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|1-7: The Discovery of the Electron|
the Cathode Rays]
Put a pair of plates (electrodes) in a glass tube and apply a several kV high voltage potential between them. When the gas pressure in the tube becomes very low (lower than 0.1 atmospheric pressure), an electric discharge will take place. This is called vacuum discharge. We can see a striped pattern in the tube. An example is shown in the picture below.
When the gas pressure in the tube becomes about 0.000001 atmospheric pressure, the striped pattern will disappear and the inside of the tube will become dark. However this does not mean the discharge stopped; a current is still passing between the electrodes. Namely, something is flowing between the electrodes. These are called the cathode rays.
The equipment to generate the cathode rays was named Crookes tube after its inventor W. Crookes (UK, 1832 - 1919).
In order to investigate the properties of cathode rays we place a cross in the Crookes tube as seen in the following figures and set a screen of a fluorescent substance beyond the cross also inside the tube. Then we can watch a shadow of the cross on the screen. This implies that the cathode rays emitted from the cathode toward the anode travel in straight lines to the screen.
[What are the
J. J. Thomson (UK, 1856 - 1940) investigated the true nature of cathode rays (1897).
There have been
three famous British physicists
in the history
Do not confuse them.
(1) W. Thomson (William Thomson, 1824 - 1907). "Lord Kelvin". The unit of absolute temperature K is given after his name.
(2) J. J. Thomson (Joseph John Thomson, 1856 - 1940). Under consideration on the present page. J. J. Thomson had many achievements including not only the discovery of the electron but the proposal of the atomic model discussed later and many other discoveries.
(3) G. P. Thomson (George Paget Thomson: 1892 - 1975) A son of the above J. J. Thomson. G. P. Thomson confirmed the diffraction of electrons with the use of a metsllic crystal to prove the wave nature of electrons independently of the works of US physicists, C. J. Davisson and L. H. Germer.
The apparatus used
by J. J. Thomson
the properties of
is schematically shown
in the following
Its basic idea
is essentailly the same as that
of the Crookes tube.
J. J. Thomson presumed that the cathode rays that came out of the negative electrode (cathode) were a collection (beam) of the same particles with negative charges. If they are so, these particles emitted from the cathode would be pulled by the positive electrode (anode), pass through a small hole at the center of the anode, travel in a straight line, pass between the plates, P1 and P2, and finally arrive at a screen of fluorescent substance to make a small spot on it.
If the electric potential is so applied that the upper plate is negative and the lower is positive, then the beam would be curved downward and the spot on the screen would move downward. J. J. Thomson observed this phenomenon, and he also found that the spot neither spreaded greatly nor faded.
We can conclude from these results that J. J. Thomson's presumption is correct; namely, the cathode rays are a beam of the same kind of "particles" with a negative charge.
of the Cathode Rays]
Using the experimental set up discussed above, we can measure the charge-to-mass ratio e /m of the "particle" in the cathode rays. Here e is the charge of the particle and m is its mass. To explain the details of the method to evaluate the charge-to-mass ratio, we need some mathematical expressions, which we will explain on another page:
1-7-A: Measurement of the Charge-to-Mass Ratio of Cathode Rays
Using the method explained in the other page (1-7-A), J. J. Thomson measured the charge-to-mass ratio of the particle in the cathode rays and got a value of
A more precise estimate is
J. J. Thomson confirmed that an almost constant value of e /m was always obtained under various experimental conditions. He therefore concluded that cathode rays are a collection of the same kind of particles.
[The True Nature of
the Cathode Rays,
Let us compare the charge-to-mass ratio of the particle in the cathode rays with that of the hydrogen ion.
As seen in
Faraday's Law of Electrolysis
on Page 1-6, a charge of is necessary to separate 1 gram equivalent element through the electrolysis.
Consider the case of hydrogen for example. Because the valence of hydrogen is 1 and the atomic weight of hydrogen is about 1, 1 gram of hydrogen ions have a charge of . Namely the charge-to-mass ratio of the hydrogen ion is about . Accordingly,
This inplies either that the mass of the "particles" in cathode rays is about 1/1800 of the mass of a hydrogen ion, or that the "particle" can carry 1800 times more charge than a hydrogen ion. J. J. Thomson thought that the latter is not plausible. Therfore he assumed the former and named the particle in the cathode rays: electron (1897), whose mass is by a factor of about 1/1800 than the lightest atom, hydrogen.
[The Mass of the Electron]
A little later than J. J. Thomson's research on the electron, the value of the elementary charge e was clarified by the Millikan oil drop experiment as explained on Page 1-6;
Thus it is quite natural that the charge of the electron would be equal to the elementary charge e. Accordingly, the mass of the electron is determined.
Today, the data on the electron are precisely measured as
[Electron as a Common
Constituent of Atoms]
Various experiments showed that the properties of cathode rays do not depend on the kind of gas in the discharge tube. Moreover, when a metal is heated to a very high temperature around a large number of electrons are emitted from it. They are called thermal electrons.
O. W. Richardson (UK, 1879 - 1959) investigated the details of the thermal electrons. He considered that the electrons in atoms in a highly heated metal were strongly shaken and rushed out of the metal. Thus he confirmed that the electron is one of the common constituents of atoms.
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