Combinations of Bells and Pushes.
One of the wires in a bell circuit may be replaced by the ground ([Fig. 26]). Connection may be made to a gas or water pipe or to a metal plate buried deep in damp earth. Any wire fastened to such a plate must be thoroughly soldered to it or a voltaic action will be set up, which will eat it away at the point of contact.
Fig. 26
When one bell is to be rung from two or more points the pushes are to be connected in multiple ([Fig. 27]) as if they were in series; all would have to be closed to complete the circuit.
Fig. 27
Fig. 28
If two bells are to be operated from one push they may be in series ([Fig. 28]), but in this case one of them must be arranged for single stroke. If both were vibrating bells the armature of one would not vibrate in unison with the other armature and the result would be irregular contact breaking and intermittent ringing.
Fig. 29
A preferable connection for two or more bells and one push is [Fig. 29], where the bells are in multiple. This requires more current than the series method.
Fig. 30
Fig. 31
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.