150 



BELL SYSTEM TECHNICAL JOURNAL 



an aluminum cup riveted to the assembly panel. The windings are 

 dried and impregnated and the space between the coil assembly and 

 container is filled with insulating compound. 



The mutual inductance of a typical coil varies as shown in Fig. 8 

 as the cores are moved. The range, with the shunts disconnected, is 

 approximately + 1-6 to — 1.6 microhenries, which is covered in about 

 16 turns of the screw (a total core travel of 0.5 inch). With the shunts 

 connected, the effective mutual inductance at a given setting becomes 

 less as the frequency rises, the two components. Ma and Mb, varying 

 with frequency as shown in Fig. 9. To determine the proper values of 

 Ls and Rs for the shunt, allowance must be made for the complex 

 mutual inductance inherent in the coil due to proximity effect within 

 the windings. 



— cr 



4 6 8 ID 12 14 l( 



NUMBER OF TURNS OF ADJUSTING SCREW 



Fig. 8 — Mutual inductance of trial balancing coil. 



18 



20 



The series inductance of the balancing coil without shunts varies as 

 shown in Fig. 10. As the cores are moved, the inductances of windings 

 l-A-B-2 and S-C-D-6 (Fig. 5) behave as shown by their respective 

 curves, one increasing as the other decreases. The sum of these two 

 curves is shown by the dotted line, and the measured value of 1-5-6-2 

 is shown by the solid line. It is seen that the overall self-inductance 

 of 1-5-6-2 is constant to within ±0.1 microhenry. The difference 

 between the curves (about 0.1 microhenry) is caused by the slight 

 mutual inductance existing between winding \-A~B-2 and 5-C-D-6, 

 which is negative owing to reversed winding direction in this side of 

 the balancing coil. The measured inductance around 3-7-8-4 would 

 slightly exceed that obtained by adding the inductances of the two 

 sections owing to positive mutual inductance between the two ends. 

 These end effects could be reduced by greater separation of the two 

 sets of windings, but this refinement is not necessary. 



