234 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1954 



1 .32 Cost of Power Purchased from Power Companies 



The power used by the switching apparatus corresponds to a larger 

 amount of power furnished by the power company, since the power 

 machines are not 100 per cent efficient. Its cost may be stated as 



Co 

 Cp= -, 

 e 



where Co = average marginal cost of power furnished, 

 e = conversion efficiency. 



The annual power bill will then be 



Co E'h 

 e 1000/2 



dollars. 



Converting from annual charges to first cost, through the same steps as 

 above, gives 



R 



U power — Cp, — p^ , 117) 



or CP2 dollars per watt-hour-per-year. 



1.33 Total Power Cost 



With both power plant costs and purchased power costs now stated 

 on the same basis, they may be combined to give the total power cost, 

 Cp: 



where Cp represents the dollars per watt-hour-per-j-ear for power con- 

 sumed by a particular relay, and is the eciuivalent first-cost value of 

 power. 



Thus a method is available to find the value of the power that magnet 

 coils may need — to be balanced against the first cost of the coils (Section 

 1.1). Procedures for best results are given later. 



1 .4 THE EQUIVALENT MANUFACTURING COST OF THE FUNCTIONING TIME 

 OF A RELAY 



Another large part of the investment in every crossbar-tj^pe central 

 office is the common-control equipment, most important of which is the 

 "marker." This eciuipment has the sole duty of selecting the proper 

 path for each call, and sending it on its Avay. After each such function, 



