The first evidence for sub-atomic particles came from experiments with the conduction of electricity through gases in sealed glass tubes at low pressures.

Associated with the flow of electricity in such a tube are rays which originate from the negative electrode--the cathode. Thus these tubes have been called cathode ray tubes. From careful experiments with cathode ray tubes J.J. Thomson demonstrated in 1897 that the rays consist of a stream of negatively charged particles which he called electrons. He was able to measure the charge/mass ratio of these particles and found this to be the same regardless of what gas was in the tube or what metal the electrodes were made from.

In 1909 Robert Millikan used the classic oil drop experiment to determine the charge on these particles.

Using the smallest charge obtained and Thomson's charge/mass ratio the electron mass is roughly 1/2000 the mass of the lightest atom. Thus there are obviously particles smaller than atoms.