exerted upon the pointed body. This is shewn by a very pleasing
experiment called the electric fly. This fly is composed of four small
wires, fastened into a metallic cap, similar to those used in
sea-compasses, so that the wires may easily move upon a point, in a
horizontal direction. They should be exactly balanced, and have their
ends, which must be very sharp, all bent in the same direction. Now if
this fly be placed on an insulated point and electrified, its sharp ends
will become luminous in the dark, and it will revolve in a direction
contrary to that in which the ends are bent; or if it be placed on an
uninsulated point and brought near the electrified prime conductor, the
same effect will follow.
It is to be observed, that the fly will move round in the same
direction, whether electrified positively or negatively. The cause of
this seeming contradiction depends upon the repulsive power existing
between bodies possessed of the _same_ electricity; for the air opposite
to the points acquires a strong electricity, analogous to that of the
points, it is therefore repelled, and replaced by other air, which is
also electrified and repelled. Hence a continual stream is produced,
blowing from the points, and that equally, whether the electrization be
positive or negative; and as action and re-action are equal and in
contrary directions, the points, repelling the air, must themselves be
repelled, and in the opposite direction; which causes the fly to be
always turned one way, that is, in a direction contrary to that in which
the air is moved.
In vacuo no motion is produced, because there is no air on which the
electric matter can act when it issues from the points.
In like manner, if air be confined in a receiver, the motion of the fly
soon ceases, because the fluid cannot pass through the air and the
glass. But on applying the end of a finger to the outside of the
receiver, opposite one of the points of the fly, the motion will begin
again, and by moving the finger occasionally round the glass, it may be
continued till most of the glass is charged.
The cause of this motion is, that when the finger is applied to the
outside of the receiver, the glass, loosing part of its natural quantity
of electricity from that side, (i. e. when the fly is electrified
positively, and vice versa if negatively) takes up the fluid from the
air on its inner surface. Hence the air becomes capable of being again
electrified by the point and this renews the motion.
We have already stated that if a pointed wire be presented to a
conductor _positively_ charged, it will be illuminated with a star or
globe; and if the conductor be negatively charged, the illumination will
have the form of a pencil or divergent cone. F. Beccaria explains this
in the following manner. I suppose, says he, that the star is occasioned
by the difficulty with which the electric fluid is extricated from the
air, which is an electric; suppose for instance that a pointed wire is
presented to a body positively electrified; the electric fluid is first
communicated from that body, to the air between it and the pointed wire,
and then the wire must extricate it from the air.
The pencil is occasioned by the force with which the fluid, issuing from
the point, passes through the contiguous air to that which is more
remote, i. e. by dividing the contiguous air, and not by affixing itself
to it.
Beccaria likewise remarks, that if two equally sharp pointed bodies are
brought near the prime conductor, they will appear luminous at only half
the distance that one of them would. They will also discharge it in half
the time.
It will not be improper to remark here, that when a point not
electrified is opposed to one electrified positively, both points will
have small globular lights upon them; but if a positive one be opposed
to one negatively electrified, they both preserve their own
characteristic properties.
From the above the following conclusions may be drawn.
First, That pointed bodies attract the electric matter more or less
easily, and at a greater or less distance, according to their acuteness.
Second, That pointed bodies have the power of attracting electricity as
well as of repelling it, in a greater degree than conductors of any
other form.
We shall treat farther of pointed conductors under the article
_Thunder-house_.
CHAP. VIII.
_Of electric attraction and repulsion._
No satisfactory theory of electric attraction and repulsion has, so far
as our knowledge extends, ever yet been given. The phenomena have been
differently accounted for, as the writers have embraced different
opinions in regard to positive and negative electricity. One mode of
explanation has been adopted by those who believe, with Franklin, that
positive electricity is only an accumulation of the electric fluid in a
body beyond its natural state; and that negative electricity is nothing
more than a deficiency of this fluid in a body. Another mode of
explanation is given by those who maintain, in opposition to Franklin,
that positive and negative electricity are either two distinct fluids,
or else vibrations of the same fluid—the positive electricity always
rushing out of a body, and the negative always rushing in. Those who
maintain this hypothesis endeavour to support it by the easy solution
which they affirm it gives to the phenomena of electric attraction and
repulsion. But after a careful examination of this theory, we think
that, so far from being satisfactory, it is scarcely intelligible. We
therefore do not choose to introduce it into our epitome, as affording
any solution of the difficulties that occur on this part of our subject.
We are besides of opinion that the evidence in favour of a single fluid
is conclusive, as we shall show when we come to discuss the theory of
electricity. Yet we confess that we cannot, on this theory, offer a
rationale of electric attraction and repulsion, that satisfies
ourselves. It is therefore the demand of candour, and in the spirit of
the Newtonian philosophy, to avow explicitly that this part of our
subject is yet involved in much obscurity. In the mean time we are
acquainted with certain facts, and with the clear explanation which they
give of certain phenomena.
1. That bodies positively electrified, repel each other.
2. That bodies negatively electrified, also, repel each other.
3. That bodies positively electrified, attract those which are
negatively electrified.
4. That bodies either positively or negatively electrified, induce a
contrary electricity in bodies in their natural state, brought within
the sphere of their action.
This statement is easily verified by experiment, in the following
manner.—By flaxen or hempen threads, suspend, from the prime conductor,
two balls made of cork or elder-pith, so that they touch each other. On
charging the conductor, these balls, being both electrified positively,
will immediately repel each other, and be separated to a considerable
distance.—Remove one of the balls, take it in your fingers, and bring it
near to the one which remains positively electrified, and the two will
immediately rush together; because there are now two substances of which
one is electrified positively, and the other negatively.—Again. Suspend
two balls, of the kind just mentioned, from an insulated cushion of an
electric machine, and let them touch each other. Put the machine in
motion and the balls, which are now both electrified negatively, will
repel each other and separate, as in the case first described.
In attempting to explain the first of these phenomena Dr. Franklin once
supposed that there was an electric atmosphere round each of the balls
positively electrified, the particles of which atmosphere, by mutually
repelling each other, separated the balls. He also supposed that as
bodies negatively electrified, or not having their proportional quantity
of the electric fluid, are always strongly disposed to receive it, this
would account for the fact that when one of these bodies was brought
near to one that had more than its proportional quantity, the two would
naturally rush together; the one to impart, and the other to receive the
fluid. But at this time he was not acquainted with the fact, that two
bodies negatively electrified would repel each other. When this was
discovered he candidly acknowledged the utter deficiency of his theory,
in regard to electric attraction and repulsion. Some of his friends and
followers, however, have endeavoured still to maintain it. But we think
that though their zeal has been greater, their success has not exceeded
that of the Doctor himself: and we have already stated that other
theories are equally, if not more defective, than that of Franklin. Let
us then leave the explanation of electric attraction and repulsion to be
made when future and fortunate discoveries shall have furnished the
means of making it, and let us proceed with the application of known
facts and principles.
A pleasing exhibition of the phenomena of electric attraction and
repulsion, may be made in the following manner.
Take a glass tube, and after having rubbed it, let a small light feather
fall from your fingers, at the distance of eight or nine inches from
it.—The feather will be immediately attracted by the tube and stick very
close to its surface for some seconds, after which it will be repelled,
and if the tube be kept under it, the feather will continue floating in
the air, at a considerable distance from the tube, without coming near
it again, except it touch some conducting substance; and if you manage
the tube dexterously, you may drive the feather through the air of the
room at pleasure.
The cause of this phenomenon is obvious. The feather, at first, not
being electrified, rushes to the excited tube. There it becomes
electrified and is then repelled, and cannot approach the tube again,
unless it first touch some conducting substance; because it cannot part
with its electricity while floating in the air, and therefore cannot
acquire a contrary electricity; consequently it must remain in a state
incapable of being again attracted by the excited tube.
There is a remarkable circumstance attending this experiment, which is,
that if the feather be kept at a distance from the tube by the force of
electric repulsion it always presents the same part towards the tube.
The reason of this phenomenon is, that the equilibrium of the fluid in
the different parts of the feather being once disturbed cannot easily be
restored; the feather being an electric, or at least a very bad
conductor. When the feather has acquired a quantity of electricity from
the tube it is plain that, by the action of the excited tube, that
superinduced electricity will, for the most part be forced to that side
of the feather which, at first, happened to be farthest from the tube;
hence that part will always afterwards be repelled the farthest.
This experiment may be agreeably varied in the following manner.—A
person may hold an excited tube of glass, within a foot and a half of a
stick of sealing-wax, or any other electric negatively electrified, held
by another person; a feather let fall between these differently excited
electrics will leap from one to the other alternately, and the two
persons will seem to drive a shuttlecock by the force of electricity.
Another experiment calculated to shew the phenomena of electric
attraction and repulsion is the _electric spider_.
Cut a piece of cork in the shape of a spider, and run a few short
threads through it, to represent the legs; this done, suspend it by a
silk thread from the ceiling of the room, or any other support, so that
the spider may hang mid-way between the knob of a jar and the knob of a
wire fastened to the table, or to the outside coating of the jar when
not charged; let the place where the jar stands be marked; then charge
and replace it. The spider will now begin to move from knob to knob, and
continue this motion for a considerable time.
In this case, the knob of the jar is charged positively, and the spider,
being in its natural state, is attracted by it; the knob then
communicates to it some of its electricity, and the spider becoming
possessed of the same electricity with the knob, is repelled by it, and
immediately runs to the other knob, which communicates with the negative
coating, or with the table, where it discharges its electricity and is
again attracted by the knob of the jar. This attraction and repulsion
continue till the jar is discharged, when the spider finishes its motion
and seemingly expires.
CHAP. IX.
_Of the Leyden phial._
This consists of a glass phial, jar, or bottle, coated on the outside
and inside with tin-foil, rendered adhesive by paste or gum water. About
two inches of the glass at the top are left without any metallic
covering, to prevent a communication between the outside and inside
coatings, while the electricity is collecting.—The mouth of the phial or
jar is furnished with a cork which receives a wire, ending in several
ramifications which touch the inside coating. The upper end of this
wire, which should extend a convenient distance above the mouth of the
jar, is furnished with a metallic ball.
When the phial or jar is to be charged, it may be held in the hand or
placed on an uninsulated table, with the knob of the wire touching the
prime conductor. The inner surface of the glass now acquires the same
electricity with the prime conductor, and the external one acquires a
contrary electricity by means of its uninsulated coating.
When a phial similar to the one above described is highly charged, a
spontaneous discharge will usually take place over the uncoated surface,
and seldom through the glass. But if the uncoated surface be left larger
than from two to three inches, the phial is more apt to crack and become
useless, by the charge passing through the glass. There is not however
an absolute certainty that a jar which has once discharged itself over
its surface will not, at another time, break by a discharge through the
glass.
It was long disputed whether the discharge of the Leyden phial resided
in the coating or in the electric. The following experiment clearly
decides, that its residence is in the electric.
Upon an uninsulated plate of metal, lay a plate of glass considerably
larger, so that there may be a rim of three or four inches projecting
beyond the metal. Upon the glass lay another piece of metal, of the same
size with the first, and so as precisely to cover it.
Let this instrument be charged, by connecting the upper metallic plate
with the prime conductor. Then separate the metallic plates from the
glass; and upon examination the glass will be found to possess the
contrary electricities on its opposite sides; that side which during the
electrization communicated with the prime conductor will have a like
electricity with it, and the other the contrary.
Discharge the electricity of the metallic plates, and replace the whole
apparatus in its former situation.—Take a discharging rod, formed by a
piece of bent wire with a metallic ball at each end; touch the under
plate and bring the other end of the wire near the upper plate. The
consequence will be, that a strong and loud spark will pass between the
upper plate and the discharging rod; the electricity of the glass will
be discharged, and there will afterwards remain no signs of electricity,
either in the glass, or the metallic plates.—Hence it appears that the
electricity resides in the glass, and that the coatings, whether in a
plane or spherical form, are of no other use than to convey the electric
fluid to the glass; to keep it equably distributed over the surface; and
to form a communication between the different parts of the electrified
glass, so that the discharge from them may be simultaneous.
When the discharge of a coated electric is made through the body of a
living animal, it occasions a sudden motion, by contracting the muscles
through which it passes, and gives a disagreeable sensation commonly
called the _electric shock_.
CHAP. X.
_The electrical battery—and experiments performed with it._
When a greater degree of electric force is required than a single jar is
capable of giving, the electrical battery is made use of as part of the
apparatus, which takes its name from the formidable effects it produces.
This battery consists of a number of coated jars, placed in such a
manner that they may all be charged at the same time, and discharged in
an instant; so that the whole force of electricity accumulated in them,
may at once be exerted on the substance exposed to the shock.
In discharging electrical jars, the electricity goes in the greatest
quantity through the best conductors, and by the shortest passage. Thus
if a chain and a wire be made to communicate at the same time with the
outer coating of a jar, and be both presented to the knob of that jar,
the greater part of the charge will pass by the wire, and very little by
the chain, because the latter is a worse conductor than the former, on
account of its discontinuation at every link. When the discharge is made
by the chain only, sparks are seen at every link, which is a proof they
are not in contact.
The force of an electric shock is not affected by the inflections of a
conductor through which it passes, though it is sensibly weakened by its
length. Hence, when the circuit or communication between the two sides
of a Leyden phial is formed by one person applying his hands to the
different sides, the shock is stronger than when it is formed by many
persons joining hands. Yet a considerable shock was given by the Abbè
Nollet, in the presence of the king of France, to one hundred and eighty
men; who formed an electrical circuit.—They were all shocked in the same
instant.
Doctor Watson and many other gentlemen of eminence in science, were at
the pains of making experiments of the same kind. They found, by means
of a wire insulated on baked wood, that the electric shock was
transmitted instantaneously through the length of 12,276 feet.
Electricity transmitted in large quantities through living vegetables,
destroys their vegetable life.
When transmitted, in the same form, through animals, it generally puts
an end to animal life; though it is said that there are individuals who
are not affected by it. Possibly the reason why some persons are not
killed by very large electric shocks is, that their muscular system, or
bodily organization, has something peculiar which protects them.
If an electrical circuit be made by means of imperfect conductors, as a
slender piece of wood, a wet pack-thread, the discharge will be made
silently.
If a small interruption of an electrical circuit be made in water, on
making the discharge, a spark will be seen in the water, which never
fails to agitate it and sometimes breaks the vessel in which it is
contained.
A strong shock from a battery, sent through a slender piece of metal,
instantly makes it red hot. Usually it is melted in whole or in part. If
the fusion be perfect it is reduced into globules of different
magnitudes. In this experiment it is a little remarkable that the parts
of the metal at which the fluid enters and issues, are most likely to be
melted.
If the metal be enclosed between pieces of glass, the shock will force
the melted metal into the substance of the glass, so that it cannot
afterwards be removed, without scraping off part of the glass with it.
In this experiment the glasses which enclose metal are commonly broken
to pieces.—It is seldom that they resist the force of a strong shock. If
the glasses enclosing metal be pressed by a heavy weight, a small shock
is often sufficient not only to raise the weight, but to break glasses
of considerable thickness. When the pieces of glass are not broken, they
are marked by the explosion with the most lively prismatic colours,
which lie sometimes irregularly, and sometimes in their prismatic order.
Gun-powder may be fired by a charge from three square feet of coated
glass. The powder is to be put into a quill, and then a wire is to be
thrust into each end so as nearly to meet, and afterwards these wires
are to be made a part of an electrical circuit.—A less charge of
electricity will be sufficient if iron filings be mixed with the
gun-powder.
When a shock somewhat less than is sufficient to melt a piece of metal
is sent through a chain, a black dust, in the form of smoke, is seen to
proceed from the chain. This dust is probably some of the metal itself,
partly calcined, and by the violence of the explosion forced from it. If
the chain be laid upon a piece of paper, glass, or other electric, this,
after the explosion, will be found stained with some indelible marks,
and often shew evident signs of having been burnt.
What is more remarkable in considering the effects of electricity on
metals is, that it often, in a considerable degree, revivifies their
calces or oxyds. In making experiments of this kind, the metallic calx
or oxyd is to be made a part of an electrical circuit, through which a
strong shock is to be sent: when the calx or oxyd will be found in a
measure restored to its metallic state: the electric shock having, as it
appears, taken away from the oxyd a portion of its oxygen.
The electric shock when passed through the magnetic needle, sometimes
destroys its magnetic virtue, and sometimes reverses its poles. It is
affirmed that two ships sailing together on the same voyage, were led,
from the effect of lightning on their needles, to steer exactly opposite
courses, after the storm in which they were exposed to the lightning had
subsided. When the charge of ten, eight, or even a less number of square
feet of coated glass, is sent through a sewing needle, it will often
give it polarity, so that it will traverse when laid upon water. In this
experiment it is remarkable that if the needle be lying east and west,
that end of it which communicated with the positive coating will point
towards the north; but if the needle be struck while lying north and
south, that end of it which lay towards the north, will, in any case,
point north; and the needle will acquire a stronger virtue in this than
in the former case. But if the needle be placed perpendicular to the
horizon, and the electric shock be given to either point of it, the
lower extremity will afterwards point north.
The electric explosion taken upon the leaves of certain flowers changes
their colour.
If the ball of a thermometer be placed in a strong current of
electricity, the mercury or spirit will rise several degrees.
If a thin bottle be exhausted of air by means of an air pump, it will
receive a considerable charge of electricity, by applying its bottom to
an electrified prime conductor. In performing this experiment the bottle
is to be held by the neck or near the mouth, and the electric matter
will pass through the vacuum, and along the inner surface of the bottle,
to the hand, from that end of it which is nearest to the prime
conductor. The luminous appearance exhibited by this experiment is
exceedingly beautiful in the dark, especially if the bottle be of any
considerable length. It exactly resembles those lights which appear in
the northern sky, and which are called streamers or the aurora borealis.
If one hand be applied to the part of the bottle which was before
presented to the prime conductor, while the other remains at the neck, a
shock will be felt, at which instant the natural state of the inner
surface is restored by a flash, which is seen pervading the vacuum
between the two hands.—The principle on which this experiment depends
will be explained hereafter.
CHAP. XI.
_A description of the electrophorus, and some of its phenomena accounted
for._
The electrophorus is a machine, consisting of two plates, usually of a
circular form. At first the under plate was of glass covered with
sealing wax; but there is little occasion to be particular, with regard,
either to the substance of the lower plate, or to the electric with
which it is covered; a metallic plate however is preferable to a wooden
one, though the latter will answer very well. This plate must be covered
with an electric: pure sulphur answers nearly as well as the dearer
electrics gum lac, sealing wax &c.
The upper plate is made of brass, or a piece of paste-board covered with
tin foil or silvered paper, which must be nearly of the same dimensions
as the electric plate: this plate must be furnished with an electric
handle, which, by means of a metallic or wooden socket is fastened to
its centre.
This instrument was invented by Mr. Volta, an Italian philosopher. The
manner of using it is as follows.