The cathode ray tube that Thomson used was an evacuated glass tube with a cathode and an anode for applied high voltage, two parallel electrodes for the deflecting electric field and two spools with electric wires creating the deflecting magnetic field.

Heating the cathode with an additional electric circuit liberated the cathode rays, which were accelerated between the cathode and the anode, and then deflected by the electric and magnetic fields.

It is particularly easy to estimate the charge over mass ratio when the bending force of the electric field and magnetic field are equally large but acting in opposite directions. In this case the electrons go straight forward, unaffected by the bending forces.

J. J. Thomson studied electric discharge rays systematically for many years and concluded that the cathode rays were the same, irrespective of which cathode material or gas that was used. He tried many, but the e/m ratio always came out the same. The particle he had discovered, the electron, existed in all the different cathode materials he examined. It is actually a constituent of all the matter we are surrounded by.

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