AND ITS APPLICATION TO TORSION BALANCES. 
219 
repeat the experiment with another current ; and the quantities of electricity 
circulating round the wires will be directly proportional to the number of 
degrees through which the thread has been twisted. By this contrivance it is 
obvious that the currents always act with the same mechanical advantage on 
the needle ; and consequently their deflecting forces, which are counter- 
balanced by the elastic force of torsion, must be directly as the number of 
degrees through which the thread has been twisted. In the common galvano- 
meter, the deflecting force acts with diminished mechanical advantage as the 
needle deviates from the coil. When it has been deflected nearly 90 degrees 
from the original position, an additional power will produce scarcely any effect, 
and consequently the instrument ceases to give indication of a more energetic 
current. 
This instrument appears to me well adapted to many interesting investiga- 
tions connected with voltaic electricity; but these coujd not properly be 
introduced in this paper, and may therefore form the subject of another com- 
munication. 
7. We shall now describe another application of the elasticity of glass 
threads for ascertaining the weight of very minute portions of matter. The 
chemist in some of his most refined analyses has frequently to ascertain the 
weights of minute portions of matter, which by the ordinary process is a work 
of time and labour. A balance of the most delicate and perfect kind is also 
required, which, from its expense, frequently prevents the young chemist from 
prosecuting his experimental researches. 
The balance which I am now to propose may be made at a trifling expense, 
and it will give the weights of small quantities of matter to a degree of 
accuracy seldom attained by the most perfect hydrostatic balances. 
Prepare a small wooden beam, very light and about a foot or fifteen inches 
long, into the centre of which fix a steel blade having a smooth edge, like the 
blade of a fine penknife. To one extremity of the blade and in the line of its 
edge a fine thread of glass is to be securely fixed, whilst the other end of the 
thread is to be secured with sealing-wax in the centre of a small cylindrical 
key, passing through the centre of a vertical circle divided into degrees, or any 
convenient number of equal parts. To the other end of the fulcrum, and also 
in the line of the edge, a fine thread formed of a few fibres of untwisted silk is 
2 f 2 
