280 W. WEBER ON THE BIFILAR MAGNETOMETER. 



The equality of these opposite moments of rotation, or the 

 equiUbrium of the instrument in this position, is the result. 

 Whether the true position of equiUbrium coincide with the cal- 

 culated one or not, is proved immediately by an observation 

 of the scale, which ought to be the same as before. For the 

 miri'or has been turned (together with the whole apparatus) 

 the angle z in the direction of the daily motion of the sun ; but 

 having been turned by its independent motion the same angle z 

 in the opposite direction, it consequently retains its first posi- 

 tion, and the point of the scale is unchanged. 



14. If, however, the observation shows an alteration of the 

 scale, it follows that the supposition in the first experiment — 

 of the magnetic axis of the bar being in the magnetic me- 

 ridian — was not accurately fulfilled. The amount of the enror 

 can be calculated, and the experiments repeated. This calcula- 

 tion wUl be still more accurate and certain, if a corresponding 

 experiment has been pre\aously made, proceeding precisely as 

 described in (13.), only making all the rotations in the contrary 

 direction. 



15. When the required coincidence has been obtained, the 

 magnetometer remains in its transverse position. Its time 

 of vibration is then, according to a simple theorem, the geo- 

 metrical mean between the times of vibration / and t, and 

 the observations of changes of intensity can be arranged like 

 those of the changes of declination. The changes of inten- 

 sity are obtained in divisions of the scale. If we desire to con- 

 vert them into fractions of the entire intensity, these are ob- 

 tained by multiplying the arc value of the scalar divisions (ex- 

 pressed in parts of the radius) by 



cot g = ^2_^9 ? 



for the value in arc of the parts of the scale, expressed in parts 

 of the radius, gives immediately the changes of intensity in parts 

 of the directive force, which, under the prescribed conditions, is 

 S cos z. If we divide this directive force by the whole intensity, 

 i. e., by S sin z, we obtain by multiplying the value of the arc 

 by the quotient, cot z — the changes of intensity in fractions oi 

 the whole intensity. 



