AUTOMATIC TELEPHONE EXCHANGES 255 



are built up. In fig. 46 a nerve and telephone unit are 

 contrasted. In a telephone system a unit is made up 

 of (1) a transmitter, the instrument which takes up our 

 words and turns them into electrical messages ; (2) a 

 long stretch of copper wire, which carries messages from 

 the transmitter to the exchange ; (3) an electric battery, 

 which pours an electric current along the wire and thus 

 makes it alive ; (4) an exchange terminal. These four 

 elements make up a telephone unit. Each subscriber is 

 connected to an exchange by such a unit. If one sub- 

 scriber wishes to communicate with another, then the 



EXCHANGE TERMINAL 



Fig. 46. — Plan showing the corresponding elements in a telephone and in a nerve 

 system. A, Unit of a telephone system. B, Unit of a nerve system. 



exchange terminals of their respective units have to be 

 linked up. Engineers have discovered a method of 

 having connections made at exchanges by automaton 

 operators, but the automaton has to be worked by the 

 subscriber who wishes to dispatch a message. In the 

 local exchange from which our example is drawn, a living 

 operator receives the subscriber's call and by means of 

 switches or connecting plugs puts him through. The 

 caller's message thus passes over the elements of at least 

 two units to reach its destination. (5) A receiver to 

 convert electrical impulses into sound waves, and thus 

 render messages transmitted along wires into words. 

 The corresponding elements in a nerve system are shown 

 in fig. 46, B. They are : (1) a transmitter, a contrivance 



