26 



THE BELL SYSTEM TECHNICAL JOl'RNAL, JAXI'ARY 1954 



coil constant is wholly determined by the coil dimensions and the type 

 of insulation used. Thus the power required for relay operation depends 

 upon (1) the load characteristic of the contact arrangement, (2) the 

 magnetic characteristics which relate the pull to the ampere turns, and 

 (3) the coil dimensions as defined by S/m , which determines the coil 

 constant A^ /R, and thus the relation between ampere turns and power 

 input. 



The external dimensions of relays are largely determined by the coil 



2 300 







t 



OPERATED 

 POSITION 



0.01 0.02 0.03 0.04 ^ 0.05 



ARMATURE TRAVEL IN INCHES 



RELEASED POSITION 

 (AGAINST backstop) 



Fig. 1 — Typical load and pull characteristics of a wire spring relay with 12 

 transfer contacts. 



