ELECTRICAL INVENTIONS 



221 



In a great city, where there are hundreds of thousands of sub- 

 scribers (there are over 6,000,000 in Chicago), there must be a 

 number of centrals, for since most calls are between neighbors a 

 nearby exchange can care for these without the expenditure 

 necessary to carry all the wires to a single office. When you 

 call a person in a distant part of the city, the local central con- 

 nects your wire with that of the 

 distant exchange, and the operator 

 there plugs the wire from your local 

 central into that of the subscriber 

 with whom you wish to talk. There- 

 fore, it is necessary when calling to 

 give not only the desired number but 

 also the name of its local exchange 

 (Fig. 90). 



Recently the automatic switch- 

 board is being introduced to replace 

 the operators at central, for an electric 

 device is cheaper and more depend- 

 able than a person, and the task of an 

 operator is very fatiguing. These new 

 automatic centrals will free human 

 beings for more worth-while tasks. 

 It seems very remarkable that a 

 mechanical contrivance can so effi- 

 ciently replace the intelligent action of the central operator. 



The electric bell has a hammer attached to an iron bar so 

 mounted that it will be forcibly drawn to the magnetized soft 

 iron core of a coil when an electric current is sent through the 

 latter. As the bar moves, the hammer strikes the bell. It will 

 be seen that the principle of operation is very much like that of 

 the sounder or receiver of the Morse telegraph (Fig. 91). In the 

 bell, however, an ingenious device causes the hammer to strike 

 the bell repeatedly. The current goes to the coil through two 

 points which are in contact when the hammer is at rest, but 



FIG. 91. Diagram of an elec- 

 tric bell. 



