500 CONSTANTS OF COILS. 



needle experiences from the same currents, when the actions of the 

 coils are in the same or contrary directions ; we shall have 



H tan 8 = XI + XT, 

 H tan<$ 1 = XI-XT. 



From this follows, if ft is the ratio - -^ , 



tan 6 



The ratio is equal to unity if the same current traverses both 



coils. If the coils are shunted on a circuit, this ratio is equal to 

 the inverse ratio of the resistances g and g' of the corresponding 

 branches. We may then modify these resistances, so that the 

 current is null in the second experiment, and we have 



C.L.COR/. x.l_ 



X' I f 



If the coil B to be compared is a tangent galvanometer of 

 known dimensions, which enables us to calculate X in absolute 

 value, equation (3) will give X'. 



When the needle is not infinitely small in comparison with its 

 distance from the windings which the current traverses, the values 

 of X and X' the ratio of which follows from experiment are not 

 exactly those corresponding to the points P and P'. We may then 

 introduce a correction (746 and 796) for the length of the needle. 



1089. SURFACE OF A COIL. MAGNETIC MOMENT. The value 

 of the electrodynamical surface of a coil, or its magnetic moment 

 for unit current, in the case of a cylindrical coil of mean radius a 

 (727), is 



/ 



( 



V 



+ 7), 

 3" 



y being the term of correction. 



The value of the surface S may be deduced from the intensity of 

 the field of the coil at a point situated in a principal position, and 

 sufficiently far from the centre ; the component X of the field will be 



