Encyclopaedia Britannica > Volume 8, DIA-England

ELECTRICITY.
658
Electrical cal position, at any required angle, by means of a joint at
Apparatus, m ; and all the insulating portions are carefully varnished
v—'V'w/ with a solution of shell-lac in alcohol.
Mr Snow Harris's Electrometer.
Mr Snow The object of this electrometer, an account of which
Harris’s has been kindly communicated to us by its inventor, is to
electrome- measure directly the attractive force of an electrified bo-
Plate dy, in terms of a known standard of weight, estimated in
CCXYI. degrees on a graduated arch, xy, fig. 13. An insulated con-
Fig. 13. ductor,/, is fixed on a varnished rod of glass, fg^ resting by
the intervention of a wooden ball on the extremity of a
micrometer screw, s, by the aid of which the conductor
may be raised or depressed through given intervals to
within the hundredth of an inch of any required point. A
moveable and similar conductor, m, made of light wood,
hollowed and gilded, is suspended immediately over the
former from the periphery of a small brass wheel W, fig. 13
and 14, by means of a fine silver thread attached near its
vertical arm, and passing from thence over its grooved cir¬
cumference, as shown. This conductor, m, is counterpois¬
ed by a short cylinder of wood,/>», fig. 13, 14, suspended
in a similar manner from the opposite side of the wheel by
means of a silk thread, and resting partly in water con¬
tained in the glass vessel N, fig. 13.
The extremities of the axis of the wheel W, fig. 13, 14,
are turned to extremely fine pivots, and rest on two large
friction wheels, after the manner represented in the figures,1
by which great freedom of motion is obtained.
There is a fine index of light straw, Wc, attached to the
extremity of a small steel needle, inserted diametrically
through the circumference, which indicates on the gra¬
duated arc xy the force exerted between the conductors
m,f. The weight of this index is accurately poised by a
small globule of brass, n, fig. 14, moveable on a screw,
cut in the opposite arm of the steel needle carrying the
index.
The centre of the wheel W is accurately placed in the
centre of the arc xy, which, with its radii of support, is
made of varnished wood; the graduated scale being of card,
board, or ivory. This arc is the sixth part of a circle, di¬
vided into 120 equal parts, 60 in the direction cx, and 60
in the direction cy ; the centre c being marked zero.
Fig. 14. Fig. 14 represents the wheel W, with the suspended
conductor and counterpoise, the index and its balance-
weight, together with the lines of suspension, passing free¬
ly over the circumference, and fixed at the point ii.
The various wheels above mentioned, with the gradu¬
ated arc, are sustained on a projecting metallic rod, pass¬
ing through a glass column B. The column is secured by
means of the rod to a sort of double stand, M, fig. 14, sup¬
ported on three levelling screws. The interval between
the plates of this stand contains the glass vessel N and
the micrometer screw s. The upper plate has a circular
hole, p, through which the cylindrical counterpoise passes
into the water n. The levelling screws serve to regulate
the position of the counterpoise through the hole ; so that
when it hangs in it centrally, the whole is accurately ad¬
justed.
The gravity of the suspended conductor m being in the
above arrangement opposed by that of the counterpoise,
it may be so far considered as existing in free space de¬
void of weight, and will therefore become very readily Electrica
moved by any new force applied to it. It may conse-Apparatus
quently be caused to approach to or recede from the fixed
conductor f, by the operation of forces acting in either of
these directions; the motion will, however, be speedily
arrested by the cylindrical counterpoise n, which becom¬
ing either further immersed in, or otherwise raised in the
water, furnishes, in the greater or less quantity of water
displaced, a measure of the force. In this way the force
may be estimated either in degrees or grains of actual
weight; since the number of grains requisite to add to
either side, in order to advance the index in either direc¬
tion a given number of divisions, may be immediately
found by experiment; and which, as the sections of the
cylinder are all similar, will be found to increase or de¬
crease with the degrees of the arc. Thus, if one grain ad¬
vances the index in either direction five degrees, then two
grains will advance it ten degrees, and so on.2
In the application of this instrument to electrical in¬
quiries, the force to be measured is first communicated to
the fixed conductor f, a free communication being estab¬
lished between the suspended conductor m and the ground,
or otherwise with the negative side of the jar or battery,
should the attractive force be derived from this species of
accumulation; this is readily effected through the brass
work of the apparatus, in connection with the rod passing
through the interior of the glass column B.
For the repulsive force we connect the conductor f as
before, and suspend m by a silk thread; in which case it
will, after being electrified similarly to f, recede from it;
but this method of experiment is evidently more compli¬
cated than the former, and liable to fallacy. The dis¬
tance between the conductors m, f corresponding to a
given force, is easily ascertained by means of the degrees
indicated on the arc x y. In the instrument above de¬
scribed, each degree corresponds to a variation of distance
between the conductors equal to the ’Ol of an inch. If,
therefore, at the commencement of any given experiment,
we first bring the nearest points of the conductors m,f in
contact, the index being in zero, and then depress the in¬
ferior conductory a given distance, known by means of the
micrometer screw s, then all subsequent distances may
be readily determined between these points.
It is now only requisite to observe, that the interior of
the cylindrical counterpoise is hollow, in order to weigh
it accurately, and cause it to hang vertically in the water;
and there is a small hemispherical cup, p, fixed on its stem
for the reception of small adjusting weights, by which the
position of the index at 0 of the scale is regulated with
great nicety. With respect to the form of the conductors
m,f, they are generally plain circular areas, backed by
small cones, and are of about two inches diameter. Con¬
ductors of other forms, however, such as spheres and cy.-
linders, may be occasionally used when the object is to
experiment more particularly on bodies of peculiar forms.
Experiments with this instrument are remarkably clear,
considering the subtile character of the principle we have
to investigate. Thus, when the insulations are perfect,
and the atmosphere dry, the index immediately exhibits
the amount of the attractive force, and remains stationary
for a much longer time than is required to note the
result.
1 Mr S. Harris resorted to this method of employing friction rollers, as being more efficient than that in which the axis is allowed
to rest in the angle formed between the peripheries of four smaller wheels. In this case it rolls fairly on a large circumference, and is
prevented from passing off it on either side by the check-wheels, either of which, when acted on, opposes little or no resistance to
motion.
2 The counterpoise should be free from grease or varnish of every sort, and should, previously to being used, be keptdmmersed in
water: the insulation of the conductor^ also should be made extremely dry, and occasionally warmed by a stick of burning charcoal.