238
POPULAR SCIENCE MONTHLY
Fig. 18.
believed combines the advantages of the (thirteen) different systems." A diagrammatic representation of the wiring of a single circuit in this board is shown in Fig. 19. There is also shown a flexible cord attached to a plug and a wedge of hard wood having a metal plate fastened to one side. The instructions sent with the board read:
The local size gravity battery is used—one cell for each bell and for each mile of wire is sufficient. A circuit one mile long having ten bells requires about fourteen cells of battery. Two circuits may be operated by one battery if they are about equal length and I have the same number of bells on each. . . . 
Fig. 19. When any subscriber on this circuit wishes to call the central office he presses his knob twice, which rings the bell; the operator then inserts the wedge between the spring and the plate, with the metal side against the spring, and the plug into a brass strip which is connected through a set of telephones to the ground. This, it will be seen, takes off the battery and connects the telephones so that the operator can talk with the subscriber and ascertain his wants. If the subscriber wants to talk with a person on another circuit, the central office calls that person and on receiving his answer, the two circuits are connected together by inserting a wedge under each spring and putting each plug into one of a pair of brass strips which are connected together through a hand telephone by means of which the central office operator can ascertain when the two persons have finished using the circuits. Then he removes the wedges and plugs and the circuits are ready for another call.
Fig. 19. When any subscriber on this circuit wishes to call the central office he presses his knob twice, which rings the bell; the operator then inserts the wedge between the spring and the plate, with the metal side against the spring, and the plug into a brass strip which is connected through a set of telephones to the ground. This, it will be seen, takes off the battery and connects the telephones so that the operator can talk with the subscriber and ascertain his wants. If the subscriber wants to talk with a person on another circuit, the central office calls that person and on receiving his answer, the two circuits are connected together by inserting a wedge under each spring and putting each plug into one of a pair of brass strips which are connected together through a hand telephone by means of which the central office operator can ascertain when the two persons have finished using the circuits. Then he removes the wedges and plugs and the circuits are ready for another call.The instructions for the subscribers equipment read:
The circuits are run from the central office and grounded at the last
