This demonstration shows the procedure for determining the charge to mass ratio of an electron. This is similar to the procedure used by J. J. Thomson in 1897. A beam of electrons is accelerated through a potential. A pair of Helmholtz coils produces a uniform magnetic field perpendicular to the electron beam. The magnetic field deflects the beam in a circular path.

Demonstration Ideas

1. Use a permanent magnet to deflect the electron beam instead of the Helmholtz Coils.
2. The socket for the e/m tube can be rotated. By changing the angle of the magnetic field with respect to the electron beam, you can show how beam deflection is affected.

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