The discovery of the electron

Cathode rays, specific charge and Millikan

Heated cathodes emit electrons (thermionic emission); accelerated through $V$, each gains $\frac12mv^2=eV$. Thomson used crossed electric and magnetic fields ($v=\frac EB$) and magnetic deflection to find the specific charge $\frac em\approx1.76\times10^{11}$ C kg⁻¹, about 1800 times that of a hydrogen ion. Millikan balanced charged oil drops ($\frac{QV}d=mg$) and found every charge was a whole multiple of $1.6\times10^{-19}$ C.