344 ELECTRICAL MEASUREMENTS 



rangement of two concentric circular coils, the ratio of the radii 

 being 0.548 (see page 80), a practically uniform scale extend- 

 ing over about 60 may be obtained; if the coils are used in con- 

 junction with a fixed mutual inductance the scale may be ex- 

 tended to about 120. 



Ayrton and Perry Inductor. The Ayrton and Perry variable 

 standard of self-inductance has long been used for general labora- 

 tory purposes. This standard consists of two coils of slightly 

 different diameters, the smaller pivoted within the larger in such 



FIG. 204. Ayrton and Perry variable inductor. 



a manner that it can be rotated about a vertical diameter. The 

 coils are connected in series and a pointer shows their relative 

 position; as each position corresponds to a particular value of 

 the self-inductance of the combination, the dial may be graduated 

 in henrys. When the index stands at the lower end of the scale, 

 the inductance of the combination is nearly zero, owing to the 

 fact that the currents in the two coils are circulating in opposite 

 directions. When the movable coil is turned through 90, the 

 inductance becomes the sum of the self-inductances of the two 

 coils, as there is no mutual induction in this position. When 

 turned through 180 the currents in the coils are in the same di- 



