Cathode rays were an enigma during the late 1800s. Many Europeans thought that the rays emitted at the cathode in the discharge tube were an ethereal disturbance, like light. Crookes proposed in 1879, that the cathode rays were radiant matter or negatively charged particles that were attracted to the anode and repelled from the cathode (Cambridge Physics, 2015).
In Germany, Hertz conducted an experiment where he observed that the rays discharged could pass through very thin gold sheets. He also conducted another experiment to find out the impact of electric fields on the rays and wrongly concluded that the rays were not deflected by the electric fields. In 1897, J.J Thomson repeated Hertz’s experiment (Cambridge Physics, 2015).
The difference between J.J Thomson’s experiment and Hertz’s experiment was that J.J Thomson conducted his experiment in a vacuum tube. Hertz had too much gas in his discharge tube and the gas had become ionized by the electric field. The positive ions were attracted to the negative plate and the negative ions were attracted to the positive plate. This reduced the net charge on the plates and the strength of the field was not enough to deflect the cathode rays.
J.J Thomson repeated the experiment and when he passed the cathode rays through the electric field in a vacuum, deflection occurred. The fact that the cathode rays moved towards the positively charged plate allowed him to conclude that the rays must be negatively charged.
After this, J.J Thomson used two coils of wire known as the Helmholtz pair to produce a uniform magnetic field. Any beam of charged particles when passed through the magnetic field produced by the Helmholtz pair will be bent at 90 degree angle to the field into a complete circle
Using this concept, J.J Thomson positioned the coils to cause a deflection in the opposite direction to that produced by the electric field. To balance the forces