Apparatus and methods have been devised for the rapid and precise determination of the Boltzmann temperature of oxide coated cathodes in scanning beam tubes. The measuring apparatus comprises a digital voltmeter connected between cathode and collector of the tube under test by means of a high input resistance amplifier connected to have unity gain, and a variable shunt from cathode to collector consisting of tap switches and 20 precisely calibrated resistors having values from 104to 2×1010&OHgr;. The 19 temperatures determined by the 20 retarding potentials measured with this apparatus are computed on a GE 265 calculator. The interfering side effects which are encountered in practical measurements, viz., contact potential drift, contact potential instability, Ohmic and non‐Ohmic leakages, collector thermionic and photoelectric emission, heater‐to‐collector emission, and interelectrode potential minimum, nonuniform geometry, and poorly activated cathodes, are discussed, and the means taken to overcome these effects described in detail. The use of a magnetic field parallel to the electric field to effect collimation is described. It is shown that a very high field may be required to obtain the correct temperature in tubes with nonuniform geometry. It is estimated that in most cases the electron temperature can be measured with an absolute probable error of 1.7°, and that the relative probable error is approximately an order of magnitude lower.