216 BELL SYSTEM TECHNICAL JOURNAL 



eliminated and the measuring technique simplified. This required that 

 the mutual inductance be adjustable so as to bring the unbalance onto 

 the galvanometer scale. 



The mutual inductance (approximately 0.260 henrys) was con- 

 structed for convenience in four separate sections. Each section had a 

 hard wood toroidal core, a low resistance toroidal secondary winding, 

 an inter-winding shield, and sectionalized primary windings on the 

 outside. The secondary windings were connected in series with the 

 galvanometer by a twisted shielded pair of wires. The primary wind- 

 ing groups were also connected in series, and adjusted so that the com- 

 bination resulted in a mutual inductance of the right value to obtain 

 balance. To eliminate humidity as a source of error each coil was 

 painted with cellulose acetate, covered with silk tape, painted again, 

 baked 48 hours at 108° C. and finally potted in Superla wax in an 

 earthenware jar with only the tops of the terminals exposed. All con- 

 nections were made by soldering. 



During the measurements, the maximum primary current corre- 

 sponding to Hm was held constant to 0.01 per cent by comparing the 

 voltage drop across Rs with a battery of Weston standard cells. 

 Switching was automatically performed by a photocell and selector 

 switch mechanism previously described ^' ^ but not shown here. 

 These operated switches 5 and Si at the proper time and in the right 

 order. The difference in flux turns between the air core mutual in- 

 ductance and the specimen was determined in terms of the ultimate 

 galvanometer deflection as before. From this the difference in B 

 between the side of the hysteresis loop and a straight line drawn through 

 its tips could be computed for a given H. A number of values of this 

 difference A5 were thus determined for different values of H, and 

 plotted to give the hysteresis loop. 



A.-C. Apparatus 



In order to compare results obtained by the vacuum ballistic gal- 

 vanometer with those obtained with alternating currents, bridge 

 measurements of resistance and inductance were made over the same 

 range of flux densities at a number of frequencies ranging from 35 to 

 1 0,000 cylces. The secondary winding was removed and the special 20- 

 turn primary winding used for most of the measurements. Later an addi- 

 tional 60-turn winding was used for checking the measurements in the 

 low-frequency range. In either case the inductance was low enough 

 to depress any effect of distributed capacitance far below the precision 

 of the measurements. 



Measurements were made on a 10-ohm equal ratio arm inductance 



