electromagnetic alarm

An alarm which is brought into action by closing an electromagnetic circuit. This may be a burglar-alarm, in which the opening of a door or window is made to close a circuit mechanically; or it may be a fire-alarm, in which the lengthening of a rod or a change in its shape is made to close a circuit. In some cases a column of mercury is expanded by the heat and thus completes the circuit, making the hammer vibrate and deliver blows upon the bell. electro-magnetic clock, 8. 1. A clock of which electricity is the motive power. Of this kind are those of Wheat stone, of Bain, and of Shepherd, that of the last-named inventor being exhibited at the Great London Exhibition of 1851. In this clock electro-magnetism is the sole motor in moving the pendulum, driving the train, and running the striking-works, no weights or auxiliary springs being employed. ( Knight .) 2. A clock, the pendulum of which is designed to have an electric connection with that of another, so as to make them beat synchronously. Dr. Locke, of Cincinnati, carried out such a principle about A. D. 1860. By it all the clocks in a city may be made to keep the same time. ( Knight .)