CONSTRUCTION OF A NEW ELECTRICAL BALANCE. 
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hole is drilled, passing obliquely from the centre out at the side just below. The wire 
is passed through this hole, and its end finally secured by a small brass spherule s, 
fig. 4, accurately fitted to the end of the shoulder. When the brass spherule is gently 
forced into its situation, the wire is compressed, and becomes firmly united to the 
needle. Being thus secured below, it is continued through a fine central hole in the 
moveable plate rr', fig. 3, and finally attached to an adjusting cylindrical peg at a'. 
It is completely stopped at any given length by a small conical screw and nut n fixed 
in the plate, fig. 16, and through which the wire passes; by tightening the nut the 
wire is compressed, and cannot therefore twist beyond a given point. 
In order to balance the needle accurately, should it be required, the two sliders b h', 
fig. 4, are turned so as to bring the suspension hooks underneath ; this enables us to 
append to the hooks two small weights w w', and place them at such respective di- 
stances from the centre as will accurately balance the needle m n in a horizontal 
position*. 
The instrument thus prepared becomes, by means of the micrometer at x, a very 
complete torsion balance, possessing many important advantages ; amongst these is 
the means of increasing or diminishing its susceptibility by changing the length of 
the wire, Coulombe having shown that the reactive force of a wire is in a simple 
inverse ratio of its length. 
10. The reactive force obtained in the way just shown, by means of two parallel 
filaments of silk, is more perfect, in a great variety of instances, than the elastic 
force of torsion, and is besides very manageable. The deflection of the threads from 
the vertical may at all times be extremely little ; and the angular deviation of the 
needle seldom greatly exceeds 90°, although by increasing the number of stays s' s", &c., 
fig. 12, it may with safety be carried through the whole circle (7.) ; for many refined 
inquiries, however, the angle of deflection need not exceed 30°. We must, however, 
remark, that in this, as in the case of the torsion balance, an error may arise in large 
arcs by taking the arc itself as the measure of the distance between the opposed 
bodies p n, fig. 3, and the length of the lever at the extremity of which the force 
acts, as equal to radius or half the length of the needle : these errors, however, in 
the instrument we have been describing, sufficiently balance each other, or very 
nearly so ; one of the factors of the moment of the force being the cosine of half the 
angle of deflection, and consequently less than radius, or the lever upon which the 
repulsion operates, whilst the arc taken to measure the distance is always greater 
than its chord, or the real distance. 
11. It is easy to perceive that the reactive force imparted to the threads of sus- 
pension may be varied in any proportion, either by changing the position of the 
sliding bar rr', by which their length is altered, or otherwise by varying their distance 
apart ( <p .) ; or finally, by the due adjustment of small circular weights placed on the 
* A separate needle may be employed for this purpose, if desired. 
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