For showing slow A.C. on an oscilloscope or meter or for demonstrating the use of a dynamo mounted in lighting a lamp. The apparatus has a cycle dynamo mounted on a base and provided with two sets of gears driven by a hand crank for fast and slow rotation. Two 4mm socket terminals are provided for the dynamo output, connected in parallel with an M.E.S. lamp holder and 2.5V bulb. Dimensions 220x115x105mm high.