MAGNETIC FIELD OF A COIL. 499 



balance (987). Silence is broken if there is any connection between 

 the windings. 



Only coils of a very special form and construction can be sub- 

 mitted to calculation ; and such coils should, so to speak, serve as 

 standards. For ordinary coils, the constants can only be determined 

 by comparison with the former. 



1087. MAGNETIC FIELD OF A COIL. We shall obtain the 

 magnetic action of a coil at a point, for unit current, by the general 

 methods employed in determining the intensity of a magnetic field, 

 and to which we shall afterwards return ; but in comparative experi- 

 ments we ought to seek for rapid methods, which require neither a 

 constant current nor a measurement of the intensity. 



In order to determine the direction of the force which a coil B 

 exerts at a point P, it is sufficient to place at this point a magnetised 

 needle of very small dimensions, subject at the same time to the 

 action of an external field, such as the terrestrial field, and to turn 

 the coil about the point P, until the passage of a current, or its 

 change of direction, produces no deflection. The magnetic axis of 

 the needle then gives the desired direction. 



In the case of very powerful currents, the use of magnetic images 

 often gives valuable indications ; the successive elements of the iron 

 filings trace out the lines of force, and give a direct idea of the form 

 and intensity of the field. 



The ratio of the actions F and F' for unit current of two coils 

 B and B', at two points P and P', can be obtained by the method 

 used for comparing two galvanometers (875). The coils are placed 

 so that their actions at the points P and P' are sensibly perpendicular 

 to the external fields or, at least, that the projections X and X' of 

 the actions of the coils, on a plane perpendicular to the axis of 

 rotation of the needles, are perpendicular to the projections H 

 and H' of the external fields, and very small needles are placed 

 at these points. The deflections 8 and 8', corrected for graduation, 

 produced by the same current, which traverses the two coils, give 



X' H'S'' 



1088. In order to eliminate the ratio of the fields H and H', 

 we shall make the two coils act on the same needle (876). 



Suppose, for instance, that the coils are traversed by the currents 

 I and I' respectively. Let 8 and 8 l be the deflections which the 



K K2 



