The Discovery of the Electron: Electrical Discharges in Gases

Although standard work continued under 'J J' it was soon eclipsed by research on the discharge of electricity through gases. The glass apparatus used, hastily built and continually modified, present a great contrast to the beautifully finished pieces of brass and mahogany of the previous era. This was probably the origin of the 'string and sealing wax' tradition that was long associated with the Cavendish.

'J J' was convinced that the key to an understanding of the nature of matter was the study of discharge phenomena in gases, but for the first ten years he worked with little success. In 1895 Rontgen announced his discovery of X-rays, and Thomson very soon found that they made gases conducting. 'This was a matter of vital importance for the investigations on the passage of electricity through gases... Until the rays were discovered the only ways of making electricity pass through a gas were either to apply very great electric forces to it, or else to use very hot gases such as flames. In either case it was exceedingly difficult to get anything like accurate measurements. The results were apt to be very capricious, apparently depending upon causes which it was very difficult to locate... To have come upon a method of producing conductivity in a gas so controllable and so convenient as that of the X-rays was like coming into smooth water after long buffeting by heavy seas.' Rutherford and Thomson soon arrived at the theory of ionisation of the gas by the X-rays which is accepted today.

At very low pressures Crookes discovered that a different type of radiation, cathode rays, passed through the discharge tube. 'J J' showed that the rays were deflected by a magnetic field and that a Faraday cylinder placed out of the normal path of the rays received no charge, but received a large negative one when the beam was deflected by a magnet into the cylinder. This seemed to prove that they were negatively charged particles, but Hertz had found that the rays were not deflected by an electric field and thought that they were flexible electric currents flowing through the ether. Eventually Thomson realised that the absence of electric deflection was due to the residual gas in the tube being ionised by the rays. The ions were attracted to the electric plates and cancelled out the applied field. 'The thing to do was to get a much higher Vacuum. This was more easily said than done... However after running the discharge through the tube day after day without introducing fresh gas, the gas on the walls and electrodes got driven off and it was possible to get a much better vacuum. The deflection of the cathode rays by electric forces became quite marked... This result removed the discrepancy between the effect of magnetic and electric forces on the cathode particles: it did much more than this, it provided a method of measuring the velocity of these particles, and also m/e where m is a mass of a parciel and e its electric charge.' 'J J' found that m/e for the cathode rays was about 1/1000 times that of the Hydrogen ion and postulated that they were very light, negatively charged 'corpuscles' which were a universal constituent of matter.

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