MAGNETIC LOSSES AT LOW FLUX DENSITIES 



217 



comparison bridge,^ and were verified at low frequencies using a 1-ohm 

 ratio arm bridge. Calibration of the bridge and standard coils was 

 efifected by making measurements over the entire frequency range on a 

 calibrated high quality air core coil substituted for the test coil. The 

 maximum correction required on this account was approximately 0.1 

 per cent of the resistance due to the magnetic core. 



The source of alternating current was an oscillator-amplifier supply- 

 ing approximately 0.4 watt undistorted power, calibrated for these 

 measurements against the Laboratories' standard frequency. The 

 current was adjusted by the insertion of resistance in series with the 



5 6 7 8 9 



H IN OERSTEADS X 10^ 



Fig. 2 — Core permeability as measured by the ballistic galvanometer. 



primary of the bridge input transformer, and was measured by means 

 of a thermocouple between the transformer secondary and the bridge. 

 The bridge unbalance was amplified by means of an impedance 

 coupled amplifier for the 10-ohm bridge, and by means of a resistance 

 coupled amplifier for the 1-ohm bridge. The amplified unbalance was 

 observed by means of head phones at frequencies above 200 cycles, and 

 by means of a vibration galvanometer at lower frequencies. The d.-c. 

 balance required bridge current of about 3 m.a. in the test coil winding, 

 and had the same precision as the a.-c. balance, viz., db 0.0002 ohm. 

 The inductance readings were corrected for the air space within the 

 winding, and had a relative accuracy of about 0.03 per cent, and an 

 absolute accuracy of approximately 0.1 per cent. 



