[B] For a full explanation of self-induction see No. 1 of this series.
Continuous-ring Bell.
In some classes of bell work, such as burglar alarms, it is desired that the bell when once started shall continue to ring until stopped by the person called. In this case a continuous-ringing bell is needed, such as in [Fig. 8].
Fig. 8
When the push P is pressed, the current flows in the usual way through contact screw L, armature spring A, magnet coils M M, battery B, back to P, and the bell rings. But on the first forward movement of the armature it releases the spring contact S, which flies forward and makes contact at U. The circuit is now from B, through M M, to A, thence through L and S, to U and back to B.
The bell will continue to ring until the spring contact S is moved back and caught by the projection on the armature A.
A continuous-ring attachment is also made and sold in most electrical supply stores, which is complete in itself and can be applied to any bell.
Fig. 9