MAGNETIC LOSSES AT LOW FLUX DENSITIES 



215 



spiral core with successive turns insulated from each other by painting 

 with a suspension of fine quartz powder in CCI4 immediately prior to 

 winding. The core had an effective magnetic diameter c^ = 11.22 cm., 

 and cross-sectional area of alloy A = 3.96 cm.-. It was annealed in 

 pure hydrogen for one hour at a temperature of 1000° C. 



In order to protect the annealed core from mechanical stress during 

 subsequent winding, it was placed on felt in an annular bakelite box 

 which held it without constraint. The box was wound with a 20-turn 

 magnetizing coil using a flat tape composed of 28 parallel strands of 

 insulated wires connected together at the ends. This winding prac- 

 tically covered the box with a single layer of wire, and gave a uniform 

 magnetizing force. It was employed as the magnetizing coil in both 

 the ballistic and the a.-c. bridge measurements. For the ballistic 



SHIELD 



^G 



^ 



Sa 



MUTUAL ^a p 



f-vw 



'\mJ 



jwn 



■wv 



Rs 



I TO I 

 POTENTIOMETER 



Fig. 1 — Ballistic galvanometer circuit, showing adjustable air-core mutual inductance 

 in series-opposition with the test coil. 



tests, a layer of insulation was applied over the primary winding before 

 applying the secondary winding. This insulation consisted of two 

 wrappings of silk tape interspersed with a tinfoil sheath which formed 

 a grounded electrostatic shield between the secondary and the primary 

 winding. The foil was cut to avoid a short-circuited turn. The 

 toroidal secondary winding consisted of 5000 turns No. 19 silk-covered 

 enamelled copper wire. 



D.-C. Apparatus 



In the former experiment,^ the specimen was compared with a fixed 

 air core mutual inductance in terms of the galvanometer deflection and 

 the primary currents required to obtain approximate balance. For 

 this experiment, the circuit was modified so that the same current 

 flowed in the primaries of both the specimen and the air core mutual 

 (Fig. 1). Thus variable thermal effects in the primary circuits were 



