210 MEASUREMENT OF CURRENTS. 



the maximum of the two degrees of sensitiveness is when the rotation 

 of the coil is near 90. 



833. TANGENT GALVANOMETER. For galvanometers, properly 

 so called, the coil is fixed, and the needle takes a direction of 

 equilibrium under the combined influence of the current and the 

 external field. 



If the plane of the coil makes the angle a with the external field, 

 and the current I deflects the needle through the angle 8, we have 



IMG cos (8 + a) = HM sin 8, 

 or 



H sin 8 



(7\ 1= * 



Gcos(a + 8)' 

 The angle a is generally zero, and therefore 



H 

 (8) I.-tanS; 



the intensity is proportional to the tangent of the deflection. This 

 method is also due to Pouillet. 



For the law of tangents to be correct, the term of correction <, 

 which should be introduced into the value G (822), must be very 

 small ; or, in other words, the length of the needle must be small 

 compared with the mean radius of the coil. 



We have, for the tangent galvanometer, 



S a = cos 2 8, 



S =-sin 28 ; 

 2 



the absolute sensitiveness is a maximum for very weak deflections, 

 and the relative sensitiveness is so for the deflection 45. 



834. If the direction of the field is not parallel to the mean 

 plane of the coil, we may correct the defect of adjustment by 

 observing the fresh deflection 8' obtained after reversing the current. 

 Assuming that the value of G is the same for the two positions of 

 the needle, the two equations of equilibrium are 



IGcos(8 + a) = H sin 8, 

 IG cos (8' - a) = H sin 8'; 



