CONSTRUCTION OF A NEW ELECTRICAL BALANCE. 
421 
The pulley p, fig 1 . 13, employed in these experiments was extremely delicate. The 
small scale f, in which the weights were placed, weighed the *1 of a grain, and Avas 
suspended by a filament of the thread of a silkworm ; that part of it passing over the 
pulley being particularly slender and flexible. The •01 of a grain was by this 
means rendered very sensible on the index. The approximations in this Table are 
sufficiently close to show that the reactive force of the threads is as the angle of de- 
flection (Exp. A.). 
8. Upon these data the electrometer represented in fig. 3 has been contrived, which 
the following description will, I hope, render sufficiently intelligible. 
a. Fig. 3, a " d is a cubical box or cage of about 14 inches square and 10 inches 
high ; its vertical faces consist of large panes of glass loosely inserted in the grooved 
edges of light mahogany pillars about *2 of an inch in diameter ; li c, d l represent 
two of the columns, and d h one of the glass faces. The upper edges of these panes 
project above, so as to be easily removed when requisite. 
The base of the cage, d k , consists of a clamped square of well-seasoned maho- 
gany ; the boundary a " h If of the upper part or roof is a stout framework of the 
same wood, through which the glass faces pass : the frame is firmly connected to 
the base by the intervention of the wood columns ; each column is connected to the 
framework above by short brass shoulders fixed in their upper ends, and screwed 
into small plates of brass at the angular parts of the frame, as at the points h l ; the 
horizontal grooves for the passage of the glass sides are continued through these 
plates in common with the woodwork of the frame ; the base k d is secured beneath 
by similar shoulders fixed in the lower ends of the columns : these last pass freely 
through the base at the lower angles k c d into a flattened spherical nut, by which the 
whole is hove tight ; each nut terminates in a cylindrical brass leg about an inch and 
a half in length and a quarter of an inch thick, as represented at k 1 c' d'. When the 
nuts are hove tight, the skeleton of the cage is very firm and complete : the cylin- 
drical legs keep the instrument raised for about two inches above the table ; they 
also serve to steady it on four pillars, as at k' c' d', each five inches in length, and 
upon which the whole instrument is occasionally elevated for the purposes of experi- 
ment. The pillars last mentioned are united below to a clamped square of mahogany 
supported on levelling-screws, and have holes drilled into their upper ends for the 
reception of the cylindrical feet just mentioned. 
(3. An insulating needle mn, figs. 3 and 14, ten inches in length, is suspended within 
the cage by two vertical filaments of silk, a b, a! V : it is connected with an index 
v v', by means of a vertical rod o u, fixed to its centre. This index is about nine inches 
long, is set at right angles to the direction of the needle m n, and points out on a gra- 
duated circle, v h' v', the angle of deflection from the point of rest. The graduated 
circle v h ' v', figs. 3 and 14, upon which the angular deflections of the needle are 
thus shown, is about nine inches in diameter : it is supported on four small pillars 
about an inch high, its centre being coincident with the central point at u. 
