The Charge-to-Mass Ratio System reproduces one version of J.J. Thomson's landmark experiment, providing an accurate measurement of the charge-to-mass ratio of an electron. And, since the electron tube can be rotated through 90°, students can also make a more general study of the behavior of electrons in a magnetic field.
This apparatus also has deflection plates so students can study the effect of an electric field on moving electrons.
A large, helium-filled electron tube is mounted between a pair of Helmholtz coils. The tube contains an electron gun, which generates a focused beam of electrons. A measured current is applied to the Helmholtz coils so that the magnitude of the magnetic field within the electron tube can be calculated. A measured accelerating potential (V) is then applied to the electron gun. The magnetic field (B) deflects the electron beam in a circular path with a radius (r) that is measured using the illuminated mm scale. From these measured values, the charge-to-mass ratio of the electron is calculated:
e/m = 2V/B2r2
(The details of the calculations are fully described in the manual.)