DEVELOPMENT OF THE TELEPHONE 



a metallic back and front. The front of the box is 

 made of very thin metal and acts as a diaphragm, the 

 interior of the box being filled loosely with hard carbon 

 granules. The current from the battery passes from 

 the diaphragm through the carbon granules to the back 

 of the chamber, the vibrations of the diaphragm caus- 

 ing vigorous vibrations of the granules, thus producing 

 long sounds. A modification and improvement of this 

 chamber has been made by A. C. White in America, 

 and this is the transmitter, known as the "solid back" 

 transmitter, now in general use on long-distance tele- 

 phones in the United States. 



TELEPHONE EQUIPMENT 



The question of equipment and operation of telephones 

 over any amount of territory large or small, is one 

 that is constantly occupying the attention of engineers, 

 as the almost universal use of this instrument renders 

 even the slightest improvement in the facility and di- 

 rectness of communication a matter of the greatest 

 importance to both the public and the telephone 

 companies. 



:< Three distinct types of telephone equipment have 

 been developed," explains a recent writer in the Elec- 

 trical Review, "the magneto or local-battery, the 

 common-battery or lamp-signal, and the automatic 

 system. The first two may be further subdivided into 

 the transfer and the multiple systems. . . . 



"The trunking or transfer system is a development 

 of the original transfer system. It is due to an effort 



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