Automatic Switch

— A switch which opens and closes automatically at required times. One form of switch used as a cut out for dynamos consists of an electromagnet, fixed upon a slate base, and an iron armature fixed to the ends of the pivoted levers of the switch. The electro-magnet is included in series with the switch an armature circuit, and while the pre ure of the machine to which the instrument is connected remains at its normal value, the current flowing in its coils is sufficiently strong to enable it to hold up the iron armature against its pole pieces. If from any cause the voltage of the machine is reduced, the current flowing in its armature is decreased also, until when it falls below a certain minimum value at which the automatic switch is arranged to act, the strength of the electro-magnet has been so far diminished that it can no longer hold up the armature against the weight of the levers, and these latter therefore drop and switch the machine out of circuit.