218 BELL SYSTEM TECHNICAL JOURNAL 



Referring to Fig. 8 the space or area available for winding is 



A =LA. (2) 



From equations 1 and 2 the total turns possible arc 



The total resistanco of the winding is the product of the resistance 

 of the mean turn Rm and the total turns .V, 



Ri = RmN. 



The length of the mean turn for a round core, Fig. 8, is 



2Kt+I)=^^^^+^)- 



and if r is the resistance per unit length we ha\e 

 Rm = Tr{Ci + A)r, 



whence the total resistance is 



R,^w{C2+^)rN; 

 or substituting the value of A^ from equation 3, 



K. 



For a core of rectangular cross section equation 3 holds for the total 

 number of turns and it will be found that the equation for total 

 resistance is 



where p represents the perii)liery of thu core in inches. 



Tempi;r.\tire Ch.\r.\cteristics 



The critical circuit conditions with respect to the rela\' winding 

 specify either constant wattage, constant voltage or constant current. 

 The constant \'o!tage circuit is one in which a change in resistance 

 of the relay winding materially affects the current How. The con- 

 stant current circuit is one in which a change in resistance of the 

 relay winding does not materially affect the current flow. An ap- 

 proximate constant wattage condition is one in which a resistance 



