400 



USE OF THE GALVANOMETER. 



sine the corresponding ordinate p c. Now turn the coil to the left at 

 an angle w 31. A deflection 31 p' is produced; therefore we have for 

 the point c' of the curve, the abscissa w 31 -\~ 31 p' = w »', and the 

 ordinate p' c' == sin 31 p'. Proceed in this way until w 31 z=z 90°, 

 the deflection will then be 0°; and hence the abscissa =. 90°, and the 

 ordinate = 0°. Turn the coil in the same manner to the right of the 

 meridian, and mark the corresponding angles of deflection until the 

 angle between the magnetic needle and the coil of wire is reduced to 

 0°. This completes the observations. The sine of the angle of de- 

 flection which corresponds to the last position of the coil, represents 

 the first ordinate Ma of the curve. 



In general terms the process is as follows: Place the coil at an angle, 

 w, with the magnetic meridian; the magnetic needle will then make 

 with the coil an angle, ??, and with the magnetic meridian an angle, 

 a = n -f- m, or n — m, according as m lies upon the same side of the 

 magnetic meridian as n, or the contrary. If the several values of 

 m and n be now distinguished by accents above or below, according 

 as they belong to the former or latter position of m, we shall have 

 the following results: 



This determines the form of the curve which represents the effect 

 of the coil of wire on the magnetic needle for a current of a certain 

 strength, by means of the co-ordinates of the magnetic curve 31 N 

 whose greatest ordinate, N M, is arbitrarily assumed. 



The form of the curve must next be determined for currents of any 

 other strength. If it were necessary to repeat the process just de- 

 scribed for every possible strength of current, the process would of 

 course be entirely useless. But such a repetition is not necessary; 

 the form of the curve determined for one intensity, enables us to de- 

 termine its form for every possible intensity. 



It is evident that when the force of the current varies without any 

 change in the relative distance and inclination of the coil and the 

 needle, the influence of the coil upon the needle must vary in direct 



