The apparatus is used to determine the ratio of the charge to the mass of an electron. A 'Magic Eye' vacuum tube is mounted on a strong plastic base which is fitted with colour coded 4mm sockets to accept 100 to 250V d.c. anode supply and 12V a.c./d.c. for the filament. The 'Magic Eye' tube has an equipotential cathode, simple control grid structure and a cone shaped anode co-axially aligned with the cathode. The path of the electron flow is visible on the surface of the anode.