428 
MR. HARRIS ON THE ELEMENTARY LAWS OF ELECTRICITY. 
small stage at o' ; we may by either of these methods, taken separately or conjointly, 
vary the reactive power between very wide limits. In the instrument above men- 
tioned a force so small as the 2 -^ oTo dth part of a grain for each degree is readily attained, 
and it may be diminished to the s ~dth. 
12. The construction of this instrument having been completely described, a brief 
experimental explanation will suffice to make its practical application to the purposes 
of an electrometer easy and intelligible. 
Let it, for example, be required to investigate the laws of the repulsive force ex- 
erted between the insulated bodies p m, figs. 3 and 14, under the variable conditions 
of distance, intensity of charge, and the like. 
The instrument being elevated on its vertical pillars, as represented in fig. 3, is 
first levelled, and the central rod o u made, by a due adjustment of the plate £ £', fig. 16, 
to hang freely over the vertical pivot at u , at whatever angle the frame ax a may 
be turned : the disc m of the needle is now adjusted, so as just to touch the fixed 
disc p in every point, or nearly so, the forked lever w being employed to retain 
the index v v' at zero. The fixed indexes I and £' y are then also set at zero of their 
circles A £ i and v h v', the forked lever iv arresting, without undue pressure, the 
discs m and p in contact, when the index v v' is at zero. We now proceed to electrify 
the discs by means of any insulated charged body introduced within the cage, either 
through the circular hole H or under one of the glass sides, which can be raised for 
the purpose ; this done, we turn the circle A £ I through any given number of degrees 
in a direction contrary to that in which the needle would be repelled, the forked 
lever w, figs. 3 and 14, being gently eased away until the repulsive force becomes ex- 
actly balanced. In this case, as is evident, the distance between the repelling sur- 
faces is measured by the number of degrees of the graduated circle v h 1 v', intercepted 
between the vertical and horizontal indexes t ' y , v v' , and the force itself by the 
number of degrees of the same circle contained between the final position of the 
index v v' and zero. Suppose, for example, the repelling discs were actually cir- 
cumstanced as in fig. 14, then their distance apart is expressed by the ar c y v ! , and 
the force, by the arc z v 1 , the point z being zero of the card. If the point y coin- 
cided with z, then both the distance and force would be expressed by the arc z v'.. 
If the index t' y be not employed, we estimate the distance of the bodies by means 
of the angular quantity indicated on the brass circle A £ I, which must be in 
this case added to the angular deflection of the needle from zero. Let it now be 
required to examine the repulsive force at any other distance less than the former. 
We have only to move the circle A £ I in the same direction as that in which the 
needle is repelled, and we immediately change the relation between the arcs y v' 
and z v 1 , the receding of the needle being at the same time gently checked by the 
fork of the lever w. We thus eventually obtain a new distance and force, measured 
in terms of the same graduated circle. 
In this kind of inquiry we may, as it is evident, obtain deflections ol the needle 
