This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
in Fig. 4. The two bells, D, are shown connected in parallel, which requires more wire than if they were connected in series. If they be connected in series, one or the other should have its make-and-break contact closed. The bell whose circuit remains unchanged will intercept the current for the other bell in series with it. The operating of the bells is more satisfactory, however, when they are in parallel, and each taking current from the battery independent of the other.
The diagram, Fig. 5, shows the proper connections for operating two bells from two independent push buttons, each push button operating a particular bell. Any number of bells operated from any number of push buttons, all of the bells being rung from any one of the push buttons, are connected as shown in Fig. 6. Such a circuit can be used as a fire alarm or time call in a factory, the operation of the circuit being controlled from any one of a number of different points.
The proper connections for what is called a return-call circuit is shown in Fig. 7. The circuit is so arranged that the bell at one end is controlled by the push button at the other end. Such a circuit can be used in transmitting signals in either direction. A ground return-call circuit is shown in Fig. 8.
Ill: Wiring Diagrams for Return-Call Bells
In the circuits shown in Figs. 7 and 8, only one battery is needed.
The connections of a two-wire metallic return-call circuit are shown in Fig. 9. A special push button must be used in this circuit, and in this case two batteries are used instead of one, as in Figs. 7 and 8. This circuit may be changed to a ground return-call circuit by using the earth as a conductor instead of either wire. There are, of course, numerous other methods that may be used in connecting call bells, but the connections shown in the diagrams are perhaps the most common.