240 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1954 



Cost of power: 



Tp = 



cpEJi 



Cost of functioning time: 



Cr = 



qct 

 wp 



(18) 



(20) 



Examination of these equations will show that, in nearly every case, a 

 cheapening of the cost of the structure leads to more expensive (i.e., 

 poorer) performance, and vice versa. However, the availability of these 

 relations, together with accurate values for the constants, gives the in- 

 formation needed to vary the relay design to the point where maximum 

 over-all economy (i.e., the cheapest central office) can be realized. 



In the following sections, some examples of methods for finding this 

 optimum point, and how much it is worth to find it, will be given. 



Part II — ■ Design for Maximum Power-Plus-Coil Economy 



Among the more common problems facing the designer of switching 

 circuits is the selection of the relay magnet which gives best economy in 

 use. On the one hand, eciuation (1) has shown that the relay coil cost 

 will be less if the resistance is less (but power consumed is more) . On the 

 other hand, e([uation (18) shows that the equivalent first cost of that 

 part of the power plant furnished to operate the relay increases as the 

 resistance is reduced (but coil is more expensive). These opposite ef- 

 fects are indicated in Fig. 9 where the curve labelled Cw shows how wind- 



RESISTANCE 



Fig. 9 — Power and winding costs. 



