Mass of Electron apparatus is a 'Magic Eye' tube mounted on a base with terminals to accept both heater and plate voltages. A fan of electrons can be seen by viewing through the end of the tube. An Air Cored Solenoid is placed over the tube and a current passed through the solenoid. The field at the centre of the solenoid causes the fan to curve (distort) and this curvature is measured to permit the approximate mass of an electron to be calculated.

The Air Cored Solenoid is not supplied with the Mass of Electron apparatus. It must be purchased separately.