172 REPORT—1863. 
sured by means of a magnetometer from Kew by the ordinary method. The 
correction found was 
C=-+tan p 
='00780. 
The small magnet generates induction currents in the coil which react on 
the magnet, and tend to turn it in the direction in which the coil revolves. 
If there were no horizontal magnetic force due to the earth, the coil would 
drag the magnet round after it. In the actual case it makes the deviation 
greater than it should be by :0078. 
D. Correction for torsion of the fibre 
ey taal 
= —:00132. 
This correction depends on the relation between the stiffness of the fibre 
and the directive force of the suspended magnet. The fibre was a single 
fibre of silk 7 feet long; the magnet was a steel sphere ,5, inch diameter, 
and not magnetized to saturation. The correction for torsion was therefore 
much larger than if a stronger magnet had been used. 
KE. Correction for position of suspended magnet. 
Let the centre of the magnet be at a distance £ above or below the centre 
of the coil, » north or south of the axis of motion, and & east or west of the 
axis, then there will be a correction, 
3 ra)sinta {43 ®) 
7. (1—4 cot? a) sin! a {45-435 
Here a=156-6 millimetres, and the place of the magnet was so adjusted that 
it could not vary one millimetre in any direction without the error being 
observed. Hence this correction is negligible. 
F. Correction for irregularity in the magnetic field due to iron or magnets 
near the instrument. 
Let ¢ be the time of oscillation of a magnet at the centre of the coil ¢,, and 
t, at distances z above and below that point, then 
eat eceaM 
Sil) So raee ene 
E=-+ 
F=+4 
This correction may also be neglected. 
G. Correction of scale-reading. The quantity observed is tan 2g, the quan- 
tity to be found is tang. The correction to the value of R is 
ies 
ar D 
H. Correction for electromagnetic capacity of the coil. 
Let L be the value of the electromagnetic capacity, the correction is 
_182L(2L_, 
4D°GK\GK J 
In the actual coil L was found by calculation =397750 metres, and by a 
rough experiment=398500 metres, 
Now GK=560245 metres. 
The correction is therefore—F = (0°596234)=H. 
This correction is of the same form with G, and may be taken along with it. 
