214 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY l!Jo4 



In the latter category, the changes may be magnetic or mechanical. 

 The mechanical changes appear in the value of the operated load, and 

 are practically negligible in relays of the AG type, because of the lift-off 

 actuation used. In this they compare favorably with F-type relays, 

 which are liable to load changes through stud and contact wear. The 

 magnetic changes include "ageing" of the magnetic material, a gradual 

 change in permeability and coercive force which occurs with magnetic 

 iron. Hydrogen annealed silicon steel, used in the AG relay, is free from 

 this defect. The other magnetic changes are those appearing in the pole 

 face configuration and finish thickness as a result of wear. No significant 

 changes of this type have been observed in the AG relay, which is there- 

 fore highly stable in maintaining the performance of its initial adjust- 

 ment. 



In use, the release time varies inversely with the sleeve or slug tem- 

 perature. As shown above, the 140°F temperature rise allowed for coils 

 in normal operation will, if experienced by the sleeve, reduce the release 

 time by about 25 per cent. Allowance for variation in this range must be 

 made in planning the applications of slow release relays. Relays fitted 

 with slugs are less affected by coil temperatures than are those using 

 interior sleeves. The slug arrangement, with an insulating air gap be- 

 tween slug and coil, would be essential to a slow release relay designed for 

 closely controlled release time. 



The initial differences from nominal performance may be sub-divided 

 into those arising from the adjustment tolerance, and those resulting 

 from using release current as the adjustment criterion. Only the former 

 would apply if the direct timing criterion were used, provided a procedure 

 was employed in which the sleeve temperature was fixed in measure- 

 ment. A tolerance range of 50 gm wt is a 25 per cent variation for a 

 representative load of 200 gm wt, and corresponds, from (16) to about a 

 12 per cent variation in NI + (NI)c ■ If (NI)c is about ecjual to N^I, 

 the variation in the latter is about 25 per cent, which results, from (9) 

 in a similar variation in the release time. A smaller tolerance range could 

 be attained by more expensive adjustment means than the buffer spring 

 of the AG relay, with a corresponding reduction in the range of time 

 variation. 



With release current as the criterion, the value of AU in (9) is subject 

 to the adjustment variation, of the order of 25 per cent for the case 

 cited above. In addition, the release time is subject to variations in G, 

 <p" — (po , and f(z). In the optimum range of operation where f(z) is 

 nearly constant, variations in this term are minor. <po is subject to the 

 relatively large percentage variations in {NI)c and (Ri , but the net effect 



