MAGNETIC EFFECT OF ELECTRIC CURRENT. 



105 



When no current flows through the coil the suspended magnet 

 points in the direction of the earth's horizontal field H' . A cur- 

 rent of / abamperes in the coil produces a magnetic field of which 

 the intensity at the center of the coil is 

 H = 2irZIIr and of which the direction at 

 the center of the coil is at right angles to 

 H'. This field H combines with H' to 

 give a resultant field R, Fig. 69^, in the 

 direction of which the suspended magnet 

 now points, 9 being the angle through which 

 the magnet is turned by the current. From 

 Fig. 69^ we have 



H 



H * of coil 



Fig. 69b. 



or, substituting 2irZI\r for H and solving for /, we have 



_ rH' 

 I = ~ tan 9 



(3&0 



This equation gives the value of 7 in abamperes when r is in 

 centimeters and H is in gausses, the values of r, H' * and Z 

 being known and < being observed. 



If 7 be expressed in amperes, then equation (36^) becomes 



'amp. 



tan <f> 



A serious fault in the tangent galvanometer is that the earth's 

 horizontal field H' is never known accurately because it is con- 

 tinually changing in value. When it is desired merely to measure 

 the ratio of two currents, however, the value of H' need not be 

 known (provided it does not change while the following obser- 

 vations are being taken). One current 7j is sent through the 

 galvanometer, and the corresponding deflection (f> l is observed, 

 giving 



*See Art. 42 and Chapter X. 



