chap. x. ] SINE GA L VA NOME TER. 9 7 



magnetic needle, whose length is small in comparison 

 with the radius of the circle. The needle is surrounded 

 by a horizontal circle marked in degrees ; and it 

 points to zero when the galvanometer is in proper 

 position. When a current traverses the wire the 

 needle is deflected. In its new position it is acted 

 on by the force of the earth's magnetism tending 

 to bring it back to zero, and by the repulsive action 

 of the current, which tends to set it at right angles. 

 It accordingly takes up a position between the tw T o, 

 and this position is such that the tangent of the angle 

 of deflection is proportional to the intensity of the 

 current. When the amount of deflection caused by 

 the current is great the proportion is not accurately 

 maintained. To meet this the sine galvanometer 

 is constructed, in which the vertical circle is movable 

 round a vertical axis ; in this form the sine of the 

 angle of deflection is proportional to the intensity of the 

 current. 



Now the great objection to the use of either of 

 these forms of galvanometer is that they always 

 require a comparatively strong current to influence 

 the magnet. As already indicated, multiplication 

 of the number of turns of the wire in the circle 

 surrounding the magnet will increase the effect upon 

 the needle, and the greater the multiplication, there- 

 fore, the weaker may the current become without 

 losing influence on the needle. But this multiplication 

 has its limits, since every turn interposes resistance, 

 and consequently weakens the current. The great 

 cause of the non-sensitiveness of the needle is, how- 

 ever, the directive action of the earth, since as soon 

 as the needle moves out of the magnetic meridian this 

 force comes into play, tending to bring the needle back 

 again. It was, therefore, a great step in the production 

 of sensitive galvanometers when Nobili, in 1827, de- 

 vised a method for diminishing as much as possible 



H 7 



