MAGNETIC EFFECT OF ELECTRIC CURRENT. 113 



i FIBRE 



light connecting rod and mirror, and coils containing many turns 

 of fine wire may be made to indicate distinctly a current as small 

 as one million-millionth of an ampere (io~ 12 ampere). 

 The Kelvin galvanometer may 



* JFrBRE 



be used for the approximate meas- 

 urement of very weak currents, be- 

 cause the deflection, within a small 

 range, is proportional to the cur- 

 rent. 



COIL I 



COIL II 



Fig. 78. 



Fig. 79. 



SUSPENDING 

 WIRE 



61. The sensitive galvanometer 



(D 1 Arsonval type). A coil sus- 

 pended in a magnetic field is acted 

 upon by a torque when a current 

 flows through the coil, and the torque is given by the equation 



(38), namely: 



T= abZIH 



as explained in Art. 58. If the coil is suspended by fine wires 

 this torque will turn the coil more or less, and, in order that the 

 coil may be perceptibly turned by a very weak 

 current, the suspending wires (which serve to 

 lead current to and from the coil) must be very 

 fine, the number of turns of wire in the coil must 

 be great, and the magnetic field H in which the 

 coil is suspended must be intense. In order to 

 obtain a quick movement of the coil it is impor- 

 tant to have its breadth b moderately small. 

 Figure 80 shows the essential parts of a sen- 

 sitive galvanometer constructed according to 

 these principles. It consists of an elongated 

 coil of fine wire suspended in the strong field 

 between the poles of a magnet. This type of 

 galvanometer is due to D'Arsonval. It is not 

 so sensitive as the galvanometer of the Kelvin 

 type, but it is scarcely affected by outside magnetic influences. 

 9 



Fig. 80. 



