To use only one wire for a return call bell connect up as shown in the diagram, using a closed circuit or gravity battery, B. The current is flowing through both bells all the time, the same as the coils of a telegraph sounder, but is not strong enough to ring both connected in series. Pressing either push button, P, makes a short circuit of that bell and rings the one at the other end of the line. --Contributed by Gordon T. Lane, Crafton, Pa.

Wiring Diagram