426 
MR. HARRIS ON THE ELEMENTARY LAWS OF ELECTRICITY. 
this the zero of the circle can be nicely set to any given point. The whole is then 
finally fixed in the required position by two small screws at s s', fig. 1 5, passing 
through the interior ring against the edge of the plate within. 
g. Each quadrant of the graduated circle i f is divided from 0 to 90°; and there is 
an index x projecting from the moveable plate above immediately over the divisions, 
indicating the angular quantity it has been turned, and consequently the amount of 
the deflective force impressed upon the needle m n through its threads of suspension. 
This arrangement, especially represented in fig. 15, enables us to retain the brass 
circle A I t and the frame a x a! in their respective situations, without interfering 
with the required circular motions or the opening into the cage below for the threads 
of suspension (£.). 
a. The frame a x a! may vary from one foot to three feet, according to the views 
of the experimentalist ; it is slightly but firmly constructed of two light brass tubes, 
x a, x a! ; these are received upon two short vertical rods, screwed into the plate x, 
and are united by a cross piece a a! above. The extremities of this cross piece carry 
spring sockets for the two adjusting pegs a a! already mentioned. 
r. The brass plate for regulating the length and distance of the suspension threads 
(|S.) is steadied between two parallel pieces r r' by means of small screws which tie 
them together ; sufficient space being allowed for placing it accurately in the centre. 
The compressing pieces are united at each end by cross bars, and are soldered to 
short tubes which slide with friction on the vertical rods x a, x a!, as shown in fig. 16. 
v. The small stays s s', fig. 3, inserted at given distances between the suspension 
threads, are formed and applied in the following manner. A short piece of finely 
grained cork, g g', fig. 10, of the required length, about - 2 of an inch wide and T of 
an inch in thickness, is cut nearly through at each extremity, and a horizontal por- 
tion g a, g' a', removed, leaving a raised portion a! a in the centre, exactly equal 
to the given distance between the threads. The filaments of suspension being each 
passed into the eye of a fine needle, are lightly smeared with a little oil, and run 
through the cork in the points of intersection of the depressed portion g a, g 1 a', and 
the central straight line parallel to its sides. The stays will now hang with sufficient 
friction between the threads, and may be placed in the points required. 
<p. In order to facilitate any required change in the sensibility of the instrument by 
varying the distance between the threads of suspension (5.), several sets are prepared 
at different distances apart, terminating above and below in small loops, as repre- 
sented in fig. 12. The upper ends being passed into the eye of a fine needle, are 
easily run upwards through the holes in the plate r r, fig. 3, at the given distances, 
and hooked on to the stout threads a a' above, and to the hooks b V of the needle 
below. 
9. When we wish to employ this instrument as a balance of torsion (1.), the needle, 
with its index, &c., is suspended by a metallic wire in the following way : — there is a 
small shoulder at the extremity of the rod u o, figs. 3 and 4, through which a small 
