2l6 MEASUREMENT OF CURRENTS. 



In the former case, if the distance x is sufficiently great, the term 

 of correction may be put in the form 



/ 2 



When the needle is in the mean plane, at a distance r from 

 the centre, we have (795) 



/V i . n A i+cos 2 8 



7 = - 



The tangent galvanometer is one of the instruments best suited 

 for measuring currents in absolute value. In that case, inde- 

 pendently of the constant G, which should be calculated as a 

 function of the dimensions of the coil, it is necessary to determine 

 the intensity H of the field in absolute value. We shall examine 

 subsequently the methods used in this latter determination. 



>838. Fig. 161 represents a tangent galvanometer, with movable 

 coils, constructed entirely of wood. Each of the two coils is wound 

 on a frame which, sliding on two wooden rails, can be fixed by a 

 screw, and can be adjusted by a two-way screw. The distance 

 of the frames may be determined by means of two verniers. 



The needle, provided with a mirror and suspended by a fibre 

 of silk, is placed on the axis of the coils. The reading of the angle 

 is effected by means of a telescope and a graduated scale. As the 

 whole of the apparatus turns about a vertical axis, it may be adjusted 

 so that for the normal position the needle is parallel to the plane 

 of the coils. 



The condition of Von Helmholtz's galvanometer is satisfied by a 

 suitable displacement of the coils. In any case, the length of the 

 needle is only a tenth of the diameter of the coil, and the small 

 correction for the law of tangents may be obtained with sufficient 

 approximation. The sensitiveness is a maximum when the two 

 coils are in contact with the tube containing the suspension wire. 



839. ORDINARY GALVANOMETERS. In an instrument which is 

 not intended for absolute measurements, the object aimed at is to 

 get great sensitiveness as well as rapid and easy readings. 



The sensitiveness is proportional to G, and is inversely as H. In 

 order to increase the factor G, we must bring the windings as closely 



