The apparatus shows the path of a stream of electrons in a rarified gas discharge. The electron beam can be deflected with a common magnet.

Crookes tube apparatus (see figure)
Horseshoe magnet
2000 V neon sign transformer
Wires with alligator clips to make necessary connections

This demonstration can be part of a discussion of the structure of the atom. It can be shown in conjunction with the airglow demonstration (Demo-036) which deals with the electrical conductivity of rarified gases. This demonstration shows the electron flow associated with such a discharge.

Connect the tube to the transformer with the supplied wires. Turn on the transformer; the phosphor should show the path of the eletcron stream. If not, reverse the connections. Point out to the class that the glowing line is not a beam of light, but a stream of negatively charged particles. Demonstrate this with the magnet. Hold the magnet over the tube and slowly lower it until it nealy touches. The electron beam will deflect strongly; a light beam would not be moved.

At the simplest level, this demonstration reveals the path of the electric current through the rarified gas discharge. Historically, a device like this was used to make the first quantitative measurement of an electron property, the charge to mass ratio (e/m). This was accomplished by balancing the applied voltage on the tube against magnetic field strength until there was no deflection (null experiment). The measurement of the electron charge e by Millikan (the oil drop experiment) allowed the first determination of the electon mass from the e/m ratio.