Electron Charge-to-Mass Ratio System


e-m Apparatus, complete with power supplies which can also be used in other experiments


e-m Apparatus

e-m Apparatus

SE-9629   - 

Perform accurate measurements of charge-to-mass ratio experiments. Also perform electrons in a magnetic field studies. Includes power supplies which can also be used in other experiments.

Also Available:

Helmholtz coils for e/m

Helmholtz coils for e/m

SE-9626   - 

Helmholtz coils (without tube) - for classrooms already equipped with power supplies and e/m Tubes.
Replacement e/m Tube

Replacement e/m Tube

SE-9651   - 

Replacement e/m Tube for PASCO's Electron Charge-to-Mass Ratio System


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.

How It Works

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.)