AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
135 
The short needle was as usual provided with an aluminium index, and by means of 
this, reading’s on a graduated circle traversed by the ends of the index could be 
depended upon to within y^-tlis of 1°. The needle was suspended by a platinum wire 
ToVoth of an inch in diameter and 12 inches in length, the upper end of the wire being 
secured to a torsion-head provided with a vernier, which moved over a graduated 
circle; by means of the vernier a torsion amounting to six minutes could be measured. 
The fine platinum wire hung in the axis of a brass tube which was provided near its 
lower extremity with a small glass window, so that the illuminated image of a 
vertical wire focussed by a lens could be received on a small mirror and reflected back 
on a scale placed at a distance of 1000 of its own divisions from the mirror. The 
mirror was attached to a piece of stout brass wire which was connected at its lower 
extremity with a needle, and at its upper extremity was clamped to the lower end of 
the platinum ware. 
The effect of the torsion on the deflection was determined by a series of careful 
observations made by turning the torsion-head through different angles, first in one 
direction and then in the opposite, and noting the corresponding deflection of the 
needle. Thus, in one experiment, the torsion-head having been turned through 100 
degrees from left to right, there was produced a deflection of 57° 6', and when the 
head was turned from right to left, the deflection was 56° 46' on the opposite side of 
the zero point; the mean of the two deflections of the needle is 57°, and therefore the 
force of 1° of torsion of the suspension wire would = 
sine 57° x H 
(100-57) 
= ’0195 XH; where H 
is the horizontal force of the earths magnetic action on the needle. 
In order to be able to determine in absolute units the value of the deflections of the 
needle, a new Daniell’s cell of large size was charged with a saturated solution of 
sulphate of copper and a semi-saturated solution of sulphate of zinc. This cell was 
allowed to rest for two hours after having been charged, and was then short-circuited 
for half an hour. After this time the cell was connected up with the thick copper 
wires of the galvanometer, and the deflection of the needle produced, observed by 
means of the mirror and scale, when external resistances of 0 ohm, 10 ohms and 
20 ohms were successively introduced. The deflections were reversed in each case by 
reversing the battery-current and the mean values of the two deflections in the 
| different directions were for 0 ohm, 324 divisions of the scale; for 10 ohms, 110 
divisions; and for 20 ohms, 30 divisions; these deflections corresponding to 8° 59', 
3° 8', and 0° 52' respectively. By comparing the deflection with 0 ohm and 10 ohms 
in circuit, the internal resistance of the battery, together with the resistance of the 
connecting wires, was found to be 1'055 ohm, and by comparing the deflections with 
10 ohms and 20 ohms in circuit, this same resistance was calculated to be 1’058 ohm ; 
therefore, 1’057 ohm was assumed to be the true resistance. Again, taking the 
electromotive force of the Daniell thus charged to be 1*12 volt, or l’12xl0 8 
electromagnetic units, and the resistance of 1 ohm to be 10° of these units, the formula 
