A SHIELDED BRIDGE FOR INDUCTIVE IMPEDANCE 



157 



TABLE I 



Capacitance and Phase-angle Tests between Two Double-silk-covered No. 

 38 A. W. G. Wires, Each 88 in. (224 cm.) Long, Wound in Bifilar Fashion 

 on a Glass Tube One In. (2.54 cm.) in Diameter and then Treated with 

 Various Materials. Samples Dried before Testing. 



From the standpoint of percentage capacitance change (reckoning 

 from the minimum temperature and frequency conditions as being 

 those at which initial adjustments would be made), untreated and 

 paraffin-treated silk insulation were found to be appreciably superior 

 to any of the other materials. The change due to absorption effect 

 (about three per cent for these two) was considered the more impor- 

 tant, as normally variations in temperature would not be very large. 

 As would be expected, the untreated silk had the lowest phase-angle 

 effect and also the smallest capacitance. Assuming that a method 

 of excluding moisture could be devised, it was concluded that an 

 untreated silk-insulated winding would be the best to use, although the 

 paraffin treatment was also considered promising. Discounting the 

 fact that the two ratio coils would change in the same direction 

 though not necessarily by the same amount, it was decided that a 

 satisfactory factor of safety would be provided if it were assumed 

 that the coils might become unbalanced by one half the observed 

 change in one coil; that is, by about 13^2 per cent. In order that 

 such unbalance should not exceed 0.1 mmf., the capacitance of each 

 coil would need to be not more than about 6.0 mmf. It should be 

 noted that this limit applies to the true inherent capacitance and not 

 to the resultant of the coil capacitance and inductance. 



Considering now the variation in resistance over the frequency 

 range of the bridge, it can be shown, following the methods of Curtis 

 ^nd Grover, that the effect of a capacitance of the value noted 



