**Dy′namom′eter,** strictly an instrument to measure a force tending to produce motion. Thus, a spring-balance is sometimes called a dynamometer. The term, however, is commonly applied to an instrument to measure power. Power is rate of doing work. Work is force overcome through distance. The unit of work used in engineering is the work done in raising a pound-weight one foot against gravity, called a foot-pound. When a machine does work at the rate of 33,000 foot-pounds in one minute, the power is one horse-power (H. P.). The power-unit used by electricians is a kilowatt (K. W.), equal to 1⅓ H. P. A dynamometer is technically used in engineering for any device which measues H. P. One of the simplest and most common forms is the Prony brake. A block B is clamped fairly tight on the pulley O of the machine. The machine then does work against friction. The force is balanced by the weight W on the lever-arm AO. From the speed of rotation, the lever-arm and the force W, the power is directly calculated. For the many forms of dynamometers, see J. J. Flather’s text-book on *Dynamometers.* A wattmeter measures electrical power in watts. It is an instrument for measuring the product of the pressure (volts) and current (amperes), which gives the watts directly. The best known forms are those of Siemens and the modified forms of Weston.