212 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1954 



Optimum Design Conditions 



In considering the design features that are favorable to slow release 

 operation it is convenient to refer again to the expression for the release 

 time given by equation (9): 



Avhere f{z) is the function shown in Fig. 5 to be nearly a constant in the 

 range of interest. It is also convenient to refer to the expression for the 

 pull given by the equation (16): 



2t{NI + {NDcY d(S{a 



F = 



(R? dx 



Together with reference to the operate requirements, these two equa- 

 tions indicate the characteristics that are important for slow release 

 operation. 



The winding space determines the value of G attainable, as shown by 

 the relations of Section 5. It limits the combined values of Gs and Gc , 

 of which the latter controls the operate power sensitivity, while the 

 former, from (9), determines the release time. Thus both the operate 

 sensitivity and the release time attainable vary directly with the winding 

 space and hence with the over-all size of the relay. 



From (9), the attainable release time is nearly proportional to ^", and 

 hence to the cross section of the core, assuming (po/(p" to be small. The 

 external core dimension is the internal dimension of the sleeve, so that 

 increases in (p" are offset by decreases in G if this dimension alone is 

 varied. In any case, sufficient section must be provided for (p" to have a 

 margin over the field required to operate the maximum load. 



In telephone use, the slow release relays are a minority group in a 

 relay population which must, for maximum economy in manufacture and 

 use, have common overall dimensions and as few differences as are con- 

 sistent with the requirements of specific uses. Thus the core section, 

 winding space, and over-all dimensions reflect an optimum choice for 

 the whole relay population, and not for the slow release relays alone. 

 The latter are distinguished by as few special features as are essential 

 to their special function. In the AG relay these are: the armature, the 

 heat treatment of the magnetic parts, the sleeve and coil, and a buffer 

 spring for load adjustment. 



The material and its heat treatment determine the iron reluctance and 

 the coercive mmf, 4t(NI)c , of which the latter is the more important 

 quantity. It enters the timing relation indirectly in the minor term <po , 



