This animation describes J. J. Thomson’s second experiment involving the deviation of an electron beam in a vacuum tube, called a Crookes Tube.

A partial vacuum (less than 10^-6 atm) is maintained in the tube. A high voltage (between 10 and 100 kV) is applied between two electrodes.  The very intense electric field that results from this accelerates the few ions present in the tube which, via collisions, ionize other particles. The lower the pressure, the more the electrons thus liberated and accelerated travel great distances until they strike the screen  at the opposite end of the tube.

By studying the deviation of this beam, J. J. Thomson, in 1897, isolated a new elementary particle carrying a negative charge – the electron.

This apparatus constitutes the first particle accelerator. As a result of his work, Thomson proposed a completely new model of the atom (Thomson’s Plum Pudding Model) that one of his students, Ernest Rutherford, would improve upon 10 years later.