CHAPTER VII 



MEASUREMENT OF INDUCTANCE AND CAPACITY 



STANDARDS OF INDUCTANCE 



For carrying out the methods of measurement described in 

 this chapter, standards of inductance and capacity are required, 

 and convenience dictates that, in many cases, they be made 

 adjustable. These standards may be divided into two classes: 

 primary standards, whose values are calculated from their di- 

 mensions; and secondary standards, whose values are deter- 

 mined experimentally. 



The subject of the calculation of primary standards of self- 

 and mutual inductance is beyond the scope of this work. Read- 

 ers are referred to the papers of Rosa and Grover, who have col- 

 lected and tested all the available formulae and have published 

 them together with illustrative examples in the Bulletin of the 

 Bureau of Standards. 1 



Standards of Mutual Inductance. Primary standards of 

 mutual inductance which have a single fixed value are useful 

 in the calibration of ballistic galvanometers, also in the calibra- 

 tion of variable working standards of mutual inductance which 

 are used in the laboratory. 



The considerations governing the design are: 



1. The value must be accurately calculable from the geo- 

 metrical dimensions. 



2. The construction must be such that permanence is assured. 



3. The value must be sufficiently large to give high sensitivity 

 when comparisons are made. 



4. The resistances of the coils must be kept as low as possible. 



5. Eddy-current effects must be eliminated as far as possible. 



6. The capacity effect between the primary and the secondary 

 must be a minimum. 



7. The bobbins upon which the coils are wound must be free 

 from magnetic materials. 



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