In the 1890s, physicists were debating whether there existed any particle smaller than the atom, and if how, what it was. Previous scientists had imagined that the smallest possible particle was a hydrogen atom (also now known as a single proton). The British physicist J. J. Thomson was the first to suggest that a particle might exist that was less than a thousandth of the size of a proton. By experimenting with the rays that were emitted from cathodes (the electrode attached to the negative end of a voltage supply), he found that these rays were vastly less heavy than even hydrogen atoms. He initially called these tiny, negatively charged particles 'corpuscles', but they became known by the preferred name of 'electrons'.
Editors. "Electron discovered by Joseph Thomson". The Literary Encyclopedia. First published 30 August 2013
[http://www.litencyc.com/php/stopics.php?rec=true&UID=6379, accessed 21 January 2017.]