Experiment to Measure the Increase in Electron Mass with Velocity

An experiment is described for measuring the mass increase with velocity of relativistic electrons. A beta-ray spectrometer is used to momentum-analyze electrons emitted from a Tl²⁰⁴ source, and a semiconductor detector is used to measure the energy of the electrons. The energy calibration of the semiconductor detector is established absolutely. The experimental results serve to verify the predictions of special relativity, and provide precise values for the electron rest mass and the speed of light.