The bell used is that common with different forms of electrical instruments. It consists of a gong and a clapper vibrated by the combined action of an electro-magnet and a spring. The magnet, when the current passes, draws the clapper to itself and in doing so opens the circuit; this destroys its magnetism and allows the spring to carry the clapper back. This "make" and "break" action, rapidly repeated as long as the current is passing, produces a continuous ringing of the bell. Reference to Fig. 2  will make this movement clear.
One end of the wire of the coils of the magnet M M is secured to the binding-post B, and the other to the post C. The arm of the clapper k is a rather stiff spring, which in its normal position holds the armature e carried by it from the poles of the magnet. It then presses against the spring r, attached to the post D. The posts A and E holding the wires from the battery are respectively connected with B and D by metallic strips. The current enters at A, traverses the coils of the magnet M M, passes through the armature e, and out by way of spring r and posts D and E. In doing so, the soft-iron cores of the magnet are magnetized and attract the armature e. This in moving breaks its contact with the spring r, and interrupts the current. The clapper then springs back into position. In the bell now generally used the ringing continues not only while the door or window is open, but until the indicating parts of the annunciator are restored to position.
- Figs. 2 and 3 are reproduced, through the kindness of Mr. George B. Prescott, from his works on "The Electric Light," etc., and "Electricity," etc.