Describes the electrical properties of semiconductor materials, the effects of adding various controlled impurities, and the way in which semiconductors can be used for rectification or as amplifiers. Examples of both applications are shown, ranging from the minute transistorised transmitter in a radio pill for medical research to the banks of semiconductor rectifiers on heavy duty electric railway locomotives. Sketches the manufacturing process involved and examines the possibility of using semiconductors for power generation, central heating and refrigeration.