Free Charge Carriers in a Vacuum – Thomson Tube

Investigate the deflection of electrons.

The experiment investigates the deflection of electrons in electric and magnetic fields. For different anode voltages UA, the beam path of the electrons is observed when the deflection volt- age UP at the plate capacitor is varied. Additionally, the electrons are deflected in the magnetic field of the Helmholtz coils by varying the coil current I. The point at which the electron beam emerges from the fluorescent screen gives us the radius R of the orbit. When we insert the anode voltage in the following equation, we can obtain an experimental value for the specific electron charge, the magnetic field B is calculated from the current I.

A velocity filter (Wien filter) is constructed using crossed electrical and magnetic fields. Among other things, this configuration permits a more precise determination of the specific electron charge. At a fixed anode voltage UA, the current I of the Helmholtz coils and the deflection voltage UP are set so that the effects of the electric field and the magnetic field just compensate each other. The path of the beam is then virtually linear.

Electron deflection tube
Tube stand
Pair of Helmholtz coils
High-voltage power supply 10 kV (2)
DC Power Supply 0...16 V/0...5 A
Safety connection lead 25 cm, red (2)
Safety connection lead 50 cm, red
Safety connection lead 50 cm, blue
Safety connection lead 100 cm, red (3)
Safety connection lead 100 cm, blue (3)
Safety connection lead 100 cm, black (2)