To ring two bells from either one of two points, the arrangement in [Fig. 30] will answer. It requires only two wires or one wire and ground return, but two batteries. As both bells are in multiple both will ring, the one nearest the push being depressed ringing the loudest. This is a disadvantage. If the series arrangement in [Fig. 31] be selected, one bell must be arranged for single stroke. Both bells will ring with equal power.
Fig. 32
In [Fig. 32] only the distant bell rings, the circuit having only one battery but three wires, or two wires and ground return.
A plan where two batteries are needed but only two wires, or one wire and ground is in [Fig. 33]. Double contact or three-point pushes are necessary here, making one contact when depressed and a second one when not being touched.
Fig. 33
In this figure only the distant bell rings.
Faults in Bells.
On examining many electric bells it will be noted that only one binding post is insulated from the frame when the latter is of iron ([Fig. 34]). As the armature spring S is in electrical connection with the frame F by reason of its metal screws and support, the circuit may run from the insulated post U to the magnet coils, thence through the insulated contact screw C through the armature spring (when it is making contact) and through the frame to the uninsulated post I.