222 



BELL SYSTEM TECHNICAL JOURNAL 



to its temperature limit in a constant voltage circuit, would overheat 

 in a constant wattage or constant current circuit. A relay properly 

 designwl to work at a safe temperature under a constant current 

 condition, would be unnecessarily large and expensive in a constant 

 voltage or constant wattage circuit. It is, therefore, evident that 



Fig. 10 — Relation Between Copper Volume and Percentage Loss in Effective Ampere 



Terms 



exact rules can not be given for the correct proportioning of spool 

 and winding dimensions from a purely design standpoint without 

 consideration of the circuit in which the electromagnet is to operate. 

 Some general design features, however, can be indicated which will 

 enable preliminary assumptions to be made that can be refined as 

 the design is worked out for its particular operating conditions. 



Spool Dimensions 



Certain important facts regarding spool dimensions arc indicated 

 in Fig. 11. The spool dimensions for the winding may be investi- 

 gated by assuming that a definite radiating surface must be used to 

 dissipate the heat, and then determine the relative values of winding 

 depth, length, and volume in terms of the core diameter. The volume 

 of wire used in the spool is taken as a measure of the first cost and a 

 variation in the length of the coil is reflected in the leakage flux which 



