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.)
- 1x Helmholtz coils for e/m (SE-9626)
- 1x Tunable DC Power Supply (Constant Current) (SE-9622)
- 1x DC Power Supply II (Constant Voltage) (SE-9644)
- 1x Red and Black Patch Cords
|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|
|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 e/m Tube||SE-9651|
|Replacement Mirror Scale — e/m Apparatus||SE-9649|
Electron Charge to Mass Ratio