30. E/M and Helical Electrons

This apparatus is used in an 8E lab experiment. A spherical electron tube is mounted inside of a Helmholtz coil setup. The electrons are projected to circle in the magnetic field, and you measure the radius of the circle, the current to the Helmholtz coils, and the accelerating voltage to determine e/m. The electron path is made visible by dim ionization of the residual gas in the tube.

If the tube is twisted a little inside the coils, the circle becomes a helix. This is a very pretty demonstration of the path of charged particles in a magnetic field.