Bifilar or Horizontal Force Magnetometer. xxxiii 



In order to reduce the variations of the horizontal component given, pages 229 

 to 238, in the volume for 1843, to their true values in parts of the whole horizontal 

 component, they must be multiplied by the factor 1-316. 



41. The bifilar magnet was adjusted November 10 d 1843, when the angle v was 

 found = 68° 18', the bifilar scale reading 173. The angle v remained unaltered, 

 excepting for short periods during disturbances, till January 1, 1846. 



42. During considerable disturbances the collimator scale, which contains too 

 small an angle, goes out of the field of the reading telescope, it was found necessary 

 in these cases to turn the arms of the torsion circle until it reappeared ; afterwards 

 the arms of the torsion circle were turned to their original position : experiments 

 were made in the end of 1842, during periods of slight change, which shewed, after 

 turning the arms of the torsion circle a few degrees in either direction, that on re- 

 curring to the original value of v, the scale readings were unaltered. If ft be the 

 small angle through which the arms of the torsion circle are turned, n be the scale 

 reading minus 170 (the adopted scale zero), then N, the number of scale divisions 

 from the zero (corrected for temperature) for the same force when (3 = 0, is ob- 

 tained from the formula,* 



o • 



^ Sill — . r\ . i r\\ 



2 / , 0\ cos (v + fi) 



N = -cos(»+£ ) +n i ^ + tg' 



a cos v \ 2 / cos v 



= A + nB +t <{ 



/3 is considered negative when v is diminished, n is negative when the reading is 

 below the zero (170), t is the temperature of the magnet minus 26°, and q is the 

 temperature coefficient in scale divisions. 



43. The arms of the torsion circle were turned during the disturbance, Novem- 

 ber 16, 1844,f as follows : 



Sc. Div. 



Nov. 16 d 6 h 20 m — 37 m j8 = + 1° 8' A = + 59-0 B = 0-950 

 16i 6 h 38™— 45™ (3= + 2° 43' A = + 136-4 B = 0-880 

 16 d 6 h 46m__i7d 3 h j3= +i° 13--5 A = + 63-7 B = 0-946 



17 d 3 h . The arms of the torsion circle were turned to their original position. 



44. The mean time of one vibration of the bifilar magnet, is between 26 s and 27 s : 

 the natural arc of vibration is generally very small, and when considerable, the time 

 of vibration was found less than from large artificial vibrations, j 25 s has been used 

 in the observations for 1844. 



* Introduction, 1843, p. xxxiii. 



f In 1847, it was found more convenient during disturbances, to bring the scale of the bifilar 

 magnet into the field of the reading telescope, by means of a small deflecting magnet placed on a 

 beam of wood at known distances from the centre of the bifilar magnet. The effect of the deflecting 

 magnet was afterwards determined with the aid of a second deflecting bar. 



J Introduction, 1841-2, pp. xxviii., xxix. 



MAG. AND MET. OBS. 1844. % 



