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XX. Inquiries concerning the Elementary Laws of Electricity . Second Series. 
By W. Snow Harris, F.R.S. 8$c. 
Received May 11, — Read June 16, 1836. 
l. Having frequently had occasion to employ the balance of torsion invented by 
Coulombe, on the indications of which so many important deductions in electricity 
rest, I have endeavoured at various times to free it from certain defects of a mecha- 
nical kind, and render it more completely available to the purposes of an electro- 
meter. Some of the difficulties experienced by the experimentalist in the use of this 
valuable instrument had already engaged the attention of our talented countryman 
the late Professor Robinson of Edinburgh, who proposed an ingenious method of 
checking the swinging of the needle, generally set in motion whenever we turn the 
micrometer or electrify the insulated balls. In the course of these inquiries I was 
led to the construction of a new species of balance ; it may be termed, from the pe- 
culiar mechanical principle on which it depends, a bifile balance. The reactive force 
of this instrument is not derived from any principle of elasticity, as in the balance of 
torsion, but is altogether dependent on gravity; it seems generally available in ex- 
perimental physics, is extremely well adapted to the measurement of small forces of 
repulsion, and to researches in electricity and magnetism, and is easily converted 
into a common torsion balance when required, free from the difficulties before alluded 
to. A description of this new instrument, together with an account of some further 
researches into the elementary laws of electricity, may, I hope, be acceptable to the 
Royal Society. 
2. If a needle m n, fig. 1, (Plate XXVIII.) be suspended by two equal and similar 
vertical filaments of silk without torsion, a h, a ' h\ placed parallel to each other at equal 
distances from the centre c c, and fixed at the points a a\ it is evident that its posi- 
tion of rest will be horizontal, and in the vertical plane passing through the two 
threads. Whenever, therefore, we turn the needle from this position about the ima- 
ginary axis c c', the lines of suspension will become deflected from the vertical, so 
that the distance c c will be shortened. We have hence a reactive force derived from 
the weight of the needle, which becomes imparted, as it were, to the threads of sus- 
pension ; since the centre of gravity of the mass will again tend to rest in its previous 
position, and will be in a similar condition to that of a body falling down a very 
small circular arc. If therefore the needle be freely abandoned to this reactive force, 
a vibratory motion will arise, by observing which we may determine by the formulae 
for oscillating bodies the nature of the reactive force producing the oscillations. 
mdcccxxxvi. 3 H 
