342 



ELECTRICAL MEASUREMENTS 



To eliminate eddy-current effects, all conductors which carry 

 large currents must be made of insulated strands, and all metal 

 frames, etc., near the coils must be avoided. 



In the past inductance coils have frequently been wound on 

 bobbins of serpentine. It has been found, however, that a coil 

 so wound has an inductance which depends, to a slight extent, 

 upon the strength of current flowing in the conductor, thus 

 showing that the permeability of serpentine is not unity and 

 that it depends on the magnetic field in which the serpentine is 

 placed. 



T 

 i 



6 



i 



Primary 



10 

 Secondary 



t 



-.-A 



-14^8 



Primary 



\ 



FIG. 203. Campbell single valued primary standard of mutual inductance. 



For primary standards of self-inductance it is customary to 

 use single layer coils which are wound on accurately ground 

 cylinders of marble, or of plaster of paris which has been impreg- 

 nated with paraffin. Such a construction facilitates the exact 

 determination of the geometry of the coil. In a standard of 

 mutual inductance, this construction is not admissible for both 

 the primary and the secondary coils, since the product of the 

 primary and secondary turns must be large, about 100,000 

 for a mutual inductance of 0.01 henry. 



Campbell Fixed Standard of Mutual Inductance. 2 It 

 is highly desirable that the arrangement of the primary and 

 secondary coils in a standard of mutual inductance be such that 

 errors arising from slight displacements of the coils from their 

 supposed relative position will be reduced to a minimum. By 



