complete telephone sentence, "Mr. Watson, come here, I want you," 

 was successfully transmitted on March 10, 1876. 



Almost from the hrst, the efforts of inventors to develop successful 

 telephone transmitters made use of this principle, and while many 

 variable resistance elements were tried out with some degree of success, 

 transmitters employing granular carbon, the resistance of which varies 

 with pressure, were the most satisfactory. Such a transmitter devised 

 in 1878 by Runnings of England using powdered "engine coke" was 

 extensively used commercially. Better performance was provided by 

 the design in 1890 by White of the Bell System of the so-called "solid- 

 back" transmitter. This principle and the use of carbon transmitter 

 elements have survived through numerous improvements in transmitter 

 design and are applied to millions of telephone instruments in use today. 



The telephone station of today includes, generally speaking, a 

 transmitter of the variable resistance type, a receiver based on the 

 principle of Bell's original discovery, both of these instruments being 

 modern in design, includes induction coils, condensers, etc., necessary 

 for electrically associating the transmitter and the receiver with each 

 other and with the telephone line and, in addition, includes such items 

 as a bell, a switchhook, etc. w^hich are necessary for signaling and 

 control of the telephone circuit. 



Telephone Switching Systems 



Very early in the practical use of the telephone, it became evident 

 that the full usefulness of this method of communication required the 

 development of means by which any subscriber could quickly obtain 

 connection between his telephone and any other telephone rather 

 than being limited in his conversations to one other subscriber or 

 a small group of other subscribers connected together on the same 

 telephone circuit. The difficulties which would be encountered with 

 a telephone plant consisting of large numbers of stations connected 

 to one circuit are obvious, the outstanding disadvantage being that 

 only two subscribers could carry on a conversation over the circuit 

 at one time. These difficulties led to the development of telephone 

 switchboards at which connection could be made between lines to any 

 two subscribers in a given town or city. 



As technical developments made toll service between different 

 cities possible, means were needed for the rapid connection of any two 

 subscribers in different cities. It would obviously be impracticable 

 to connect together at the same switchboard subscribers in distant 

 cities, and switching systems were adopted so that toll connections 

 between any two subscribers in different cities could be established 



