TELEPHONE SYSTEM OF THE UNITED STATES 9 



ber of calls handled throughout each hour of the day and in this way 

 the forces are adjusted to the work to be done. 



In order that the demand for telephone service may be met promptly 

 as it develops and further that plant additions may be along sound 

 and economic lines, calls for careful planning. To this end the funda- 

 mental plans prepared for the different exchange areas forecast the 

 telephone development from 15 to 25 years in the future. Such 

 fundamental plans show the proposed central ofifice locations, the 

 boundaries of the districts to be served by each office, and the plan 

 of the underground conduit system. They are based on analysis of 

 the existing market for telephone service; the forecasted market at 

 a future date, considering both growth and distribution of population; 

 expected changes in wage levels; estimates of the amount of service 

 that will be sold under probable future rate conditions; and other 

 factors. 



Station Apparatus 



One of the most important parts of the telephone plant is the appa- 

 ratus installed on the subscribers' premises known as the station 

 apparatus. Of this equipment the telephone transmitter and tele- 

 phone receiver are fundamentally important elements and continued 

 research work has been carried out to improve the efficiency, clarity 

 of reproduction and reliability of these instruments. As a result of 

 improvements in transmitters, receivers and induction coils the over- 

 all efficiency, for example, of the station apparatus has since 1912 

 increased by a factor of 6.5. At the present time commercial trans- 

 mitters when fully energized by direct current, are capable of deliver- 

 ing electrical energy in the form of voice currents 200 times as great 

 as the acoustic energy of the voice of the speaker by which the trans- 

 mitter is actuated. For the most important part of the frequency 

 range used in speech this ratio of output power to speech power is 

 considerably greater. That is to say, the transmitter acts as a high 

 ratio amplifier. 



In the Bell System the type of station equipment most generally 

 in use is the desk stand. As the result of extensive development work 

 it has been possible to produce a hand set which has transmission 

 characteristics equal to those of the desk stand equipped with the 

 best instruments heretofore in use. The hand set development in- 

 volved the solution of difficult problems, the principal of which were 

 to prevent singing or distortion of quality on account of the rigid 

 connection between receiver and transmitter and to make the trans- 



