e/m Experiment Apparatus

(Thomson’s method)

Holmarc's e/m apparatus (Model No: HO-ED-EM-03A) is designed for the measurement of the charge to mass ratio e/m, of the electron. This equipment also facilitates the demonstration of effects of magnetic fields on a moving charged particle. It consists of a fine beam electron tube known as e/m tube, a pair of Helmholtz coils and the electronic control box. The e/m tube generates the fine electron beam by applying the accelerating voltage. The electron tube is placed between a pair of Helmholtz coils. If we apply current to the Helmholtz coil uniform magnetic field is generated and as a result the fine electron beam changes its path to circular. By measuring the accelerating potential, the Current through the Helmholtz coils and the radius of the circular electron beam, we can find out the e/m ratio.

The electronic control unit provides the energy requirements for the tube and Helmholtz coil. The Helmholtz coil is a pair of coils separated by a distance of the coil radius. The e/m tube generates a fine electron beam by applying accelerating voltage. Electron collisions with the gas molecules excite the gas molecules and produce light emission in a cylindrical sheath around the electron beam. As the excited molecules relax, the electron paths become visible.

Experiment: