Encyclopaedia Britannica > Volume 8, DIA-England

ELECTRICITY. 659
Ilectrical By varying the superficial dimensions of insulated con-
pparatus. ductors, and the quantity of electricity accumulated on
them, we may, by the help of the above instrument, de¬
duce many curious and important laws of electrical action.
It is, however, first requisite to explain a method of charg¬
ing simple conductors with comparative quantities of elec¬
tricity; for without an accurate measure of quantity, little
can be effected in almost any department of this branch
of science.
late Simple conductors may have comparative quantities of
CXVI. electricity disposed on them, by abstracting small sparks
ig. 15. from an insulated charged jar, fig. 15, either immediately
on the given substance, or otherwise, on an insulated
ig. A. transfer plate, p, fig. A. An insulated jar charged with a
given accumulation, as estimated by the unit of measure,
which will be presently described, is of singular import¬
ance in researches with simple conductors; for series of
sparks may be obtained from it of such slow convergence,
that many successive terms may be considered as equal.
Thus, an insulated metallic disc, d, being placed in con¬
nection with the electrometer, fig. 13, or with the elec¬
troscope, fig. 14, was electrified many times in succes¬
sion to precisely the same amount, by sparks drawn on
an insulated plate from the positive coating, the negative
side of the jar after each contact being restored to a neu¬
tral state. When a portion of the charge was abstracted
so as to sensibly decrease the quantity in the jar, then a
new point is arrived at, from which another series of
sparks can be obtained, differing extremely little in quan¬
tity; and this process may be continued to a low point of
accumulation on the jar.
The quantity given off by the positive coating is de¬
pendent on the dimensions of the abstracting conductor,
and on the free state of the negative side of the jar. If
it be free for each experiment, or be otherwise connected
with a conductor of sufficiently large dimensions, it may
be observed that a conductor of a double capacity re¬
ceives a double quantity, a conductor of a treble capacity
a treble quantity, and so on. The extent to which this
process may be carried with a jar exposing about two
square feet of coating is somewhat considerable. We
only require in these experiments an extremely perfect
insulation.
In disposing given quantities of electricity on simple
conductors in this way, and investigating the attractive
force by means of the electrometer, Mr Harris arrived at
the laws formerly explained. (See page 587.)
hands Discharging Electrometer.
-.ane’s dis- This admirable instrument is shown in fig. 16. io the
llarf?ing stem AB of a Leyden jar, MN, is fixed a bent piece of
ectrome- glasS) for the purpose of supporting and insulating
ig the brass rod DE, which has two equal brass balls at its
B extremities. This rod moves through a spring tube at C,
so that the brass ball D can be placed at different dis¬
tances from the equal ball A, by which the jar is charged.
The insulated ball D is connected through the metallic
wire DE with the outside coating of the jar, by a wire, Eh.
If we bring the ball D near to A, a small electrical charge
conveyed to the jar MN will discharge itselt from A to D,
and pass off to the ground by the wire EE. 11 the distance
AD is increased, the jar must be more highly charged be¬
fore it discharges itself; so that the distance AD of the
balls is a measure of the intensity of the charge at the
time of its discharge. As long therefore as the jar has
not discharged itself, we are sure that its charge is less
than that which corresponds to the distance AD. Ihe
chief defect of this instrument arises from the occasional
interposition of particles of dust or other light conducting
materials between the balls, which occasions the discharge
to take place sooner than it otherwise would do. The arm Electrical
of glass is sometimes fixed on the top of the charging rod, Apparatus,
where a ball of wood is placed, and is bent downwards, so
as to carry the balls D, E. In this case the jar is charged
by another ball projecting from the charging rod towards
D- This electrometer is sometimes fixed to the prime
conductor, with and without ajar accompanying it.
Mr Snow Harris's Measuring Electrometer.
This elegant instrument, which we have had the advan- Mr Snow
tage of seeing in operation, is an invaluable addition to Harris’s
our electrical apparatus. According to the law of electri-measuring
cal accumulation on coated jars, the quantity added to one electrome-
side is always proportional to the quantity given off byter'
the other, and reciprocally. Elence the amount of the
accumulation may be always estimated by insulating the
jar to be charged, and observing, by means of a discharging
electrometer, the explosions of a small jar connected with
the negative coating. This process is, however, compli¬
cated in its general application; but Mr Harris has mo¬
dified it in the following manner:—Let a small jar, N, be
furnished with a discharging electrometer, w, and inverted CCXYI.
as in fig. 17, being supported by a brass rod, jpg', inserted Fig. 17.
into the ball D of the prime conductor ABC. Then, as
the electricity passes up the rod and accumulates on the
inner coating, a similar quantity is given off' from the
outer coating, which may be made to pass from a ball at
p. Now when the small jar N has been charged to a given
degree, an explosion or discharge takes place from m to
and restores the equilibrium ; and hence one measure of
electricity is marked by the first explosion. When this
has taken place, the jar is in the same state as at first;
and hence by a repetition of the process we obtain the
exact number of measures (or explosions) which pass
from the unit jar N, and are finally accumulated on the
jar J, or battery, into which the electricity passes from the
ball p. This process of charging a battery from the outer
coating of an exploding jar, instead of from the prime con¬
ductor, supersedes all electrometers, and is the best way
of measuring quantity.
Volta's Flame Electrometer.
It was observed by Mr Bennet, that a lighted candle Volta’s
placed above the cap of his electrometer, and communi-flame elec-
cating with it, greatly increased the sensibility of the in-trometer.
strument; and it appears from various experiments that
flame possesses the property of carrying off from bodies
the electricity with which they are charged. M. Volta
ingeniously availed himself of this principle in order to
bring down to his electrometer the electricity of the at¬
mosphere, the nature and intensity of which he was de¬
sirous of examining. This effect is produced by elevating
above the atmospherical conductor a lighted match or
torch.
Matteuccis Phosphorus Electrometer.
As the preceding instrument cannot be employed when Matteuc-
there is the least wind or rain, and still less during aci’sphos-
storm of hail or wind, M. Matteucci conceived the ideaph°rus
of constructing an electrometer depending on the strong romc
conducting power of the vapour of phosphorus. He pre¬
pared rods of this substance between the twenty-fifth and
the fiftieth of an inch in diameter, by melting the phos¬
phorus under water, and by blowing it while in a state of
fusion through a tube of the requisite diameter. He af¬
terwards made the rod of phosphorus project from the
fiftieth to the seventy-fifth of an inch beyond the end of
the tube. He then fixed the glass tube on a wooden pole,
and he insulated the pole by fixing to its extremity a
glass handle. The phosphorus communicated by its base