On thermionic emission and the use of vacuum tubes in the advanced physics laboratory

Two methods are outlined for measuring the charge-to-mass ratio e/m(e) of the electron using thermionic emission as exploited in vacuum tube technology. One method employs the notion of the space charge in the vacuum tube diode as described by the Child-Langmuir equation; the other method uses the electron trajectories in vacuum tube pentodes with cylindrical electrodes under conditions of orthogonally related electric and magnetic fields (the Hull magnetron method). The vacuum diode method gave e/m(e)=1.782 +/- 0.166x10(+11) C/kg (averaged over the vacuum diodes studied), and the Hull magnetron method gave e/m(e)=1.779 +/- 0.208x10(+11) C/kg (averaged over both pentodes and the anode voltages studied). These methods afford opportunities for students to determine the e/m(e) ratio without using the Bainbridge tube method and to become familiar with phenomena not normally covered in a typical experimental methods curriculum.