Xvi INTRODUCTION. 
16. The Suspension Thread and its Torsion Force. 
The errors due to the torsion of the suspension thread are produced in two 
ways. First, by the magnet moving out of the plane of detorsion ; secondly, by 
the variation of this plane, due generally to the varying humidity of the atmosphere.* 
The greatest change of declination from the mean has been within 2°; the greatest 
correction for the torsion from this change would be within 8’. The approximation 
of the line of detorsion to the magnetic meridian is probably seldom within 2” ; it is 
found on trial to have deviated from 5° to 40° from the magnetic meridian, and is so 
variable as to swallow up completely the changes from varying declinations. 
If the magnet be deflected w by turning the arms of the torsion circle w’, the 
torsion is w’—w, the ratio 
ul coefficient of torsion force H 
nw —w magnetic force x moment of free magnetism rel 
is the quantity by which the deviations of the magnet from the plane of detorsion 
should be multiplied, to obtain the decrements due to torsion force. If m be the 
observed deviation, (1 +) n = the true deviation. 
. H : 
The observations for the values of F are given below ; no use has been made 
of them for this correction, from the reason given above. 
TABLE 6.—Values of = 
Period to which the factor 
Date of the H ‘ 
Observation. Le pense 
1841. 
July 8 . 0-00086 July 8s—20 
July 17 . 0-00086 
July 30 . 0-00100 July 26—Dec. 4 
Dec. 28 0-00091 Dec. 22—Jan. 19 
1842. 
March 3 0-00099 Jan. 21—May 24 
June 6 : 0-00088 May 24—June 6 
June 18 . 0-00075 June 7—June 26 
June 21 . 0-00080 June 7—June 26 
June 27 : 0-00077 June 27—July 18 
Aug. 22 0-00162 July 20—Dec. 31 
17. The second and most important error due to the torsion force, is from the 
varying plane of detorsion, Unless when the period and extent of change is known, 
* A thick cotton cover was put over the whole Declinometer in January 1844, which seems to 
remedy this very much, 
’ 
