Hot-cathode emission in a vacuum: determining the polarity and estimating the specific charge of the emitted charge carriers

In the experiment P3.8.4.1, the current through a pair of Helmholtz coils is set so that the electron beam is incident on the Faraday’s cup of the Perrin tube. The Faraday’s cup is connected to an electroscope which has been pre-charged with a known polarity. The polarity of the electron charge can be recognized by the direction of electroscope deflection when the Faraday’s cup is struck by the electron beam. At the same time, the specific electron charge can be estimated. The following relationship applies:

The bending radius r of the orbit is predetermined by the geometry of the tube. The magnetic field B is calculated from the current I through the Helmholtz coils.