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OUR PHYSICAL WORLD 



directly by a wire. As telephones multiplied, it was evidently 

 impossible to have wires running from each instrument to every 

 other with which the owner of one might wish to communicate. 

 A central station was therefore established to which the wires of 

 all instruments were run and where they might then be connected 

 as desired. Each wire running from a subscriber's telephone to 



FIG. 90. A modern telephone exchange switchboard. (Courtesy of the 

 Illinois Bell Telephone Co.) 



" central" is bifurcated, one branch ending in a plug socket on a 

 switchboard, the other in the plug. Directly over the socket, 

 and wired to it, is a tiny electric lamp, which lights when the 

 subscriber rings up "central," and remains lighted until she con- 

 nects her receiver with this socket and learns what subscriber 

 is wanted. She then disconnects her receiver and connects the 

 plug of the desired subscriber's wire with the socket of the calling 

 subscriber. 



