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American Telephone & Telegraph: The Art of Telephony, 1916

Switching Back and Forth

The earliest switchboard installed in large cities usually stretched from the ceiling to the floor. As a center of subscriber connections, the switchboard facilitates the completion of circuits of communication. The high back panel of the switchboard has rows of jacks, each jack representing a local subscriber. This jack is wired to connect to the home or business of the local subscriber and is called a local extension. Some jacks represent lines connecting one switchboard to another, called trunk lines. Each jack has a corresponding lamp, which lights up to indicate an incoming call from a local subscriber.

The table or desk area in front of the operator (or operators, depending on the size) of a switchboard includes columns of switches, lamps, and cords. Each column is made up of a front switch and a rear switch, with the front switch wired to the front cord and the rear key wired to the rear cord. The rear cord is used to receive incoming calls to the switchboard, and the front cord to which it connects is used to connect to send an outgoing ring signal to the local extension requested by the incoming call. The front and rear cord thus complete the circuit, connecting two local subscribers.

The front and rear switches have three positions: back, normal, and forward. When a switch is in the normal position, it creates the connection necessary for the transmission of speech over a telephone line. In the forward position, the key connects the operator to the cord to which it is wired. In the back position, the switch sends a ring signal out on the cord.

When a call comes in, the lamp associated with the jack of that caller lights up. The operator responds to the call by placing the rear cord on the table in front of her into the appropriate jack on the back panel. After plugging in the cord, she moves the switch to the "forward" position, completing the circuit that connects her to the caller. She then speaks with the caller, asking where the caller would like to be connected. She then places the front cord in the jack representing the local extension specified by the caller, and switches the key into the "back" positing to ring the party being called. After she connects, the operator leaves both cords in the jacks and places the switches in the "normal" position, creating a talk path between the two subscribers.