216 APPLIED SCIENCE 



the top of the switchboard into the hole which marks the 

 termination of Mr. Jones' line and which may be distin- 

 guished 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 answer- 

 ing jack of Mr. Smith's line. Thus the trunk line operator 

 is called. Operator A gives her Mr. Harper's Rector ex- 

 change 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 



