In order to understand how a call is made through a large city exchange, it is necessary to have in mind a distinct picture of a switchboard and to understand the functions of the various operators. (See Fig. 94.) For the sake of clearness it will be well to take a single typical case.
Between fifty and ninety subscribers' lines run to each operator's switchboard. Operator A, for instance, receives all the calls from the subscribers on the Audubon exchange whose numbers are from 1 to 50. At the bottom of her switchboard there is a hole, called an answering jack, for each of these lines. Should one of these subscribers, Mr. Smith, take his receiver from the hook in order to call, a small supervisory lamp lights below the answering jack in which Mr. Smith's line ends. Operator A is thus notified that Mr. Smith is calling, and connects herself with his line by inserting one of a pair of plugs in the answering jack.
The top of operator A's switchboard contains a hole for every number on the Audubon exchange, and an additional one for a trunk line. (The operation of the trunk line will be explained later.)
Fig. 04. - Operating Room in a Small City Exchange.
Should Mr. Smith be calling Mr. Jones, whose line is also on the Audubon exchange, the operator can make the connection directly by inserting the other plug of the pair at the top of the switchboard into the hole which marks the termination of Mr. Jones' line and which may be distinguished by the number it bears.
Operator A can be called only by those subscribers on the Audubon exchange whose numbers are from 1 to 50. She can, however, call directly any of the subscribers on the Audubon exchange.
Suppose, on the other hand, that Mr. Smith is calling Mr. Harper, whose line is on the Rector exchange. In this case, it is necessary for operator A to use the trunk line in order to make the connection. The trunk line is the line which connects the various exchanges with one another, and has nothing to do with the subscriber directly.
Operator A, in this case, inserts the second plug of the pair in the trunk line hole, the first plug being in the answering jack of Mr. Smith's line. Thus the trunk line operator is called. Operator A gives her Mr. Harper's Rector exchange number, and she then connects operator A with one of the Rector exchange operators. It does not matter which Rector operator is given the call, for just as operator A can call any subscriber on the Audubon exchange, so can any Rector operator call any subscriber on the Rector exchange.
In this case it may be assumed that the trunk line operator knows that operator B on the Rector exchange is the least busy and, consequently, gives her the call. Operator B then "plugs in" Mr. Harper's number at the top of her board and thus through operator A, the trunk line operator, and operator B, Mr. Smith on the Audubon exchange is connected with Mr. Harper on the Rector exchange. In very large cities there may be an operator for each exchange who merely receives calls from the trunk line operator and apportions them to the operators on her exchange. In such case, the trunk line operator would call this apportioning operator instead of calling operator B directly.