Encyclopaedia Britannica > Volume 8, DIA-England

618 ELECTRICITY.
Phenome- and the gold leaves will diverge in virtue of this electri-
na and city? untd it lias been carried away and neutralized by
the immediate and successive contact of the molecules of
' air. Experiments of this kind present the unique case of
an indefinite medium, which is air, of which all the mole¬
cules are individually charged with an excess of electri¬
city of the same kind, adhering to their surface ; so that
the entire mass of the medium is found penetrated with
it in a proportion which varies with the altitude. Con¬
sequently the different particles of this medium can only
be at rest from the mutual compensation of their repul¬
sive forces combined with their gravity ; and the same
condition is also applicable to conducting bodies which
are immersed in it. Thus, for all these bodies the electri¬
cal equilibrium cannot take place when their natural elec¬
tricities are completely neutralized, but only when they
possess an excess of either electricity which corresponds
to the stratum where they are found, an excess which
is vitreous in the atmosphere when it is pure. If they
possess a greater excess of this same electricity, they
will act only in virtue of this excess upon each other, and
upon all the molecules of the surrounding air. They ought,
therefore, to repel one another mutually. If, on the con¬
trary, the excess of electricity which they possess is less
than that which they would naturally take in the stratum
where we place them, the mass of the medium will act upon
each of them in virtue of this difference, and their natural
electricities will be decomposed as far as is necessary to
supply what they want of the electricity of the medium.
In virtue of this addition they will repel the medium
as much as the medium repels them, and will experience
no more action from it. But they will act upon each
other with the excess of opposite electricity which they
have acquired ; and if the medium is an indefinite fluid
composed of particles susceptible of electrifying itself by
contact, this excess Mull gradually dissipate in space.
Many curious experiments would be necessary to esta¬
blish the laws of electrical equilibrium, under circum¬
stances sufficiently different from those which we have
been generally accustomed to consider.”
Electricity The electricity of clouds M7as noticed by some of the
of clouds, earliest writers on the electricity of the atmosphere.
Canton observed that certain clouds were charged with
positive and others with negative electricity; and he
noticed that the electricity indicated by his apparatus
often changed five or six times in half an hour. This
fact was confirmed, as we have seen, by the observations
of Mr Crosse. These irregularities, however, remained
unexplained till Mr Luke Howard distinctly proved that
the electricity at the circumference of a nimbus is ne¬
gative, while that of the centre is positive ; and he sug¬
gested it as an interesting subject of inquiry to ascertain
if the negative electricity was descending and the positive
ascending. Mr J. Foggo undertook this inquiry, and in
1823 he erected a conductor armed with a smoking
match, and erected from a south window. On the 12th
of March 1824 there was a brisk wind from the north-
M'est, with frequent shoM'ers all around. About three
p. m. large dense clouds, which discharged heavy showers
of hail, passed over the zenith. Between the showers the
electricity was always positive, and the leaves of the elec¬
trometer showed their maximum divergency. So poM'er-
ful indeed was the electrical state of the air, that by rub¬
bing the outside of the glass of a detached electrometer
with soft leather, the leaves opened more than forty de¬
grees. During the showers, or when the clouds were
over head, though no precipitation took place, the electri¬
city M’as invariably positive, and so strong that sparks
could at any time be drawn by the finger from the con¬
ducting wire. Mr Foggo likewise ascertained that by tak¬
ing hold of the wire he could at pleasure intercept the elec- Phenom
trie fluid from reaching the instrument, so that the charge naand
must have been received from the atmosphere or cloud. ^aws-
When the edge or the circumference of the cloud was ''T'*
nearly over the conductor, the electricity became nega¬
tive, and appeared to be fully as strong as when it was
positive. Mr Foggo, however, now found that it could
not be intercepted as formerly b}' taking hold of the wire,
or by touching it with a pointed steel rod. Hence he
concluded that the electricity was not proceeding from
the cloud ag before, but was given oft’by the earth to the
cloud. When the steel point was presented to the instru¬
ment, the divergence was so much increased as to endan¬
ger the gold leaf, and sparks were heard to pass rapidly
between the point and the electrometer, while sharp pricks
were experienced when the finger was brought near the
brass cap.
Such are the general electrical phenomena of the atmo-Identity
sphere during its ordinary changes ; but they appear with of electr
neu' splendour, and at once rouse the interest of the phi-f'ty aoc*
losopher and the dread of the vulgar, when they are exhi-^ltnil'
bited in the terrific grandeur of thunder and lightning.
We have already seen that various writers had pointed
out the identity of lightning and the electric spark; and
though Franklin has obtained the special honour of hav¬
ing been the first who brought down fire from heaven,
Arripuit fulmen ecelo, sceptrumque tyrannis,
yet he was no more the first who snatched the thunder¬
bolt from heaven, than he was the first who MTested the
sceptre from kings.
When Franklin called the attention of philosophers to
the various points of resemblance between lightning and
the electric spark, he conceived the idea of collecting the
electricity of the atmosphere by means of pointed conduc¬
tors, and of thus preserving buildings from its explosions.
One of the first philosophers who endeavoured to verify
these views was M. Dalibard, who, at the instigation of
Buffon, erected an atmospherical conductor at Marly le
Ville, about six leagues from Paris. An iron rod, forty
feet long, an inch in diameter, and pointed at its upper
end, was erected in a garden upon three large poles, and
insulated by silken strings, and a stool with glass feet. In
M. Dalibard’s absence a thunder-storm appeared on the
10th May 1752, between two and three p. m., and M.
Coiffier, who had the charge of the apparatus, drew
sparks with a crackling noise from the lower end of it.
Having called M. Raulet, the curate of the parish, this
gentleman continued for some time, and in the presence
of many of his parishioners, to draw large sparks of blu¬
ish fire from the conductor. A few days afterwards, on
the 18th May, M. Delors drew similar sparks from a rod
ninety-nine feet high, erected in Paris. The strongest
of them w’ere drawn at the distance of nine lines, and the
conductor afforded sparks even when the cloud had mov¬
ed at least two leagues from above the place of observa¬
tion. On the 19th day Buffon obtained, at Montbar, simi¬
lar evidence of the identity of electricity and lightning.
In our history of electricity Mre have already given an Observ
account of the observations of the apparatus by which
Franklin, in the month of June 1752, obtained sparks of 01
electricity from the atmosphere during a thunder-storm.
Attempts were everywhere made to repeat this remark¬
able observation; and the most successful of these was
that of M. Romas, who, according to a decision of the
Academy of Sciences, had invented the electrical kite
more than a year before it was employed by Dr Franklin.
The kite constructed by M. Romas was seven feet five
inches high, three feet in its greatest width, and with a
surface of eighteen square feet. The string was a cord