00 THE BELL SYSTEM TECHXICAL JOURNAL, JANTAHY \\)')4 



vixhie by the square of the slope value, and the latter then serves to 

 evaluate (Ro by subtracting Xc/A. On substituting these values of (Roand 

 A in (8) at x = Xc , (Ri, may be evaluated. For the case plotted in Fig. 

 7, the values thus obtained are: 



olo : 0.0300 cm"' 



A: 1.34 cm' 



(R,. : 0.0()90 cm""' 



These values of the equivalent circuit constants, substituted in (3), 

 suffice to accurately characterize the magnetization relations and thus 

 the electromagnet's performance through the low density region. 



Evaluation of High Density Relations 



For cases of core saturation, the high density relations may be analysed 

 in terms of the magnetic circuit of Fig. 5. The corresponding reluctance 

 is given by (4), in which (Re = (Re for v? = (^'. If (Re can l)e determined, 

 the three parameters determining (R^ in (5) may be e\'aluated by the 

 equivalence ec^uations (7). To determine (Rr through the high density 

 region from (6) requires evaluation of (Re , (f', and <p". Thus the analysis 

 of the high density relations rec^uires evaluation of these three quantities 

 in addition to the three parameters of the e([uivalent circuit evaluated 

 above. 



A method for evaluating (Re , </?', and (p" directly from the magnetiza- 

 tion curves is described subsequently, following a description of a pre- 

 ferred method which requires additional measurements. These measure- 

 ments are determinations of the drop (Rev? iii magnetic potential through 

 the core, and are conveniently made with the magnetomotive force 

 gauge developed by W. B. EUwood. 



Magnetomotive Force Measurements 



The EUwood mmf gauge measures the difference in magnetic potential 

 between the outer ends of the two reeds of a dry reed switch, as deter- 

 mined by the coil current required to open the switch. The end of one 

 reed is used as a probe, which is brought into contact with the magnetic 

 structure being measured. The end of the other reed, at a distance of 

 about 5 em from the probe, then lies on a relatively distant surface 

 surrounding the structure: this surface is substantially at the same po- 

 tential for any position of the gauge. Then the difference between two 

 such measurements made at different points on the magnetic structure 

 measures the difference in magnetic potential between them. 



