A SHIELDED BRIDGE FOR INDUCTIVE IMPEDANCE 



165 



every combination would be required, and as this calibration would 

 vary with frequency, a correction would be needed for each frequency 

 value used. 



In a standard of inductance to be used at high frequencies it is, 

 of course, always desirable in order to minimize capacitance effects 

 to have the inductance coils as small as possible. In the case of a 

 completely shielded decade standard this is even more important on 



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Fig. 15 — Circuit diagram of shielded inductance standard 



account of the capacitances added by the internesting shields. On 

 the other hand the coil resistances should be quite small in comparison 

 with their reactances, a requirement which tends to increase the coil 

 dimensions. In addition to the above such coils should be highly 

 stable in their inductance and effective resistance values with respect 

 to the residual effects of direct and alternating currents and of tem- 

 perature and humidity changes. Their values should also be of a 

 satisfactory degree of constancy with respect to frequency and value 

 of the testing current. 



To meet these varied requirements the coils used in this bridge 

 depart from the air core type ordinarily employed, in that they have 

 a magnetic core of high stability and efficiency. Thus the desired 

 inductance is obtained with a much smaller number of turns in the 



