When a large volume of current is used it is necessary to have a device known as a circuit-breaker, as fuses are sometimes too slow in action. A circuit-breaker is practically a switch, which, when the current exceeds a certain amount, automatically opens by means of the pressure of a spring regulated by a coil through which the current passes. When the current becomes greater than a certain amount, the coils attract an iron rod attached to a trigger and release it. This trigger comprises a spring which acts upon the switch. One current-breaker is used for each generator.