A lecture demonstration is described, which allows verification of the relativistic relation between kinetic energy and momentum,(mc2+E)2 = m2c4+c2p2. Electrons are accelerated by a small high-voltage generator (van de Graaff, of approximately 250 kv) and deflected in a magnetic field of approximately 100 gauss. The measured values of high-voltage and deflecting magnetic field are plotted on a straight line which represents the relation given above.