Fine beam tube or Double beam tube

Measurement of e/m for the electron

This tube is different from the deflection tube in that it is filled with low pressure helium gas. The advantage of this is that you can see the path of the beam of electrons since they ionise the gas which then glows as the electrons collide with helium atoms. The greater the potential difference between the anode and the cathode the further the electrons will travel through the gas before they lose their energy and are brought to rest. Put more exactly the range of the beam can be shown to be proportional to the accelerating voltage and therefore to the electron's energy.

There are also two electron guns that can fire through the tube, one giving a tangential beam and the other a radial beam.

For the tangential gun   ½ mv² = eV and Bev = mv²/R

where v is the electron velocity. So combining these two equations: