Encyclopaedia Britannica > Volume 8, DIA-England

654
ELECTRICITY.
Schweig-
ger’s mul¬
tiplier.
Electrical A will acquire a greater quantity of positive electricity from
Apparatus, the presence of the uninsulated plate B, which will be
negatively electrified. When KL comes into the position
KX, so that B touches C by the wire ?nO, its negative
electricity will pass almost wholly to C, owing to its proxi¬
mity to D, which communicates with the ground. By a
number of successive oscillations of the lever between the
two positions KL and KX, this operation may be repeat¬
ed till an accumulated charge of negative electricity has
been fixed upon C. The plate D must now be drawn
away from C by means of the slider FP, and if pith balls
are presented to C they will diverge with negative elec-
tricity.
In our chapter on the chemical agencies of electricity,
we have already described Schweiggers multiplier or gal¬
vanometer, which was used by M. Colladon in his experi¬
ments on the chemical action of ordinary electricity ; and
also the multiplier of Dr Faraday with a double helix,
which he employed in his researches on the identity of
the electricity of the machine with that of the pile. Va¬
rious improvements have been made on the multiplier by
M. Nobili, Professor Oersted, and others; but we must
reserve our account of them for the articles G alvanism
and Magneto-Electricity.
CHAP. III.—DESCRIPTION OF INSTRUMENTS FOR INDICAT¬
ING THE PRESENCE OF ELECTRICITY, AND MEASURING
ITS QUANTITY.
Indicators Instruments which are intended merely to indicate the
and mea- presence of electricity are called electroscopes, while those
surers ot whjch are intended for measuring the quantity of electri-
t ec nei y. are ca]|e(j electrometers. The earliest electrometer
which seems to have been employed was a pair of silk
threads, which indicated the presence of small quantities
of electricity by their divergence; and the Abbe Nollet
even attempted to measure the quantity communicated to
them, by determining the inclination of the two threads,
from their shadow on a board. Mr Waitz improved the
instrument by suspending small weights to the threads,
and Mr Canton perfected it by substituting the finest linen
threads for the silk ones, and by suspending from them a
pair of small balls turned out of the dry pith of the elder.
Description of Cavallo’s Electroscope.
Cavallo’s M. Cavallo made this little instrument portable by fit-
electro- ting it up as in fig. 1, where it is shown in a state of ac-
®c°Pe’ tion at B. When it is unloosed, the end B carrying the
CCXVI ka^s's screwecl and th6 balls are put into the glass
l ' tube at A, which serves for a handle. This glass case is
three inches long and three tenths of an inch wide, and
half of it is coated with sealing-wax. A cork tapering at
both ends is made to fit the mouth of the tube, and to one
end of the cork are fixed two linen threads carrying two
small cones of elder pith. The case of the electrometer at
C incloses at one end a piece of amber for giving negative
electricity, and at the other end a piece of ivory insulated
upon a piece of amber for giving positive electricity, to the
balls, when rubbed with a piece of woollen. All these in¬
struments may be greatly improved by substituting for the
pith of elder the pith of the sola, a tree which grows in
the East Indies.
Description of Bennet's Gold-Leaf Electrometer.
Bennet’s One of the most useful electrometers is that which was
gold-leaf invented by Mr Bennet, and called the gold-leaf electro-
electrome- meter. This instrument, which is shown in fig. 2, and a
section of it in fig. 3, consists of a cylinder, ABED, with
ter.
*lg‘ 2’ 3' a broad brass cap, AB. In a hole, a, in the centre of the
cap, is fixed a wedge of wood, on each side of which is Electrica
fastened by a little varnish a smooth-edged strip of goldApparatu
leaf, shown at m and n, about two inches long and a quar-
ter of an inch broad. Two pieces of tinfoil, b, c, are past¬
ed opposite each other, and within the cylinder, so as to
rise a little higher than the ends of the gold leaves, and
the lower ends of these pieces of foil are in contact with
the brass stand DEF which sustains the instrument. The
inside of the cap AB, and the upper part of the glass cylin¬
der, are sometimes coated with wax. A pointed wire, C, is
used to collect the electricity of the atmosphere. In using
this instrument, the cap AB is turned round till the surfaces
of the gold leaves are parallel to those of the pieces of tinfoil.
When no electricity is present the two gold leaves hang
in contact in the axis of the cylinder ; but if a fully elec¬
trified body is made to touch the cap AB, the gold leaves
m, n will diverge as in the figure, and their lower ends
will strike the pieces of tinfoil b, c, which will convey the
electricity to the ground.
Mr Nicholson has proposed to substitute two fiat radii
of brass in place of the tinfoil, and by moving them to and
from the gold leaves with a micrometer screw, to make
the instrument more sensible, and at the same time obtain
a kind of measure of its quantity.
Singer s Improved Electrometer.
Although insulation may be procured by coating glass Singer’s
insulators with wax, yet, as Mr Singer observes, this af-improved;
fords only a temporary defence, as moisture is eventually e!ectrome
precipitated upon them; and in removing this it is al-p^
most impossible to avoid exciting the surface of the waNucxVI.
and disturbing delicate experiments by the electricity pig, 4; jj
which is thus generated. To remove this evil Mr Singer
proposes to inclose the insulator in a narrow channel, as
the moist air in contact with it would be then limited in
quantity, and little disposed to motion. In applying this
principle to the improvement of Bennet’s electrometer,
the insulation is effected by a glass tube four inches long
and one fourth of an inch internal diameter, coated out
and in with sealing-wax, and having a brass wire five
inches long and one sixteenth or one twelfth of an inch
thick to pass through its axis, so as to be perfectly free
from contact with any part of the tube, in the middle of
which it is fixed with a plug of silk, which keeps it con¬
centric with the internal diameter of the tube. This ar¬
rangement is shown in fig. 4, 5, where A is a brass capFig. 4, a
screwed upon the upper part of the wire w, which pre¬
vents the atmosphere from having free contact with the
outside of the tube B, and defends at the same time its
inside from dust. To the lower end of the wire the gold
leaves are fastened, and the glass tube passes through
the centre of the usual cap of the electrometer, and is
cemented in it near the middle of its length, as may be
seen by the dotted lines which represent the cap. “ When
this construction.” says Mr Singer, “ is considered, it
will be evident that the insulation of the wire, and con¬
sequently of the gold leaves, will be preserved until the
inside as well as the outside of the glass tube is coated
with moisture; but so effectually does the arrangement
preclude this, that some of those electrometers that were
constructed in 1810, and have never yet (1814) been
warmed or wiped, have still apparently the same insulat¬
ing power as at first.” The electrometer constructed
upon the preceding principles is shown complete in fig. 5.1 ig.
Dr Faraday recommends strongly the use of this elec- Ur Fari
trometer; but having found from repeated experience day s^in|
that its indications are not in general well understood by ^uugin
those who have occasion to use it, he has given a very ejectroni
valuable description of the kind of charge which it receives lers
under different circumstances, and the precautions which