ELECTRICITY.
DERIVATION OF ELECTRICITY—SEALING-WAX EXPERIMENT—THE
ELECTROPHORUS—LEYDEN JAR—POSITIVE AND NEGATIVE—THE
ELECTROSCOPE—ELECTRIC MACHINES.
We have now briefly and of course imperfectly reviewed the phenomena of
Vibration, as exemplified in what we term Heat, Light, and Sound. We
now come to a most mysterious servant of mankind, as mysterious as any
Djinn of romance; viz., ELECTRICITY.
The term Electricity is derived from the Greek word _electron_,
meaning “amber”; because from amber the properties of what we call
“Electricity” were first discovered. Six hundred years before the
Christian Era, Thales wrote concerning the attraction which amber,
when rubbed, possessed for light and dry bodies. But it is to an
Englishman named Gilbert that we owe the word “Electricity,” which he
derived from the Greek, and in his works (about 1600 A.D.)
he discusses the force of the so-called “fluid.” Otto von Guerike, of
“air-pump” celebrity, and many other philosophers after him, continued
the investigation of the subject. At the beginning of the last century
great attention was paid to the Electric Machine. The Leyden Jar was,
as its name denotes, discovered by Muschenbrock, of Leyden, (though
the honour was disputed). Franklin made the first lightning conductor
in 1760. Volta and Galvani, to whose invention we owe “Voltaic
Electricity” and “Galvanism,” and Faraday in more modern times gave a
great impetus to electrical science. The great part that electricity
has been playing in the domestic history of the world since Faraday’s
lamented death, is probably known to the youngest of our readers. What
the future of this agent may be we can only guess, but even now we may
regard electricity as only in its infancy.
There are few scientific studies more attractive to the general reader
than electricity, and few admit of more popular demonstration. The
success of the late electrical exhibition in Paris, and its successor
in London at the present time, are proofs of the interest taken in this
great and mysterious agent whose origin we are in ignorance of, and of
whose nature and powers we are daily discovering more and more, and
finding there is still an immense field for its application.
Some fundamental facts regarding electricity may very easily be studied
with the assistance of every-day objects at hand. Amber was the first
substance to show attraction when rubbed, but Gilbert found out that
glass and sealing-wax, etc., possessed like properties with amber.
If we rub a stick of sealing-wax with a piece of cloth, we shall see
that it will attract some small fragments of paper placed near it.
Nothing is easier than to construct a small pendulum to show with
perfect clearness the phenomenon of electric attraction. A piece of
iron is fixed on a wooden pedestal, and supports a thread of silk, to
the end of which is fastened a little ball cut out of a piece of cork.
The stick of sealing-wax after being rubbed with the cloth will attract
the ball as shown in fig. 200.
[Illustration: Fig. 200.—Sealing-wax attracting a piece of cork.]
By means of a piece of paper we can produce a spark. I take a large,
strong sheet of drawing paper, heat it very thoroughly, and lay it on
a wooden table. I rub it with a perfectly dry hand, or with a piece
of woollen material until it adheres to the table. That done, I place
a bunch of keys in the centre of the sheet of paper, which I raise,
lifting it by two corners. If at this moment any one touches the bunch
of keys with his finger, a bright spark will be elicited. The metal is
charged with the electricity developed on the paper; if the weather
is dry, and the paper thoroughly heated several times, the spark may
attain nearly an inch in length.
We can easily construct other electrical apparatus. For instance, an
“Electrophorus,” or instrument for obtaining electricity by means of
induction, or a Leyden jar, can both be made at home. Let us proceed to
construct the former, of which we give an illustration (fig. 201).
We take a lacquered tea-tray about a foot long, and cut out a sheet of
thick wrapping paper, so that it will lie over all the level portion
of the tray. At each side of this sheet of paper we fix two bands of
paper, as in the illustration (fig. 201), so as to serve as handles.
The tea-tray should be placed upon two tumblers to support it and
to insulate it, glass being a “non-conductor.” (We will speak of
conductors and non-conductors presently.) We have now our Electrophorus
made ready for action; let us proceed to see how it will act.
[Illustration: Fig. 201—Simple Electrophorus.]
First, rub the thick packing paper over a hot fire or a stove, and the
friction must be continued for some time, until the paper has become
thoroughly dry, and as hot as possible without charring. When this has
been accomplished, place it quickly upon a wooden table, and rub it
rapidly and energetically with a clothes’ brush, dry and hard as can
be obtained. Place the paper upon the tray; touch the tray with the
knuckle, and draw away the paper by the handles fixed to it (_see_
fig. 201); a spark will result. Then if the paper be replaced upon the
tray, and the hand again presented, the same result will follow. This
experiment may be repeated five or six times, at least, with success.
We have in this tea-tray and its paper covering a real electric
machine. How can we manage to provide a Leyden jar to contain our
electricity? Nothing is more easy. Let us take a tumbler and partly
fill it with shot; insert into the glass a tea-spoon, and if all the
articles are quite dry we shall possess a Leyden jar.
To charge the jar we have thus provided we must work the Electrophorus
we have already described. While one person lifts off the paper as
directed, another must hold the glass filled with shot close to the
edge of the tray, and touch the corner with the tea-spoon; the spark
will then enter the “jar” or tumbler. We can thus charge the jar as we
please, and by presenting the finger as in the illustration (fig. 202),
we shall obtain a discharge from it.
[Illustration: Fig. 202.—A Leyden jar.]
Mr. Louis Figuier, in his “_Merveilles de la Science_,” relates that
Wollaston, meeting one of his friends one evening in the streets of
London, drew from his pocket a copper thimble, and proceeded to turn it
into a microscopic pile.[13]
In order to do this he removed the bottom of the thimble, flattened
it with a stone, so as to bring the two internal surfaces about on a
line with each other, then placed between the copper surfaces a small
strip of zinc, which was not in contact with the copper, owing to the
interposition of a little sealing-wax. He then placed it in a glass
cup, previously filled with the contents of a small phial of water,
acidulated with sulphuric acid. He next wound round the strip of zinc
and its copper covering a piece of platinum wire, the wire becoming
red through the electricity developed in the pile. The dimensions of
this platinum wire were extremely small; it was only 30/1000 of an inch
in diameter, and 1/30 of an inch in length. By reason of its small
dimensions it could not only be reddened, but fused by the little
battery.
Thus Wollaston’s friend, who was a witness of the experiment, was
able to light a tinder at the red wire. In this little battery of
Wollaston’s the copper enveloped the strip of zinc in every part;
that is to say, the negative element was on a higher surface than the
positive metal.
[Illustration: Fig. 203.-A simple compass.]
After considering Electricity, it is not impossible to approach the
study of Magnetism, and even to construct a mariner’s compass. We
shall find the method of doing so by borrowing an interesting passage
from the “_Magasin Pittoresque_.” Let us take a small cork and pass
through it an ordinary knitting-needle (fig. 203), which we have
already magnetized by placing it N.S., rubbing it gently, and always in
the same direction, with one of those little iron magnets with which
children amuse themselves. After the needle has been passed through
the cork, we also fix into it a sewing-needle, or rather a pin, the
point of which rests in one of the little holes in the upper part of
the thimble. In order to balance the magnetised needle, we thrust a
match into both sides of the cork, as shown in the illustration, and
fasten to the ends of each a ball of wax. Thus the needle, the balls,
and the pin are all balanced at once, so that the contrivance has the
appearance of the illustration.
As it is very important that with such a sensitive instrument any
agitation of the air should be avoided, the thimble must be placed at
the bottom of a common earthen pan, B D T, which should be
covered over with a piece of glass, V. To graduate the compass
a circle is described on a piece of paper. On this dial we trace the
divisions sufficiently close only at the north extremity of the needle,
and the paper is fixed underneath, as in fig. 203. Then we fix a piece
of wax at the end of the match pointing N., opposite the
northern extremity of the needle inside the basin. In this way we have
a very useful and inexpensive compass.
We may also magnetize a fine sewing-needle, and grease it by rubbing
it with a little suet. It is then capable of floating on the surface
of water running in the direction of the north pole. We might go on
multiplying indefinitely examples of physical experiments without
apparatus, but we have probably already given a sufficient number to
aid our readers in imagining others.
We have now in a simple manner shown how we can easily produce
electricity. We may understand that electrical phenomena are
produced—(1) friction between different bodies; (2) by placing bodies
which differ in contact; (3) by the transition of bodies from one
condition to another; (4) by chemical changes; (5) by animals. The two
first, and the fourth, are the most usual causes.
We know that certain substances when rubbed with silk or wool acquire
the property of attracting other substances. But in the case of a
rod of glass or stick of wax, the attraction will only be perceived
when the rubbing has been applied. But metal will behave differently.
Any part of the metal rod will continue to attract. So metals are
CONDUCTORS of electricity; while glass, wax, silk, amber,
sulphur, etc., are _bad_, or NON-CONDUCTORS. Metals are the
best conductors we have, but trees, plants, liquids, and the bodies of
animals, including men, are all good conductors of electricity. Dry air
is a bad conductor.
[Illustration: Fig. 204.—Attraction and repulsion.]
There are two kinds of electricity, known as positive and negative
(_plus_ or _minus_), vitreous or resinous. We saw in fig. 200 that we
can attract a small ball of pith or cork by a piece of sealing-wax
rubbed with flannel. If we then present a glass rod rubbed with silk
to it, it will be equally attracted, but will be at once repelled; and
after being so repelled, if we put the wax to it, it will be attracted
to the sealing-wax again. So wax at first attracts then repels the
ball, and so does glass, but either will attract the ball if presented
alternately (fig. 204). The reason for this is as follows:—
When we have rubbed the glass with silk, we charge it with positive
electricity, and when the rod touches the ball, the latter imbibes that
electricity, and flies away from the glass rod. The sealing-wax imparts
negative electricity in the same way.
The law is, that bodies charged with the same kind of electricity
_repel_ each other, and those containing the opposite kinds _attract_
each other. Positive _repels_ positive; negative _repels_ negative.
But positive _attracts_ negative, and negative _attracts_ positive.
Opposite electricities unite, and so neutralize each other that no
effect is perceived; but it must be borne in mind that all bodies
possess both electricities in some quantity, greater or less. By
rubbing we separate these electricities, the rubber becoming negative,
the rub_bee_ positive. The friction of glass supplies positive
electricity, and sealing-wax supplies negative electricity, or we can
obtain the same effect by rubbing either with certain material.
The manner in which a body is electrified depends upon its nature
and condition; but we may accept as a general axiom,—but by no means
as a law,—that when two bodies are rubbed together, that which gets
the hotter in the process takes the negative kind of electricity. In
the following list the substances have been so arranged that each is
negatively electrified by those preceding, and positively by those
succeeding it. 1, cat’s skin; 2, glass; 3, woollen stuffs; 4, feathers;
5, wood; 6, paper; 7, silk; 8, shellac; 9, rough glass. We append a
list of conducting and non-conducting bodies in their order:—
CONDUCTORS.
Metals.
Lime, coal, or coke.
Saline mixtures.
Pure water.
Vegetable tissues.
Animal tissues.
Hot air.
Steam.
Rarefied air.
NON-CONDUCTORS.
Ice.
India-rubber.
Marble.
Porcelain.
Resin.
Dry gases.
Paper.
Wool.
Silk.
Shell-lac.
Diamond.
Glass.
Wax.
Sulphur.
[Illustration: Fig. 205.—Positive and negative.]
[Illustration: Fig. 206.—The Battery.]
It should be observed that the degree of value as a conductor or
non-conductor depends somewhat upon the atmosphere. For instance, glass
is an excellent insulator, or non-conductor, when dry, but when wet
it changes to a conductor. So insulators are at times covered with a
solution of shell-lac, or fat, to keep away moisture. We may reasonably
conclude that bodies which are good and bad conductors are good and
bad conductors of electricity. Water is a good conductor, air is a
bad one; were it otherwise, electricity would escape from the ground
into the air; as it is, the air manages in some degree to retain the
electricity at the surface of bodies, for it is on the surface that we
find the electric “fluid.”
We have mentioned _electrical induction_ in a former experiment with
the tea-tray. We will now explain it more fully, as a consideration
of it will bring us to the _electric spark_, or lightning, with the
account of the discovery of the _Conductor_ and the _Electrical
Machine_.
Let us look at the illustration next below. A B is a cylinder supported
on a glass rod, and at each extremity is a small pith ball, _a_ and
_b_. The cylinder is in a neutral condition, as is evidenced at first
by the pellets being in a vertical position. But suppose we bring a
ball, C, towards the cylinder. C is charged with positive electricity,
which attracts the negative to itself, and so repels the positive away
at the opposite side. So the pellet at one side will be attracted to C,
and the other will fly in an opposite direction.
[Illustration: Fig. 207.—Electrical induction.]
Let us take another illustration. Here we have a horizontal metal rod,
_cc′_, insulated on a glass stand. Two balls of cork are attached
at both ends of the rod by metallic wires. Hold a rod of resin, _r_,
which has been made negatively electrical, and apply it to one pair of
the cork balls. The positive electricity will be attracted at _c′_ and
the negative repulsed, and fly away at _c_. If we remove the resin the
equilibrium will be again established, and the balls will fall to a
vertical position.
[Illustration: Fig. 208.—Induction.]
We can also by drawing off the negative electricity by the finger at
_c_, while the resin rod is still held to the other side, _c′_, fill
the whole of the metal rod with the positive electricity when the
finger and the resin have been removed respectively first and last. The
balls will then fly in opposite directions again, in consequence of the
repulsion exercised by the positive poles.
The “Electrophore,” or “_Electrophorus_,” we have already learned
to make for ourselves, as also the Leyden Jar. But we give cuts of
them. The former is very simple, and can be made by mixing two parts
of shell-lac and one of turpentine, and pouring the mixture upon a
metal plate. If this be rubbed with a cat’s skin when dry, and a metal
cover with a glass handle be placed upon it, it will be found that the
positive and negative electricity are collected on the lower and upper
surfaces of the plate respectively, and can be drawn away with a spark
as before, and made use of.
[Illustration: Fig. 209.—Electrophorus.]
The Leyden Jar requires a little more detailed description, as it is
to it we are indebted for our Battery. It is a common glass bottle or
jar, coated both inside and out with tinfoil nearly as high as the
shoulder, _a_ _a_. The mouth should be firmly closed with a bung of
wood, _g_ _g_; a hole should be bored in the bung, through which a
brass rod is tightly pushed. The rod, too, is topped by a brass knob,
and a brass chain is attached to the other extremity. The interior of
the tinfoil receives positive electricity, and the exterior negative
when the jar is charged from the “Electrophorus.” To discharge the jar
and create a _shock_ it is necessary to put one hand on the outside,
and the other on the knob of the jar. A brilliant spark and a severe
shock will result if the jar has been fully charged. It is as well to
be cautious when trying this experiment. The effect of the shock may be
felt by any number of persons joining hands, if one at one end of the
row, and one at the other end, touch the knob and the outside of the
jar simultaneously.
[Illustration: Fig. 210.—The Leyden Jar.]
This electric discharge is lightning in miniature, and it is to
Benjamin Franklin that the world is indebted for the discovery. The
philosopher was greatly interested in the science of Electricity and,
having retired from business, he devoted himself to the consideration
of thunderstorms. He wrote a treatise to show that points drew off
electricity, and that electricity and lightning were similar. He urged
that metallic rods might be attached to ships and buildings, so that
during thunderstorms, or at other times, the electricity might be
harmlessly carried into the ground. This suggestion he made without
being able to explain _why_ points did carry off electricity without
a spark. The reason is because there is no place to store it; it runs
away at once, without having time to collect, as in a ball.
Franklin made one or two experiments before his renowned kite-flying
arrangement, which convinced him that electricity was by no means
an agent to be played with. He endeavoured to kill a turkey by
electricity, but by incautious handling of the jars in which the
“fluid” was stored, he discharged them, and describes the result: “The
flash was very great, and the crack was as loud as a pistol; yet my
senses being instantly gone, I neither saw the one nor heard the other,
nor did I feel the stroke on my hand, though I afterwards found it
raised a round swelling where the fire entered as big as half a pistol
bullet.” On a subsequent occasion he was again struck senseless while
endeavouring to administer a shock to a paralytic patient.
It was not until June 1752 that Franklin made the experiment with
the kite, which resulted in such great discoveries. He made his kite
of a silk pocket-handkerchief, and he fixed a pointed rod upon the
upright portion of the frame at the top; the string ended in a foot
or so of silk, which was held by the philosopher, and to the end of
the hempen portion of the string a large key was tied. For some time,
notwithstanding the approach of most unmistakable thunder-clouds, his
patience was tried. But at last the strands of the hempen string began
to bristle up, and soon after, when Franklin applied his knuckle to
the key, a spark was obtained. The great discovery was made. Franklin
subsequently obtained lightning in his own house, and performed several
experiments with it.
[Illustration: Fig. 211.—The Electroscope.]
The Electroscope (fig. 211) is an instrument by which we can ascertain
whether electricity is present or not, and the nature of it. If we
bring an object unelectrified close to the ball or knob on the top of
the glass shade, the two needles, or strips of gold-leaf, which are
often used, will remain still. But if the body has been electrified
it will communicate the electricity to the rod inside, and attract to
itself the fluid of opposite quality; the same kind of electricity then
is in action in the gold-leaf or needles, and they fly apart—repel each
other. Supposing that positive electricity were first communicated, we
can cause the contraction of the leaves or wires by applying a negative
kind, which, meeting the positive, neutralizes it, and the wires
collapse.
If the electricity with which the instrument is charged be positive, by
approaching the baton to the ball, A, we shall see the wires
diverge more than before, and they will finally be discharged by the
knobs within. If the electricity be contrary to that in the baton, the
wires will approach each other, but by gradually withdrawing the baton
they will again separate, and even to a greater distance than before.
The Electric Machine is shown in the illustration (fig. 212). It
consists of a large plate of glass fixed upon a glass stand, between
wooden supports. The handle is of glass; two pairs of rubbers are
fastened above and below; the plate is turned between them, and becomes
“positively” electrified. The rubbers are covered with leather and
stuffed with horsehair, DD, and press very tightly against the
glass, so that the friction is constant. The rubbers are covered with
an amalgam made of mercury, zinc, and tin, two parts of the first to
one each of the others. A chain (of metal) connects the machine with
the ground. The conductors, PP, are united by a cross-piece,
Q, and sustained upon glass supports. At the end of the
conductors are two curved rods, CC, which are provided with
points to take the electricity from the plate, but do not touch it.
[Illustration: Fig. 212.—The Electric Machine.]
The electricity is thus stored in the insulated conductors as the
machine is turned. The negative portion is carried into the ground by
the chain from the rubbers, while the positive electricity is retained.
The longer we turn the more we shall obtain, and the quantity is
measured by an electric pendulum on one of the conductors, which flies
out by degrees as the charge increases, and indicates its power by
means of a needle it works upon an ivory index.
It is not difficult to make an electric machine out of a glass bottle.
This will furnish the glass cylinder. If a stick be run through it (for
which purpose a hole must be drilled in the bottom of the bottle), a
handle can be fixed, and the bottle mounted on a stand. A wash-leather
cushion, stuffed, can be so arranged that it will press against the
bottle as it is turned; a piece of silk should be permitted to hang
from the cushion frame over the glass. A conductor may be made from a
piece of wood neatly rounded and smoothed, and coated with tinfoil.
The ends should be rounded like “knobs.” Stick pins in to collect the
electricity (and it will be readily obtained). The cushion should of
course be well smeared with amalgam. From this, as well as from the
glass-plate machine, the “positive” electricity can be drawn off and
stored in a Leyden jar, and then discharged by the “discharging rod,”
which is represented on the cut. It may have one or two handles, and
one knob is placed outside the jar, the other near the ball surmounting
it. The glass being a non-conductor saves the operator, and some long
sparks and loud reports may be obtained.
[Illustration: Fig. 213.—Cylinder machine.]
The Electric Machine is always assumed to give off _positive_
electricity.
Sir William Armstrong’s Electric Machine is a mode of obtaining
electricity by moist steam. The design is Armstrong’s, and Professor
Faraday subsequently went into all the conditions to produce the
“fluid” by the friction of steam. The machine was something like a
small boiler supported on glass legs. A row of nozzles was fixed upon
the escape pipe so as to create a great velocity and friction in
the escaping steam. Round the nozzles was a box of cold water, for
that fluid was found necessary for the production of electricity as
demonstrated by Professor Faraday. The steam rushed against a row of
points attached to the prime conductor of an electric machine, and the
electricity of the steam was thus given off to the conductor. There are
many other forms of electric machines, but it will serve no purpose to
detail them.
[Illustration: Fig. 214.—Discharging rod.]
The _Electric Battery_ (_see_ fig. 206) is formed by a collection of
Leyden jars. The inside and outside coatings are connected in a box
divided into partitions lined with tinfoil. The rods of the jars are
also connected, as in the illustration, by brass rods, and when this
battery is charged people should be careful how they handle it, for a
shock may be produced which would cause serious injury, if not death.
The battery can be charged from the machine by a chain fastened to the
central ball, while a second chain connects the exterior of the box and
all the outside of the jars, by means of the handle, to the ground.
When electricity is at rest it is termed “static electricity,” and
when in motion “dynamic” electricity. The latter treats of electric
currents which can be sent through wires or chains. We can keep this
current moving by means of a machine, and the battery called a Voltaic
battery, from Volta. We will describe it presently. Electric currents
can be measured, for they may be of different strengths according
to the battery, and they are measured by the GALVANOMETER.
Electricity can therefore be transferred and carried by the conducting
substances, and much heat will be engendered as the “electric fluid”
passes along a wire. Lightning frequently fuses bell-wires as it
passes, and when we touch upon Galvanism or Dynamical Electricity we
shall hear more about it.
By the Electric Machine we can obtain some very powerful currents
of electricity; we can produce many pleasing effects, and perform a
number of experiments, such as making balls or figures of pith dance,
and several other easy and entertaining tricks, which will be found in
books more specially devoted to the entertainment of young people.
[Illustration: Fig. 215.—Leyden Jar.]
We have now given some explanation of the manner in which electrical
phenomena can be produced,—viz., by the Electric Machine and by the
Leyden Jar,—but we must not expect to find any electricity inside any
charged body. It has been proved that all the electricity is upon
the surface of bodies, even if in varying quantity, and that equal
quantities of electricity are always produced when bodies are excited
by friction, but the _kinds_ are different. The rubbing body is of one
kind, the body rubbed another, and consequently the forces neutralize
each other. The two forces or kinds of electricity we have seen repel
or attract each other, and we can imagine the farther they are apart
the less will be the force, and the _rate of diminution of force_,
according to distance, is ascertained by an ingenious apparatus called
a “Torsion” Electromoter, which was constructed by Coulomb, and was
frequently used by Faraday.
Perhaps some people may not be aware of the term “torsion.” It means
twisting, and “the torsion of a thread suspended vertically is the
force tending to twist the lower extremity when the upper end is turned
through an angle.” This instrument is really an Electromoter, and is
not considered suited to beginners, and it is scarcely accurate in its
workings. We need not therefore describe it in detail. There are some
excellent Electromoters, the Elliott being, we believe, the best for
use. A full and detailed description of the Quadrant Electromoter will
be found in Mr. Gordon’s treatise on Electricity.
_Recapitulation of foregoing Chapter._ So far, we have seen there is
electricity in everything, although some bodies are termed conductors
and others non-conductors; though, as in applying the terms heat and
cold, we must remember that no body is entirely devoid of electricity,
and no body is therefore an absolute _non_-conductor any more than any
object is absolutely devoid of heat. Faraday, indeed, was of opinion
that “conduction and insulation are only extreme degrees of one common
condition”; they are identical both in principle and action, except
that in _conduction_ an effect common to both is raised to the highest
degree, and in the case of insulation it occurs in an almost insensible
quantity.
We have also read of positive and negative electricities, and we must
not fancy there is any particular reason for this distinction. It was
Du Fay, whom we have mentioned, who gave the names “vitreous” and
“resinous” to the two kinds, as one was developed by rubbing glass, and
the other by rubbing resin. But, as shown by our experiments, either
kind of electricity can be excited in glass or sealing-wax, and both
kinds are produced at once. You cannot get “positive” without negative
electricity. “Positive” is the term applied to the kind produced by
rubbing glass with silk or wool; “negative” is the term applied to
the kind developed by rubbing sealing-wax, but the kind developed by
friction depends on the rubbing substance and certain conditions.
Bodies charged with the same electricity repel; if charged with
different kinds they attract each other. The more readily displaced
particles when bodies are rubbed become negatively electrified as a
rule.
Similar electricities repel each other with a force inversely
proportional to the squares of the distance between their centres,
as established by Coulomb. So if the space between any two similarly
electrified bodies be reduced by say _one-half_, the force of the
repulsion will be increased _four_ times. The rule for attraction is
similar; so when two bodies are charged with opposite electricities,
and the distance between them is increased, the attractive force is
diminished in proportion as the square of the distance between them.
Many confirmations of this theory were made by the late friend of
our boyhood, Sir W. Snow Harris, and published in the _Philosophical
Transactions_.
The following full list of conductors and non-conductors (copied from
Professor Noad’s Text-book of Electricity, and compared with De La
Rive’s Treatise) may be useful:—
CONDUCTING BODIES IN ORDER OF
CONDUCTING POWER.
All the metals.
Well-burnt charcoal.
Plumbago.
Concentrated acids.
Powdered charcoal.
Dilute acids.
Saline solutions.
INSULATORS IN THE INVERSE ORDER OF
INSULATING POWER.
Dry metallic oxides.
Oils (heavier the better).
Vegetable ashes.
Transparent dry crystals.
Ice below 13° Fahr.
Phosphorus.
Lime.
CONDUCTING BODIES IN ORDER OF
CONDUCTING POWER.
Metallic ores.
Animal fluids.
Sea-water.
Spring-water.
Rain-water.
Ice above 13° Fahr.
Snow.
Living vegetables.
Living animals.
Flame smoke.
Steam.
Salts soluble in water.
Rarefied air.
Vapour of alcohol.
Vapour of ether.
Moist earth and stones.
Powdered glass.
Flowers of sulphur.
INSULATORS IN THE INVERSE ORDER OF
INSULATING POWER.
Dry chalk.
Native carbonate of baryta.
Lycopodium.
Caoutchouc.
Camphor.
Silicious and argillaceous stones.
Dry marble.
Porcelain.
Dry vegetables.
Baked wood.
Leather.
Parchment.
Dry paper.
Hair.
Wool.
Dyed silk.
Bleached silk.
Raw silk.
The following may be added to the Insulators, viz.:—
Transparent precious stones.
Diamond.
Mica.
All vitrefactions.
Glass.
Jet.
Wax.
Sulphur.
Resins.
Amber.
Gutta-percha.
Shell-lac (or gum-lac).
Ebonite.
There are, as we know, two kinds of electricity, the _static_ and
_dynamic_; and when the latter state is _instantaneous_, it is
referred to as the “electric discharge,” which occurs when opposite
electricities seek each other, and the bodies return to a state of
equilibrium or neutralization. “These bodies, if _insulated_, obtain no
more electricity after the spark has passed; but if there be a constant
source of negative electricity supplying one, and a constant source of
negative electricity supplying the other, there will be a succession of
sparks; and if they communicate by a conductor there will be, through
this conductor, an uninterrupted neutralization of a continual reunion
of the two electricities, and this is what is termed the continuous
dynamic state or electric current” (De la Rive).
FOOTNOTES:
[13] This interesting experiment, which we have exactly verified, was
described to us by Professor Waldner, and M. A. Keppler.