Encyclopaedia Britannica > Volume 8, DIA-England

ELECTRICITY.
601
pborne-
rmd
irs.
Cork pressed against polished quartz.
4 3-9
Sect. IV.— On the Electricity produced by Cleavage and
Separation of Parts.
Phenome¬
na and
Laws.
Cork pressed against sulphate of lime.
4 1-9
When two insulated discs, one of cork and the other of
caoutchouc, are pressed against each other, the cork after
pressure is negatively electrical, and the caoutchouc posi¬
tively electrical. When the cork is pressed against the
skin of an orange, the cork is positive and the skin nega¬
tive.
When cork is pressed against Iceland spar, sulphate of
lime, fluor spar, sulphate of barytes, the cork is negative
and the minerals positive; but when cork is pressed
against kyanite, retinasphaltum, pit-coal, amber, zinc,
silver, &c. the cork is positive, and the minerals or metals
negative.
When insulated cork is pressed against any part of the
animal body free from moisture, the cork receives an ex¬
cess of negative electricity. The hair and down of ani¬
mals produce nearly as much electricity by pressure as
Iceland spar, but of the opposite kind. Cork pressed
lightly against inspissated oil of turpentine is negatively
electrified.
When two discs of the same substance, such as skin or
amadou, are pressed against each other, the one becomes
negative and the other positive.
The electricity thus developed by pressure is lasting.
Haiiy found it to continue eleven days with Iceland spar.
Sulphate of barytes of Royat parts with it instantly unless
well insulated; but a well insulated crystal retains it half
an hour. The duration of the electricity seems to be in¬
versely as the conducting power. Becquerel supposes the
internal surface of the body to be, like the Leyden jar,
charged with the opposite electricity; so that dissipation
is prevented by the action of the two electricities.
In these phenomena the electricity never appears till the
bodies are separated.
When the temperature of any body is raised, it has the
greater tendency to acquire negative electricity by fric¬
tion. In like manner, by heating Iceland spar, it may be
made to give negative electricity by pressure against cork.
If we cut a piece of well-dried cork into two pieces by a
very sharp knife, and press the cut surfaces against each
other, no electricity is developed ; but if one of the pieces
is heated slightly near the flame of a candle, and the pres¬
sure applied, each surface will, when separated, exhibit
opposite electricities. The same is true of two pieces of
Iceland spar.
rtricity R has been long known that electricity is produced dur-
separa. jng vi0]ent disruption of a body, or by tearing it asun-
der, or by separating a laminated body, or by breaking a
body across, or by crushing it, or even by cutting it into
portions.
Mr Bennet observed that when an unannealed glass tear,
or Prince Rupert’s drop, was put upon a book, it electri¬
fied the book negatively. Mr Wilson noticed that if a
piece of wood, when dry and warm, is rent asunder, one ot
the separated surfaces becomes vitreously and the other
resinously electrified. When a stick of sealing-wax is
broken across, one of the surfaces of fracture is vitreous¬
ly and the other resinously electrified.
pert’0m electricity developed by the bursting of a Prince
»ps.
Rupert’s or unannealed glass drop was found by Sir
David Brewster to be accompanied with a flash of light.
“ These drops,” says he, “have three different cleavages,
one like the lines of a melon diverging from the apex of
the drop, another concentric with the surface of the drop, ' -
and another oblique to the axis. Having laid one of these
drops upon a table in a dark room, and covered it with a
plate of thick glass to prevent any of the fragments from
reaching the eye, the drop was burst by breaking off a
part of its tail, and the whole of it appeared luminous,
so that at the instant of the fracture a quantity of faint
light, of the same shape and size of the drop itself, was
distinctly visible. The drop which gave this singular re¬
sult was made of flint-glass, and was the largest that he
had ever seen. Every other flint-glass drop produced a
distinct electrical light; but in none of them except the
large one could he see the luminous shape of the drop.
The same light appeared when they were burst under
water. The small glass drops made of bottle-glass never
exhibited any light at the moment of bursting ; but it was
almost always visible, in small sparks, in bottle-glass drops
of a larger size.” The same author observed also a bright
electric light when the water-proof cloth manufactured by
Charles Mackintosh, Esq. was separated by tearing it into
its two component pieces, which are united by a thin film
of caoutchouc. He found also that the same light was
produced by tearing quickly cotton and other cloths, and
by separating the films of mica. The same effects are
produced by breaking barley-sugar or sugar-candy.
When the plates of mica, or the laminae of sulphate of
lime, are quickly separated, each of the two plates, when
separated, carry off an excess of the opposite electricities,
the one being vitreously and the other resinously electri¬
fied. If these two plates are again placed together in the
position which they occupied previous to their separation,
and a slight pressure used to make them adhere, M. Bec¬
querel found that the same phenomena took place as at
the instant of their first separation, that is, each plate
took the same kind of electricity. This property conti¬
nued only a few moments, perhaps till the molecules had
taken their ordinary state of equilibrium, which is aided
by increasing their temperature. The effects above de¬
scribed he found to be more distinct in proportion as the
crystal was more heated previous to the cleavage.
The electrical phenomena produced by cleavage, and Cleavage,
by tearing asunder and crushing bodies, differ in degree
only from those produced by pressure, as in every case of
a separation of parts there must be an approximation of
the molecules in one direction. If we press, for example,
a piece of caoutchouc in one direction, or draw it out in
an opposite direction till it breaks, the effect of both these
mechanical actions is an approximation of the molecules
in the same direction. Hence the electrical phenomena
are nearly the same. The light produced by the collision
of hard bodies, or by the separation of the parts of bodies,
is no doubt produced by the rapid recombination of the
two electricities when developed at the points of pressure.
A very curious phenomenon was observed by Sir Da- Cleavage
vid Brewster during his numerous experiments on theof t0Paze3‘
cleavage of topazes, in which there were cavities con taint¬
ing very highly expansible fluids. His practice was to make
the cleavage plane pass through a fluid cavity, and thus
to open the cavity and allow its contents to be seen and
examined. When this was done, the most expansible ot
the two fluids flowed from the cavity upon the polished
and electrified face of cleavage, and continued to expand
and contract itself alternately, now collecting itself into
a drop, and then expanding itself into a flat disc. These
motions continued till the fluid evaporated; and the ef¬
fect was no doubt owing to the electricity produced by
evaporation, as well as to that produced by cleavage.
vol. vm.
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