  We have developed an undergraduate laboratory experiment to make independent
measurements of the momentum and kinetic energy of relativistic electrons from
a \beta -source. The momentum measurements are made with a magnetic
spectrometer and a silicon surface-barrier detector is used to measure the
kinetic energy. A plot of the kinetic energy as a function of momentum compared
to the classical and relativistic predictions clearly shows the relativistic
nature of the electrons. Accurate values for the rest mass of the electron and
the speed of light are also extracted from the data.
