SINE GALVANOMETER. 209 



the absolute sensitiveness is inversely as the number of oscillations, 

 and the relative sensitiveness, which is zero for N = , or for a very 

 feeble current, increases with N. 



832. SINE GALVANOMETER. In the sine galvanometer devised 

 by Pouillet,* the relative position of the coil and the needle is kept 

 unchanged, and the system is arranged so that the action of the 

 earth and that of the current are equilibrium. The apparatus 

 consists of a coil movable about a vertical axis on a graduated 

 scale ; the needle plays on a pivot in the centre of the coil, and by 

 means of a fiducial mark we can always ascertain whether, in the 

 various experiments, it is brought into the same position in respect 

 of the coil. With this object, a small ivory plate is fixed on the 

 needle at right angles to its direction, and on which is marked a line 

 moving over a divided scale. 



The needle being first of all adjusted to the mark, the current is 

 passed, and the frame of the coil is turned until it comes back to the 

 mark. If <5 is the angle through which the coil is turned, /3 the 

 constant angle of the needle with the axis of the coil, we have 



IMGsin/3 = MH sin 8, 



The current is proportional to the sine of the displacement of 

 the needle, which justifies the name given to the instrument. The 

 method does not allow of the measurement of currents whose 



TT 



intensity is greater than : , but this limiting value may be 



increased as much as is desired by varying the angle /?, which 

 determines the position of the mark. 



The experiment is somewhat long, but apparatus based on this 

 method of the sine have the advantage of presenting a systematic 

 graduation, independent of the conditions of construction, and may 

 be used with advantage in graduating ordinary galvanometers. 



We have again 



sn 



H cosS' 



* POUILLET. Comptes rendzts, Vol. iv., p. 267. 1837. 

 VOL, II. P 



