INDUCTANCE AND CAPACITY 345 



rection and the total inductance becomes the sum of the self- 

 inductances of the coils and twice their mutual inductance. 

 Thus the inductance of the standard may be varied continuously 

 from a minimum, which is nearly zero, to a maximum, usually 

 about 5 millihenrys. The scale is irregular and the instrument 

 is not astatic. 



Brooks Variable Inductor. 3 The Brooks Variable Inductor 

 very closely fulfills the requirements for a variable standard of 

 mutual and self-inductance. It was developed for use in testing 

 current transformers (see page 581) but is applicable in any 

 measurement where such a variable standard, having a constant 

 resistance, is necessary. 



The particular advantages of the instrument are its large 

 carrying capacity, low resistance and practically uniform scale. 

 The range of the instrument, as described, is from 125 to 1,225 

 microhenrys. 



Fig. 205 shows the instrument complete and in section, as 

 well as the form ot the coils. Referring to the diagram the four 

 coils, F, are fixed; by means of the handle, H, the two movable 

 coils, Af, can be displaced in their own plane about the axis, A. 

 The number of flux linkages between F and M can thus be varied. 

 The coils of stranded wire are arranged astatically. Current is 

 carried to the movable coils through heavy copper spirals, thus 

 eliminating all contact resistances. The cross hatching in the 

 diagram indicates the numbers of turns in the coils. The four 

 fixed coils are permanently connected in series and provided 

 with binding post terminals; likewise the two movable coils. 



When the fixed and movable elements are connected in series 

 the instrument may be used as a standard of self-inductance, and 

 when they are separated, as a standard of mutual inductance. 



The self-inductance, or scale reading, is 



L = Li + L 2 2m 



where LI and L 2 are the inductances of the fixed and movable 

 elements and m is the mutual inductance of the elements. L\ 

 and L 2 are constants, and for the instrument as described, LI + L 2 

 = 669 microhenrys. The expression for the mutual inductance 

 is therefore 



L - (Li + L 2 ) L - 669 

 m = TT~ o 



