Determining the specific charge of the electron

In the experiment P6.1.3.1, a tightly bundled electron beam is diverted into a closed circular path using a homogeneous magnetic field in order to determine the specific electron charge. The magnetic field B which diverts the electrons into the path with the given radius r is determined as a function of the acceleration voltage U. The Lorentz force caused by the magnetic field acts as a centripetal force. It depends on the velocity of the electrons, which in turn is determined by the acceleration voltage. The specific electron charge can thus be determined from the measurement quantities U, B and r according to the formula