2 ELECTRICAL MEASUREMENTS 



* radius which is large compared with the dimensions of its cross- 

 section, together with a small magnetic needle which so sus- 

 Bended at the center of the coil that it can move about a vertical 

 axis The needle is provided with a pointer which moves over 

 a scale graduated in degrees. The coil is so placed that its plane 

 is vertical and in the magnetic meridian. When no current is 



FIG. 1. Tangent galvanometer. 



flowing, the pointer stands at zero for the needle is then con- 

 trolled by the horizontal component of the local magnetic field, H. 

 On the passage of a current the coil sets up a magnetic field, 

 which at the needle is perpendicular to the plane of the coil and 

 of magnitude 



F - T 

 r i 



?i is the number of turns in the coil, r the mean radius of 

 the coil and / the current in absolute units. The needle will turn 



