80 



ELECTRICAL MEASUREMENTS 



coils from a perpendicular position. However, Lord Rayleigh* 

 has called attention to the fact that the mutual inductance of 



/O 3 

 two concentric circles, the ratio of whose radii is A/-J- = 0.548, 



is, over a considerable range, very nearly proportional to the 

 angular displacement of the axes from the perpendicular position. 



FIG. 39. Showing relation between the mutual inductance of two circles 

 and the angular displacement of their axes from the perpendicular position 

 when the ratio of the radii is 0.548. 



This is illustrated by Fig. 39 in which relative values of the mutual 

 inductance are plotted against the angular displacements of the 

 axes of the coils from the perpendicular position. 



From the figure it is seen that over a wide range - r- is practi- 



dv 



cally constant. As the turning moment due to the mutual action 



"The Inductance and Resistance of Compound Conductors," (Phil 

 Mag., December, 1886, p. 470). 



