can be obtained with the same value of e/m and with identical characteristics, whatever be the gas in the discharge tube or whatever be the material of the electrodes, we are dealing with something which is a common constituent of all kinds of substances and whose e/m ratio is absolutely constant.

The atom, there-fore, which “Lucretius regarded as indivisible, which Dalton and his contemporaries in the absence of contrary evidence treated provisionally as indivisible, is really complex and one of its constituents is this particle. “15 A cathode particle in which J. J. Thomson called a “corpuscle”. Therefore, Thomson announced the hypothesis that “we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matter…

is of one and the same kind; this matter being the substance from which all the chemical elements are built up. “16 As a result, Thomson presented three hypotheses about cathode rays based on his 1897 experiments: “1. Cathode rays are charged particles (which he called “corpuscles). 2. These corpuscles are constituents of the atom. 3. These corpuscles are the only constituents of the atom. “17 But there were some skepticism on the second and third hypothesis. The third hypothesis indeed turned out to be false and the first and second hypotheses turned out to be true though with some subtle changes in their meaning.

Thus, to prove that “If Thomson found the single building block of all atoms, than how could atoms be built up out of these corpuscles? “18 To prove this, Thomson proposed a “plum pudding” model, where the pudding represents the sphere of positive electricity and the bits of plum scattered in the pudding are the electrons. Therefore, notice that Thomson’s model has negative electron particles and a sphere of positive charge and that there are no protons in this model of the atom. Consequently, thanks to Thomson’s great discovery of the electron, the intriguing mystery of the nature of the cathode ray has been cleared up.

Also the instinct of those who felt the vacuum discharge tube held some vital secrets about electricity was proven to be correct. For it gave a tremendous stimulus to the study of the problem of the constitution of matter, where it enabled us also to investigate more deeply the meaning and the mechanism of all sorts of chemical actions and reactions. Therefore, the discovery of the electron was highly important because it can be easily affected by magnetic and electric fields, while on account of its slight mass it can be rapidly deflected or can be given high speeds by electric fields of moderate value.

19 Therefore, in Thomson’s experiments, he found his result to be the same, whether the rays were produced by a discharge in air, hydrogen, or carbon dioxide. Thus, Thomson came to the conclusion that he had proved the existence of an entirely new particle which was more than 1000 times lighter than any known atom. Therefore, he called it the “corpuscle” which we now know as the “electron. ” It was given this name later by Stoney simply to denote the unit of charge found in experiments that passed electric current through chemicals.

Ultimately, it was Thomson’s cathode ray experiments that led to the discovery of the electron. BIBLIOGRAPHY Shannon, S. J. , James I. The Amazing Electron. Milwaukee: The Bruce Publishing Company, 1946. Thomson, George. J. J. Thomson and The Cavendish Laboratory In His Day. Carden City, New York: Doubleday & Company, Inc. , 1965. 1 http://www. aip. org/history/electron/jjthomson. htm 2 George Thomson, J. J. Thomson and The Cavendish Laboratory In His Day (Carden City, New York: Doubleday & Company, Inc. , 1965) 25. 3 Thomson 25-26. 4 Thomson 28. 5 James I. Shannon, S. J., The Amazing Electron (Milwaukee: The Bruce Publishing Company, 1946)

15. 6 Shannon 16. 7 Shannon 18. 8 Shannon 19. 9 Shannon 19. 10 http://www. phy. cam. ac. uk/cavendish 11 Robert Andrews Millikan, “The Electron”: Its Isolation and Measurement and The Determination Of Some Of Its Properties (Chicago and London: The University of Chicago Press, 1924) XVI. 12 Shannon 21. 13 http://www. aip. org/history/electron/jj1897. htm 14 Shannon 22. 15 Shannon 22. 16 http://www. aip. org/history/electron/jj1897. htm 17 http://www. aip. org/history/electron/jj1897. htm 18 http://www. aip. org/history/electron/jj1897. htm.

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