This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
In Fig. 70 is illustrated a return-call system requiring one battery wire b, one return wire r, and for each room one leading wire l1, l2, etc. The annunciator board is divided into two parts, the upper part having the numbered drops, and the lower the return-call pushes.
Each room is provided with a double-contact push, such as that in Fig. 77. The tongue t makes connection normally with the upper contact c, but when pressure is put on the knob k, the tongue is forced against the lower contact c'. The return-call pushes on the lower board of the annunciator are of the same description; in Fig. 76 all these pushes are shown diagrammatically, for convenience in tracing out the circuits. The closure of the circuit in any room, as, for example, No. 4, rings the office bell and releases the corresponding drop. The path of the current is then from the push 4 through a-c-d-e-f-g-B-h-b. and back to the lower contact of the push button. On the return signal being made from the office, the annunciator-bell circuit is broken at d, and, the push button in the room being released, a new circuit is formed through k, as follows: from the battery B through g-m-r-n-o-a-c-k-p to the battery, the room bell being in this circuit.
97. In installing annunciator systems, it is usual to run the battery wire, which is No. 14 or 16 annunciator wire, through the building at some central portion. If there are many rooms, it will be advisable to splice on a length of No. 18 wire to extend from the push in each room to the battery wire. The connection from the other side of the push button to the annunciator, that is, the leading wire, should be No. 18. For the return-call system, a battery of four or five Leclanche cells is required.
All wires used in annunciator service should have distinguishing colors to prevent confusion. The battery wire may be blue, the return wire red, and the leading wires white. This arrangement will greatly simplify the connections, and reduce the liability of mistake.