piece of new white flannel, or a fox’s tail. A hard shoe brush, having
the bristles a little greased, will also excite sulphur very well. When
this plate is excited as much as possible, it is placed on a table with
the electric side uppermost; the metallic plate is then laid on the
excited electric; then the metallic plate is touched with the finger, or
with any other conducting substance, which receives a spark from it;
finally the metallic plate being held by the extremity of its electric
handle, is separated from the electric and after it is raised some
distance, it is, on examination, found strongly electrified, with an
electricity contrary to that of the electric, and will give a strong
spark to a conductor brought near it. By placing the metallic plate upon
the electric, touching it with the finger and separating them
successively, a great number of sparks may be obtained, apparently of
the same strength, and without exciting the electric again.—If these
sparks be repeatedly given to the knob of a coated jar, it will become
charged.
The action of these plates depends upon the principle already laid down
(page 22,) that an excited electric has the power of inducing a contrary
electricity in a body brought within its sphere of action. The metal
plate therefore, when set upon the excited electric, acquires a contrary
electricity, by giving its electric fluid to the hand or other conductor
which touches it, when the electric is positively electrified; or by
acquiring an additional quantity from the hand &c. when the electric is
negatively electrified.
More fully to explain the principle here considered let the following
easy experiment be made—
Electrify any insulated conductor _positively_. Then if an
electrometer[15] of cork balls be held at some distance from it, the
balls will diverge with _negative_ electricity. This may be proved by
bringing a piece of excited glass near them, as the balls will be
attracted by it. But if you present to them a piece of excited sealing
wax, they will immediately avoid it—that is, supposing the glass to be
excited always positively, and the sealing wax always negatively.
Again. Insulate, in a horizontal position, a metallic rod with blunt
terminations, and about two feet long. We shall designate the ends of
this rod by A and B. Let a cork ball electrometer be affixed to the
extremity A; then bring an excited glass tube within eight or ten inches
of the other end B—the balls will immediately diverge with positive
electricity. If the tube be removed the balls will immediately collapse,
and no electricity will remain in them, or in the rod.—But if, while the
tube is near one end B of the rod, and the balls diverge with positive
electricity, the other end A be touched with a finger or other
uninsulated conductor, the cork balls will immediately come together, as
if the rod were in its natural state: but if, in this state of things,
the excited tube be removed, the balls will again diverge, but with
negative electricity, shewing that the whole rod AB is now
under-charged.
This last experiment is thus explained.—When the rod is in its natural
state, the electric fluid proper to it is equably distributed throughout
the rod; but when the excited glass tube is brought near one of its ends
as B, the fluid belonging to that end will be driven towards A; which
extremity becomes over-charged, and the other extremity B under-charged;
yet the rod has no more electricity now than it had before, and when the
tube is removed beyond the sphere of its action, the redundant fluid of
A returns to its former place B, and the equilibrium is restored. But if
the extremity A be touched, while it is over-charged, by a conductor,
this will carry off its superfluous fluid, and leave the extremity A in
its natural state, the extremity B being at the same time negatively
electrified: and when the tube is removed, part of the fluid naturally
belonging to A goes towards B, and the whole rod remains under-charged.
CHAP. XII.
_Of electrometers._
We have already seen that it is a general law of electricity, that
similar electricities repel, and that dissimilar electricities attract
each other.—On this law all electrometers are constructed. In fact the
cork balls, which have been mentioned are electrometers, and exhibit at
once the most important phenomena for the explanation or ascertaining of
which the instruments which bear this name are constructed. Still it is
of use to see the application which may be made of this general
principle. It is applied to ascertain the quantity of the electric fluid
collected either in a prime conductor or a coated jar; and also the
state of the atmosphere in regard to electricity, and the character of
that electricity at any particular time and place.
The instruments by which these purposes are effected we shall now
shortly describe.
To ascertain the quantity of electricity in a prime conductor or jar, an
electrometer the most easily constructed and of the most general use has
been invented by Mr. Henley—called the quadrant electrometer.—Of this we
have given a representation in the frontispiece, (letter X.)
It consists of a perpendicular stem formed at the top like a ball, and
at the lower end with a screw, by which it is fastened to the prime
conductor. A graduated semicircle of ivory, horn or stiff paper, is
fixed near the uppermost end of the stem. A moveable index, made of a
slender piece of hickory, extends from the centre of the graduated
semicircle a little distance beyond its circumference, having a small
ball of cork or pith at its lower extremity.
When the conductor or jar is not electrified, the index is parallel to
the stem, but when it is electrified the index recedes more or less,
according to the degree of the electrization, which is marked on the
graduated circle.
A simple atmospheric electrometer was constructed by Mr. Cavallo in the
following manner.—
To the end of a common fishing rod, he affixed a slender glass tube
covered with sealing wax, and having a cork at its end, from which two
cork or pith balls were suspended by hempen strings. From the other end
of the rod proceeded a flaxen or hempen twine a little longer than the
whole rod and tube, with a pin attached to it, which was stuck into the
cork at the extremity of the glass tube, for the purpose of taking off
the insulation. The twine, to prevent its falling when the pin was
pulled out of the cork, was attached to the rod, by a small string,
running from it and meeting the rod at a little distance from the glass
tube.
To use this instrument, let the pin be pushed into the cork. Then,
holding the rod by the extremity farthest from the cork balls, project
it out, from a window in the upper part of the house, into the air,
raising the end of the rod to which the balls are appended, so as to
make an angle of 50° or 60°, with the horizon.—After having kept it in
this situation a few seconds, by pulling the twine, detach the pin from
the cork.—This leaves the electrometer insulated, and electrified with
an electricity contrary to that of the atmosphere. Now draw the
instrument into the room and you may examine the quality of the
electricity, by applying the knob of a phial positively charged to one
of the balls; if the ball is attracted by the knob it is negatively
electrified—if repelled, positively electrified.
The satisfaction arising from these experiments is sometimes abated,
from the circumstance that the quantity of electricity obtained in this
way, is so small that its quality cannot be ascertained. To remedy this
inconvenience Cavallo and Nicholson, have invented machines which they
denominate _doublers_ or _multipliers_ of electricity. But the structure
of these machines is complex and delicate, and the explanation of them
is long, and not easily understood without the aid of plates. Our
epitome therefore does not admit of inserting them. Those who may choose
to pursue the subject we refer to the writers above mentioned.
To prevent the inconvenience arising from wind and rain in the use of
the atmospheric electrometer, the following device has been used by Mr.
Cavallo.—Take a glass vessel open at top and bottom—cement it at bottom
to a convenient piece of wood—let the upper part be tapering like the
neck of a phial, and cement into it a glass tube, extending a little
above and a little below the neck of the larger vessel. Cover the tube
with sealing wax, both within and without the neck of the vessel, so as
to give it the appearance of one body. Into this tube cement a brass
wire extending a very little below the bottom of the tube, and flattened
at the lower end so as to be perforated with two small holes. Through
these holes insert two flaxen threads, or two very fine silver wires,
with small balls of cork or pith at the end of them, and touching each
other:—if wires are used they should be suspended by small rings at the
top, that they may act more easily. Let the top of the brass wire screw
into a brass cover on the top of the whole vessel, which cover will not
only secure the vessel against rain, but serve as a conductor to a very
slightly electrified atmosphere—conveying the fluid, first to the wire,
and by means of that to the balls, which will exhibit, within the
vessel, the state of electricity collected from the atmosphere. There
should be two narrow slips of tin foil stuck to the inside of the glass
vessel, and communicating with the wooden bottom, which will serve to
carry off that electricity which, when the corks touch the glass is
communicated to it, and which, if accumulated, would disturb the free
motion of the corks.
An useful alteration of this electrometer was made by Mr. Bennet. It
consists of slips of gold leaf or silver leaf, instead of the corks
suspended by threads or wires. These slips of leaf are to be suspended
from the cover of a cylindrical vessel, and hanging within it. The slips
of leaf are to be about two and an half inches long. This electrometer
is the most sensible instrument of the kind, manifesting in an
unequivocal manner very small quantities of electricity. But this
instrument is not as portable and easily managed as the other.—If very
fine threads, stiffened with glue, be used without any balls, they will
be found nearly as sensible as the gold leaf.
CHAP. XIII.
_The identity of electricity with lightning._
The identity of the electric matter with lightning is a discovery, which
has been of more use than any other in electricity.
That the effects of this fluid bore a great resemblance to those of
lightning, had been several times remarked by philosophers and
especially by the Abbè Nollet; but that they should be found to be
effects of the same cause, and that the phenomena of electricity could
be imitated by lightning, or those of lightning by electricity, was not
suspected, till our countryman Dr. Franklin made the assertion in 1750,
and afterwards demonstrated its truth by undeniable experiment in 1752.
This discovery is almost the only one in the whole science which has not
been the result of accident.
The Doctor had for a long time observed the effects of pointed bodies in
drawing off the electric matter more powerfully than could be done by
others.—Improving upon this, he supposed that pointed iron rods, raised
to a considerable height in the air, when the atmosphere was loaded with
lightning, might “draw off the matter of the thunder-bolt, without noise
or danger.” As he was waiting for the erection of a spire in
Philadelphia, that he might have an opportunity of ascertaining the
correctness of his hypothesis, it occurred to him, that, by means of a
common kite, he could have a readier access to the higher regions of the
atmosphere than in any other way. Preparing therefore a large silk
handkerchief, and two cross sticks upon which he might easily extend it,
he took the opportunity of the first approaching thunder storm to walk
into a field, where there was a shed convenient for his purpose; but,
dreading the ridicule which too commonly attends unsuccessful attempts
in science, he communicated his design to no one but his son, who
assisted him in preparing and raising the kite.
A considerable time elapsed before there was any appearance of success:
one very considerable cloud had passed over the kite without any effect;
when, just as he was beginning to despair, he observed some loose
threads of the hempen string to stand erect, and avoid one another just
as if they had been suspended from the prime conductor of an electrical
machine. On this he presented his knuckle to a key which was fastened to
the string, and received a spark. Others succeeded even before the
string was wet; but when the rain began to fall he collected the
electrical fire very copiously.
He afterwards had an insulated iron rod, to draw lightning into his
house, and performed almost every experiment with real lightning, that
he had before made with electricity collected by a machine. Thus a new
field was opened for the philosophy of electricity.
CHAP. XIV.
_Of the structure and use of the electrical kite._
In the structure of an electrical kite, the circumstances to be
principally attended to are those near, and on the ground. Silk being a
non-conductor, the end of the string which is held in the hand is to be
of that substance—a silk handkerchief tied to the hempen twine of the
kite will answer very well. An iron key is to be tied on the hempen
string, an inch or two above its junction with the silk, and from this
key, when the kite is electrified, the sparks are to be received into a
Leyden phial, to be used in the same manner as if it had been charged
from the electrical machine. As curiosity may prompt many to repeat the
experiments made with this kite, and as no experiments with atmospheric
electricity can be made without some danger,[16] we shall give the
substance of Mr. Cavallo’s directions (the best we are acquainted with)
relative to the forming and using of this instrument.—He observes that
the whole power of the machine lies in the string: and that in other
respects a common school boy’s kite, will answer the purpose as well as
any other. The string is made by twisting two threads of twine with one
of brass wire or copper, such as is commonly used for trimmings. When a
kite constructed in this manner was raised, the string always gave signs
of electricity except once, when the weather was warm, and the wind so
weak that the kite could not be kept up for a few minutes; afterwards,
however, when the wind increased, he obtained as usual a considerable
quantity of electricity.
Concerning the management of this kite he gives the following
directions.—
In raising the kite when the weather is very cloudy and rainy, at which
time there is much danger of meeting a great quantity of electricity, I
usually hang upon the string a chain with one extremity touching the
ground; and sometimes I use another caution besides, which is, to stand
upon an insulated stool; in which situation, I think that if any
quantity of electricity, suddenly discharged by the clouds, strikes the
kite, it cannot much affect my person. Although I have raised my
electrical kite a hundred times without any caution whatever, I have
very seldom received a few exceedingly slight shocks in my arms. In time
of a thunder storm, if the kite has not been raised before, I would not
advise a person to attempt it while the stormy clouds are over head, the
danger at such time being very great, even when every caution is used.
At that time the electricity of the clouds may be observed by means of a
cork ball electrometer, placed in an open situation.
But Mr. Cavallo with all his caution could not avoid danger in making
experiments on atmospheric electricity, as appears from the following
account of his observations on the 13th of October 1773. “After having
rained a great deal in the morning and the night before, the weather
became a little clear in the afternoon, the clouds appearing separated
and pretty well defined; the wind was west and pretty strong; the
atmosphere was in a temperate degree of heat. In these circumstances, at
three o’clock P. M. I raised my electrical kite, with 360 feet of
string. After the end of the string was insulated, and a leather ball
coated with tin foil, hung to it, I tried the power and quality of the
electricity, which appeared to be positive and pretty strong; in a short
time a small cloud passing over, the electricity increased a little; but
the cloud being gone it returned pretty soon to its former degree.
The string of the kite was now fastened by a silk string to a post in
the yard of the house; I was repeatedly charging two phials, and giving
shocks with them: while I was so doing, the electricity, which was still
positive, began to decrease, and in two or three minutes it became so
weak, that it could hardly be perceived, with a very sensible cork ball
electrometer.—Observing at the same time that a large black cloud
approaching the zenith, (which no doubt caused the decrease of
electricity) indicated rain, I introduced the end of the string through
the window on the first floor, where I fastened it by the silk to an old
chair.—The quadrant electrometer was set upon the same window, and was,
by means of a wire, connected to the string of the kite. Being now three
quarters of an hour after three, the electricity was actually
imperceptible, however in about three minutes it returned, but now upon
examination, it was found to be negative, which was evidently occasioned
by the approach of the cloud, which by this time had reached the zenith
of the kite; the rain also began to fall in large drops. The cloud came
farther on, the rain increased and the electricity keeping pace with it,
the electrometer soon arrived at 15°. Seeing now that the electricity
was strong, I began again to charge the phials and to give shocks with
them; but the phials had not been charged more than three or four times,
before I perceived that the index of the electrometer was arrived at
35°, and was still rising. The shocks now being very smart, I desisted
from charging the phials, and considering the rapid advance of the
electricity, thought to take off the insulation of the string, that if
it should farther increase it might be conducted silently to the earth,
without occasioning any bad accident.
To effect this, as I had no proper apparatus near me, I thought to
remove the silk string, and to fasten the twine itself to the chair. I
disengaged the wire which connected the electrometer with the string;
untied it from the silk, and fastened it to the chair: but while I was
effecting this, which took up less than half a minute, I received twelve
or fifteen very strong shocks, which I felt all along my arms, in my
breast, and legs, shaking me in such a manner that I had hardly power to
effect my purpose, or to warn the people of the room to keep their
distance. As soon as I took my hands from the string, the electricity
(in consequence of the chair being a bad conductor) began to snap
between the string and the window shutter, which was the nearest
conductor. The cloud was now just over the kite; it was black, well
defined, and nearly of a circular form, its diameter appearing to be
about 40°; the rain was copious but not remarkably heavy.
As the cloud was going off, I went near the string, and finding the
electricity weak, but still negative, I insulated it again, thinking to
keep it up some time longer; but observing that a larger and denser
cloud was approaching, I resolved to pull the kite in; accordingly a
gentleman, who was near me, began pulling it while I was winding up the
string, he told me he had received two or three slight shocks in his
arms, and if he should feel one more, he would let the string go; upon
which, I pulled the kite in as fast as I could myself, without any
further observation, being ten minutes after four o’clock.
N. B. There was no thunder or lightning perceived that day, nor for some
days before, nor afterwards.
The general laws which Mr. Cavallo deduced from a variety of experiments
made by means of electrical kites, are the following:
1st. The air appears to be electrified at all times; its electricity is
always positive and much stronger in frosty than in warm weather; it is
by no means less in the night than in the day time.
2d. The presence of the clouds generally lessens the electricity of the
kite, sometimes it has no effect upon it, and it sometimes, though
rarely, increases it a little. To this the above mentioned instance is a
remarkable exception.
3d. When it rains, the electricity of the kite is generally negative,
and very seldom positive.
4th. The aurora borealis seems not to affect the electricity of the
kite.
5th. The electrical spark taken from the string of the kite, or from an
insulated conductor connected with it, especially when it does not rain,
is seldom longer than the quarter of an inch; but it is exceedingly
pungent. When the index of the electrometer is not higher than 20° the
person who takes the spark will feel it in his legs; it appearing more
like the discharge of an electrical jar, than the spark taken from the
prime conductor of an electrical machine.
6th. The electricity of the kite is generally stronger or weaker,
according as the string is longer or shorter; but it does not keep any
exact proportion to it; the electricity, for instance, brought down from
a string of an hundred yards, may raise the index of an electrometer to
20°, when with double the length of string, the index of an electrometer
will not go higher than 25°.
7th. When the weather is damp, and the electricity pretty strong, the
index of an electrometer, after taking a spark from the string, or being
presented to the knob of a coated phial, rises surprisingly quick to its
former place; but in dry and warm weather it rises exceedingly slowly.
CHAP. XV.
_The structure and use of lightning rods._