eae 
BIFILAR OR HORIZONTAL ForcE MAGNETOMETER. XXX1 
39. The value of the coefficient deduced from the angle of torsion of the sus- 
pending wire is 
1847. k =a cot v = 0:00032675 x cot 69°:3’ = 0:0001251. 
The ratio of the true value of & to that determined by the angle of torsion — ea 1:08. 
40. The true values of k from 1841, obtained from the formula 1:08 x a cot v 
are given below :— 
July 114205 1841—July 237 5"1841,... . . . &=0-000128 
Aug. 4 20 1841—Sept. 7 5 1841,.... . k=0-000164 
Sept. 7 20 1841—Sept.30 5 1841, ... . . k=0:000158 
Oct. 6 20 1841—Oct. 19 28 1841, ... . . #£=0°:000141 
Oct. 19 23 1841—April27 4 1843, ... . . k=0-000135 
April 28 2 1843—Nov. 8 22 1843,... . : k=0:000130 
Noy. 10 8 1843—Dec. 31 12 1845, .... . &=0-000140 
Jan. O 12 1846, and afterwards, . ... . . k=0-000135 
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¢ 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. (See No. 43.) 
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 @ be the 
small angle through which the arms of the torsion circle are turned, 1 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 6 = 0, is ob- 
tained from the formula,* 
2 sing B cos (v+8 
N= eos (0 +5) ye ge 
a cos v cos v 
= A +n2B +t¢ 
6 is considered negative when v is diminished, n is negative when the reading is 
below the zero (170), is the temperature of the magnet minus 26°, and 9’ is the 
> p g ? q 
temperature coefficient in scale divisions. 
* Introduction, 1843, p. xxxiii. 
