2.4 CONDUCTION OF ELECTRICITY THROUGH RAREFIED GASES AND THE ELECTRON
Thermodynamic and chemical experiences which led to the introduction of molecules and atoms respectively, are naturally occurring experiences. The spring of air, as well as the various chemical reaction between substances, are part of our daily experience. The experience which led to the introduction of the electron, on the other hand, was quite artificial. It was made possible by the development in the middle of the 19th century of quite efficient methods of creating a vacuum. When such vacuum is sufficiently advanced, electricity begins to travel through a rarefied gas with little hindrance. Electricity was introduced into a glass container filled with such a gas by means of two electrodes connected to a battery and it appeared to travel out from the negative wire as a beam of rays which, upon hitting the glass, produced a green fluorescence.
That these cathode rays were indeed rays propagating through the rarefied gas in a straight line was demonstrated by the fact that a Maltese cross placed in their path leaves a shadow of itself on the glass behind it. That they were a beam of negatively charged particles shot off at great speed from the cathode, rather than a wave propagating in some medium, was established in a succession of experiments involving magnetic and electric fields. In 1897, J.J. Thomson showed that cathode rays are deviated by a magnetic field exactly as a beam of electrically charged particle would. Jean Perrin showed that when collected in a metal vessel, these particles give up a negative charge. Since cathode rays are thus negatively charged particles traveling in a straight line at high speed, their motion should be deviated by an electric field. Thomson showed that such was indeed the case.
To explore the nature of these particles, he used both electric and magnetic fields arranged in such a manner as to balance each other’s effect upon them and calculated the ratio between the charge and the mass of such a particle. He found 1.76 × 107 Coulomb/gram, a value which is 1,836 times larger than the value for the charged hydrogen atoms met with in the electrolysis of liquids conducted by Faraday. Since the charge carried by the two vehicles is the same, the cathode ray particle must be times smaller than an atom of hydrogen. These particles, introduced to explain the behavior of cathode rays, were named electrons. Since the electron is thus much smaller than an atom and atoms are electrically neutral, there must also be positively charged particles inside an atom. Thus protons were added to the growing family of things whose external relationships to each other were used to explain the inside of things.