Electron Charge-to-Mass Ratio (E/M Apparatus)

Product Code: SE-9629


In 1897, J. J. Thomson showed that the mysterious cathode rays were actually negatively charged particles—he had discovered the electron. In the same year he measured the charge-to-mass ratio of the electron, providing the first measurement of one of the fundamental constants of the universe.

The Charge-to-Mass Ratio System reproduces one version of Thomson’s landmark experiment, providing an accurate measurement of the charge-to-mass ratio of the 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.)

What’s Included

Product Specifications

Hemholtz Coil Radius 16 cm
Number of Turns 130
Maximum Current 3.5 A
Filament Voltage 6.3 VAC
Acceleration Voltage 0 – 200 V
Tube Diameter 15.5 cm

Buying Guide

Parts Available Separately
Helmholtz coils for e/m SE-9626
Tunable DC Power Supply (Constant Current) SE-9622
DC Power Supply II (Constant Voltage) SE-9644
Replacement Parts P/N
Replacement e/m Tube SE-9651
Replacement Mirror Scale — e/m Apparatus SE-9649

Support Documents

Electron Charge to Mass Ratio English 3.04 MB