198 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 195-1: 



As ^c may be read directly from the measured curve, as indicated in 

 Fig. 1, values of (J7c + ^)/(p may V)e computed from corresponding values 

 of If and J7, and plotted against the corresponding value of 3^c + 3^. One 

 such plot for a slow release relay model is shown in Fig. 8. In agreement 

 with (10), the plot is approximately linear over the range covered, and 

 the slope and intercept may be used to evaluate (R" and <p" as indicated 

 in the figure. If co-ordinate paper is used, as in Fig. 8, with radial lines 

 spaced to give a convenient scale of ip, the value of (p" is that correspond- 

 ing to the radial line parallel to the plot. The observed linearity of this 

 relation does not extend to much higher values of 9^, and the observed 

 asymptote <p" is fictitious, as the full curve is concave downwards, 

 asymptotic to a lower value of ip than that found in this way. If the ob- 

 served value of ipo does not agree with that computed from values of the 

 other constants, it is preferable to use the observed rather thad the com- 

 puted value of tpo ■ 



As shown in the preceding section, values of (R, (which is related to (R" 

 by (5)) and of cp'' — cpo may be independentl}^ determined from measure- 

 ments of release time versus release ampere turns. In cases of disagree- 

 ment, these values are to be preferred to those determined from the 

 decreasing magnetization measurements. The latter are primarily of 

 interest in checking design estimates, and in indicating the effects of 

 variations in dimensions and material properties. 



Estimation of Mag7ietic Constants 



In the "tight" magnetic circuit of a slow release relay, the leakage 

 field may be ignored, and the reluctance taken as the sum of the iron 

 reluctance and that of the air gaps, which may be designated (Re ■ The 

 estimation of (Re is discussed in the following section, in connection with 

 the pull relations. The iron reluctance is substantially that of a perma- 

 nent magnet, supplying flux to the external circuit of reluctance (Re • In 

 using (3) to characterize the magnetization relation, the demagnetiza- 

 tion in the second c[uadrant is taken as continuous with the decreasing 

 magnetization in the first c^uadrant. Thus the value of ^c is that which 

 would apply to a permanent magnet of length f, equal to that of the 

 iron path, as given by: 



?c = Hc(, (11) 



where He is the coercive force of the material. For the soft magnetic 

 materials used for slow release relays. He is of the order of 0.5 to 1.0 

 oersteds, so if ^ = 10 cm, [Fc lies in the range from 5 to 10 gilberts (4 to 

 8 ampere turns). 



