Encyclopaedia Britannica > Volume 8, DIA-England

602
Phenome¬
na and
Paws.
Wood
shavings.
ELECTRICITY.
Sifted
powders.
Eennet’s
experi¬
ments.
The experiments of Mr Wilson on the electricity of
wood shavings belong, to a certain extent, to the present
section. Having had occasion to work very dry wood that
had lain for several hours over a very large fire, he ob¬
served the shavings adhering to the tools and to every
thing that they came in contact with. When the dry
wood was scraped with a piece of window glass, the shav¬
ings were always vitreously electrified; but when it was
chipped with a knife, the electricity of the chips was vi¬
treous when the wood was hot and the knife not very
sharp, but resinous when the wood was perfectly cold.
The electricity of the knife was always opposite to that of
the chips. The surface of the shaved or chipped wood
was seldom electrified, but when it was, the electricity
was very feeble, and of the same kind as the weakest of
the other two. The wood used in these experiments was
beech and cherry tree.
Sect. V.— On the Electricity of Sifted Poivders.
As it has not been determined whether the electricity
produced by the falling of sifted powders arises from fric¬
tion, pressure, or separation of parts, we have thought it
best to describe them in a separate section.
In 1786 Mr Bennet observed that when powdered chalk
was blown from a pair of bellows upon the cap of his gold-
leaf electrometer, vitreous electricity was produced when
the cap was six inches from the pipe of the bellows, and
resinous electricity when the distance of the pipe was three
feet. The vitreous electricity first produced was changed to
resinous by breaking the stream of air in the bellows-pipe
with a bunch of wire, silk, or feathers, or by removing the
pipe so as to make air issue in a wide stream.
Powders.
Rosin.
Flowers of sulphur.
Powdered glass.
Ditto.
Steel filings.
Brass filings.
Gunpowder.
Fine emery.
Amalgam of tin and mercury.
Mercury.
Soot.
Ashes of pit-coal.
Spoon.
Glass or paper.
Ditto.
Dry paper, warm.
Brass.
Glass or paper.
Glass.
Glass.
Glass.
Glass.
Glass.
Glass.
Experi- The most accurate experiments on the electricity of
™eilgS . powders were made by Mr Singer. The following results
r inge1' were obtained by sifting the powders on the cap of a deli¬
cate electrometer, through sieves of hair, flannel, or mus¬
lin, the sieve being cleaned after every experiment.
The following bodies produced negative electricity.
Copper,
Zinc.
Tin.
Iron.
Bismuth.
Nickel.
Black lead
Lime.
Magnesia.
Barytes.
Strontites.
Alumine.
Silex.
Brown oxide of copper.
White oxide of arsenic.
Red oxide of lead.
Litharge.
White lead.
Red oxide of iron.
Acetate of copper.
Sulphate of copper.
Sulphate of soda.
Phosphate of soda.
Carbonate of soda.
Carbonate of ammonia.
Carbonate of potash.
When the plate which receives the powders at a dis¬
tance of three inches was moistened or oiled, Mr Bennet
found that the electricity was opposite to that produced
when the plate was dry.
When powdered chalk fell from one plate to another
placed upon the electrometer, resinous electricity was
produced; and Mr Bennet obtained the same result when
he used red ochre, yellow rosin, coal ashes, black lead,
powdered quicklime, powdered sulphur, flowers of sulphur,
sand, rust of iron, or iron filings.
When powdered chalk was placed on a metal plate upon
the cap of the electrometer, and blown away with the
mouth or bellows, it produced permanent vitreous elec¬
tricity ; and the same result is obtained if the chalk is
merely blown over the plate, or if a piece of chalk is drawn
over a brush placed on the plate.
When chalk or other powders were sifted upon the cap
of the electrometer, resinous electricity was produced; but
when the instrument was placed in a dusty road, and the
dust excited by a stick fell upon the cap, vitreous electri¬
city was developed.
M. Cavallo repeated these experiments with some im¬
portant variations. He insulated a metallic plate upon a
glass stand, and having connected it with a cork ball elec
trometer, he made the dry powder fall from a spoon about
six inches above the plate. The electricity communicat¬
ed to the plate was conveyed to the electrometer, and its
nature indicated in the usual manner. When the powder
was of a conducting nature, like the amalgam of metals, it
was placed in a glass phial, or upon a plate of wax; and
sometimes the spoon was insulated, in which case it was
always found to possess an electricity opposite to that of
the plate. In this manner M. Cavallo obtained the follow¬
ing; results:
Phenoi
naan*
Law)
Cavallt
experi¬
ments.
Electricity of Plate.
Negative.1
Negative.
Negative.
Po-sitive.
Negative.
Positive.
Negative.
Negative.
Negative.
Positive.
Negative.
Negative.
Carbonate of lime.
Muriate of ammonia.
Common pearl ashes.
Boracic acid.
Benzoic acid.
Oxalic acid.
Citric acid.
Tartaric acid.
Cream of tartar.
Strength of Ditto.
Strong.
Less strong.
Weaker.
Very weak.
Oxymuriate of potash.
Pure potash.
Pure soda.
Rosin.
Sulphur.
Sulphuret of lime.
Starch.
Orpiment.
The following bodies produced positive electricity.
Wheat flower. Wood charcoal.
Oat meal. Sulphate of potash.
Lycopodium. Nitrate of potash.
Quassia. Acetate of lead.
Powdered cardamum. Oxide of tin.
Mr Singer obtained the following results by bringing
an insulated copper plate repeatedly in contact with ex¬
tensive surfaces of powders spread upon a dry sheet of
1 Strongly positive when the spoon was insulated.