matter, long rods of iron, or other metals, have been made use of, with
a view to protect buildings from the effects of lightning. This is the
most practical and important part of our whole subject, and deserves to
be treated with the utmost attention. Iron and copper are the metals
which, on account of their conducting power, their cheapness, and the
quantity required for a lightning rod, are principally used. Copper is
preferable to iron. Care should be taken that the rod be not less than
half an inch in diameter. It is best to have it, if possible, of one
continued piece. If this be not practicable, the pieces should be
screwed into each other; or at least so constructed that the rust will
not separate the perfect metal of one piece from that of another;
because metallic rust is almost a non-conductor of electricity. The rod
should be fastened to the house by wooden cramps or staples, rather than
by those of metals of any kind; because wood is neither so good a
conductor of electricity, nor so likely to promote the rust of the metal
which it touches. The rod should be raised above the top of the building
or chimney to which it is attached, at least five or six feet. The point
or points should be made very sharp, and for a few inches should taper
off in the form of a pyramid, having all the corners or edges sharp. It
is not of much importance whether there be, or be not, more points than
one. If the means afterwards to be mentioned be not used to preserve the
points from rust, it may be of use to gild them; and the gilding should
extend downwards a foot or more. It is better to paint the point of a
rod, than to leave it wholly unprotected against rust. The lower end of
the rod should be driven or sunk at least five or six feet into the
ground, and in a direction from the building. If it can be connected
with the water of a spring, a well, or a cistern, it will be so much the
better. At powder-mills, arsenals, and all depots of inflammable
materials, it is better to attach the rod to a post, raised for the
purpose, a foot or two from the building, than to the building itself.
If the building be large, there should be a rod at each end; and it is
an additional security, if these rods be connected by a piece of metal,
running from the one to the other, on the roof of the house. If there be
but one rod, it should, in this country, be put on the western end of
the house; because thunder storms oftenest arise from that quarter. If
the position of the house affords but little choice in this respect, the
rod should be placed either on the kitchen chimney, or as near to it as
possible; because smoke and heat are conductors, and in the summer,
smoke and heat seldom ascend from any other chimney than that of the
kitchen. When there is a copper spout to a house, the rod, if
convenient, may be connected with it as a part of the conductor. In this
case however, care should be taken to make the connexion complete, both
at top and bottom. Large barns and barracks, have even more need of a
rod to preserve them from lightning than a dwelling house, because the
vapour which ascends from them when filled with vegetable substances,
imperfectly dried, is a powerful conductor.
Ships, and all vessels which have high masts, have as much need of
conductors as houses on the land. Copper conductors are in every view
the best for ships, as they will not contract rust from sea water. A
conductor, of this metal, should be attached to the highest mast of the
vessel, and extend three or four feet above its top. It should be
inserted into the side of the mast, so as to leave the surface smooth,
be carried across the deck and over the side of the ship to the keel; so
that it may terminate where the lower extremity may be always under the
water. Chains are often used as conductors to ships, but they are far
inferior to a piece of metal, whose parts are not separated.
In the above directions it has been our aim to show in what manner
structures may be best and most effectually protected against danger
from lightning, and whenever it is practicable the best means ought
certainly to be used. But it is to be remembered that where means the
most effectual cannot be applied, those of an inferior kind are not to
be neglected. A small rod, however pointed or fastened to a house, is
unspeakably better than none, and a chain should always be used in a
ship, if a rod cannot be obtained. In ninety nine cases out of a
hundred, any metallic conductor, reaching from the top to the bottom of
a structure, will preserve it from destruction by lightning, and save
the lives or property of the inhabitants, when the whole might otherwise
have been destroyed.
The points of rods have often been found melted by lightning, and both
they and the lower extremities are often injured by rust. For an
effectual method of preventing both these inconveniencies, the public
are indebted to Robert Patterson Esq. professor of mathematics in the
University of Pennsylvania, and director of the Mint of the United
States.—His memoir on the subject is as follows:—
“From the instances which now and then occur of houses being struck with
lightning, that are furnished with metallic conductors, and the frequent
instances of these conductors having their tops melted off by a stroke
of lightning, it appears that this admirable contrivance for guarding
houses against the dangerous effects of lightning is, in some degree,
still imperfect. Some improvement seems yet to be wanting at both
extremities of the rod—at the upper extremity, to secure it against the
accident of being melted, which renders it afterwards unfit to answer
its original intention, viz. drawing off the electricity, or lightning,
from the passing cloud, in a silent imperceptible manner, for it is only
_pointed_ conductors that possess this property—and at the lower
extremity, to afford a more ready passage for the fluid into the
surrounding earth.
The first of these intentions, would I am persuaded, be effectually
answered by inserting in the top of the rod a piece of _black lead_, of
about two inches long, taken out of a good pencil, and terminating in a
fine point, projecting but a very little above its metallic socket; so
that if the black lead point should happen to be broken off by any
accident, of which however I think there can be but little danger, still
the point of the rod would be left sharp enough to answer the purpose of
a metallic conductor.
This substance is well known to be infusible, by the greatest heat, and
hence its use in making crucibles; nor is it evaporable as remarked by
Cronstedt, in his mineralogy, Sect. 231, except in a slow calcining
heat, to which it could never be exposed at the top of a lightning rod.
At the same time its power as a conductor of electricity is perhaps
equal, or but little inferior, to that of any of the metals. A line
drawn on a piece of paper by a black lead pencil, will as I have often
experienced, conduct an electric explosion seemingly as well as a
similar line of gilding would do, and that without ever loosing its
conducting power, which is not the case with gilding.
The second intention is, to facilitate the escape of the electric fluid
from the lower part of the rod into the surrounding earth.
It is in many cases impracticable, from the interruption of rocks or
other obstacles, to sink the rod so deep as to reach moist earth, or any
other substance which is a tolerably good conductor of electricity. Nor,
even if this were practicable, would it, I presume, be alone sufficient
to answer the desired intention. Iron, buried in the earth, and
especially in moist earth, will presently contract a coat of rust, which
will continually increase till the whole is converted into rust, but
rust of iron, and indeed the calx of all metals is a non-conductor, or
at most but a very imperfect conductor of the electric fluid. Hence it
is easy to see, that in a few years after a lightning rod has been
erected, that part of it which is under ground will contribute little or
nothing towards the safety of the building. Besides, the surface of this
part of the rod is too small to afford an easy and copious discharge of
the electric fluid into the surrounding earth, when this is but an
imperfect conductor.
As a remedy for these defects I would propose, that the parts of the rod
under ground be made of tin, or copper, which are far less liable to
corrosion or rust, by lying under ground than iron.—Or, which perhaps
would answer the purpose better, let this end of the rod, of whatever
metal it be made, be coated over with a thick crust of black lead,
previously formed into the consistence of paste, by being pulverised and
mixed with sulphur (as in the manufactory of the ordinary kind of black
lead pencils) and then applied to the rod while hot. By this means, the
lower part of the rod would, I apprehend, retain its conducting power
for ages, without any diminution.
In order to increase the surface of the lower part of the conductor, let
a hole or pit, of sufficient extent, be dug as deep as convenient; and
into this pit let there be put a quantity of _charcoal_, round the lower
extremity of the rod. Charcoal possesses two properties, which, in a
peculiar manner, fit it for answering the purpose here in view.—(1st.)
It is a very good conductor of electricity and, (2d.) It will undergo
little or no change of property by lying ever so long in the earth. Thus
might the surface of that part of the conductor, in contact with the
earth, be increased, with little trouble or expense to any extent at
pleasure; a circumstance which every one acquainted with electrical
experiments, must acknowledge to be of great importance to the end here
proposed.”
The following experiments with a thunder-house, shew the utility of
lightning rods, and ascertain what termination of the rod best answers
the end proposed.
_To shew the effect of lightning on a house not furnished with a
conductor, or when the conductor is discontinued._
Provide yourself with the model of a house made of tin, four inches in
breadth, six long, and about five in height. Let there be a chimney
placed in the roof equidistant from both ends, and let a glass tube pass
through it, the upper extremity of which must reach a little above the
chimney, and the lower one come within an inch of the floor of the
house.—Let a small wire pass through the bore of the glass tube, the
upper end of which must extend a small distance above the orifice of the
tube, having its extremity, which must be pointed, furnished with a
screw, on which a metallic ball is to be fastened. The other end must
likewise have a ball fixed upon it.—The instrument being thus prepared,
fill the house with cotton, and sprinkle a little powdered rosin on that
part of it, which is immediately between the lower knob of the wire, and
the floor of the house. Then connect the lower part of the instrument
with the outside coating of a pretty large jar.—From the prime
conductor, in order to represent the clouds, suspend a small scale beam,
having two balls of metal or wood coated with tin foil, in the place of
the scale dishes, nicely balanced. The knob of the jar being connected
with the prime conductor; bring the ball on the wire extending through
the glass tube, under one of the balls representing a cloud.—Now charge
the jar. The cloud will be attracted by the ball on the wire—the
electricity of the cloud will be discharged—and if the experiment
succeeds, the contents of the house will be set on fire.
_The effects of lightning, when a house is furnished with a pointed
conductor._
Repeat the above experiment with this variation: unscrew the ball from
the upper extremity of the wire of the house, so that it may remain
pointed. Place the house under the cloud as in the former
experiment.—You will now find it impossible to charge the jar: or if you
charge the jar before the house is placed under the cloud; the cloud,
instead of being attracted by it, will be repelled, and the jar will be
discharged without any explosion, and without firing the cotton.
These two experiments evince that _pointed_ conductors are more proper
to secure houses from the effects of lightning that those terminating
with a ball or knob, and that if the pointed conductors fairly act on
the cloud the security is complete.
CHAP. XVI.
_Of animal electricity._
The electric power, observed by the ancients only in amber, and perhaps
the tourmaline, was in process of time found to be in glass, rosin,
silk, and several other substances. By degrees it was discovered, that
very strong signs of electricity were exhibited by a number of animals.
The experiment of producing sparks of electrical fire, by rubbing the
back of a cat in frosty weather, proved that electricity might exist in
a very active state in the bodies of animals, without injuring their
functions. From animals of an inferior kind a transition was made to the
human species. Some people were observed to have a remarkably bright
lustre of their eyes, others were found to be so strongly electrified
naturally, that a very sensible electrometer was perceptibly affected,
when brought near them.—Others, it is affirmed, were found so sensible
to the presence of electricity, as to be affected by a flash of
lightning, though so distant that the thunder could not be heard. But
what principally claims our attention in regard to this part of our
subject is, that there are unquestionably certain animals which can at
pleasure give an electric shock, of sufficient force to kill other small
animals, and that in fact they often do it. We shall describe only three
of the most remarkable of these electric animals—the Gymnotus
electricus, the Torpedo, and the Silurus electricus.
The Gymnotus is a genus of fishes, belonging to the order of apodes.
They have two tentacula at the upper lip; the eyes are covered with the
common skin.—There are five rays in the membrane of the gills; the body
is compressed, and carinated on the belly with a fin. There are five
species; the most remarkable is the _electricus_, commonly called the
_electric eel_. This species is peculiar to the Surrinam river, and they
inhabit the most rocky parts of it, at a considerable distance from the
sea.—The most accurate description of this fish, is in the Philosophical
Transactions, for 1775, where Alexander Garden M. D. gives an account of
three of them brought to Charleston in South Carolina. The largest was
about three feet eight inches long, and from ten to fourteen inches in
circumference, about the thickest part of the body. The head was large,
broad, flat, and smooth, impressed here and there with holes, as if
perforated with a blunt needle, especially towards the sides, where they
are more regular. There are two nostrils on each side; one is large,
tubular, and elevated above the surface; the other small and level with
the skin. The mouth is large, but the jaws have no teeth, so that the
animal lives by suction, or by swallowing its food entire.
The eyes are small, flat, and of a blueish colour, placed a little
behind the nostrils. The whole body from a few inches below the head,
was distinguished into four longitudinal parts, clearly divided from
each other by lines. The carina begins a few inches below the head, and
widening as it proceeds, reaches as far as the tail, where it is
thinnest. The situation of the _anus_ is very remarkable, being an inch
more forward than the pectoral fins. Across the body, there are a number
of small bands, annular divisions, or rather rugæ of the skin; by means
of which the fish seems to partake of the vermicular nature, having the
power of lengthening and shortening its body like a worm, and by means
of which it can swim backwards as well as forwards.—For an anatomical
description of this fish, see the appendix to the 2d. vol. of Mr.
Cavallo’s “Complete treatise” page 303.
The Gymnotus has the astonishing property of giving the electric shock
to any person or number of persons, either by the immediate touch of the
hand or by the mediation of any metallic conductor. The shock is
interrupted by the intervention of a non-conducting substance. If the
animal be touched only with one hand, a kind of tremor is felt in that
hand only. The power of giving shocks depends entirely on the will of
the animal.
As nature is ever provident for her creatures, both with regard to their
preservation and support, she has endowed the Gymnotus with a peculiar
instinctive faculty, so that if it be pursued by an enemy, it never
fails to communicate a shock, in consequence of which it eventually
makes its escape. In obtaining food it likewise makes use of its
electrical property by which it kills small fish, and afterwards devours
them.
But the most remarkable instinct of this fish is, that when any
substance approaches it, it is sensible whether it be a conductor or
non-conductor. In order to exhibit this wonderful phenomenon, a variety
of methods were contrived, the easiest and most satisfactory one was the
following. The extremities of two wires were dipped into the water of
the vessel, in which the animal was kept, after which they were extended
to a considerable distance, where they terminated in two separate
glasses full of water. These wires being supported by silk at some
distance from each other, the circuit was, of course, incomplete. In
these circumstances if a person completed the circuit, by placing one
hand in one of the glasses and the other in the other, the fish which
never went purposely towards the wires, while the circuit was
interrupted, would now go immediately towards them and give the shock,
and this though the completion of the circuit was made out of his sight.
The next electrical fish we are to mention is the Torpedo; a genus of
fishes belonging to the order of Chondropterygia; the species of this
genus are remarkable and numerous; but we must content ourselves with
the sixth species, called the _electrical ray_, or _cramp fish_, or
Torpedo. The head and body, which are indistinct, are nearly round, the
ventral fins form on each side the quarter of a circle, the two dorsal
fins are placed on a trunk of the tail, which is round, the caudal fin
is broad and abrupt. The eyes are small, and placed near each other;
behind each is a round spiracle with six small cutaneous rags on their
inner circumference.—The mouth is small, and the teeth are minute and
spicular.
These fish have been taken in Torbay, off Pembroke, near Waterford in
Ireland, and many other parts of Europe, with a trawl, and sometimes
with a bait; they commonly lie about forty fathoms deep. The food of the
Torpedo is fish.—For an anatomical description we refer the curious
reader to one given by Mr. Hunter, in the Philosophical Transactions,
vol. 63.
The electrical properties of this fish are remarkable; for a long time
they were considered as fabulous; but the fact having been ascertained
beyond the possibility of doubt, it was endeavoured to be accounted for,
by a variety of ingenious though unsatisfactory arguments. But when the
phenomena of electricity began to be better understood, considerable
light was thrown upon the subject; and Mr. Walsh at last, not only
explained the phenomena which generally attend it, on the known
principles of electricity, but actually contrived an artificial fish, by
which a shock very similar to that of the natural one can be given.
The electrical power of the Torpedo is conducted by the same substances
as conduct common electric matter, and is interrupted also by the same
non-conductors: but its shock will not pass over the least interception
of the circuit, not even if a chain be used. This singular fact was also
imitated by Mr. Walsh with his artificial Torpedo.
It has not been in our power to obtain a particular account of this
artificial Torpedo of Mr. Walsh.—But we know that one may be formed in
the following manner.
Let a number of small thin laminæ of talc, commonly called isinglass, or
thin sash glass, coated in the usual way, be joined together in the same
manner as in the battery. Let these be placed in the body of an
artificial fish resembling the Torpedo.—Let them then be charged, and on
being touched, the same phenomena which accompany the real Torpedo will
ensue; except that the shock of this will not be impeded by a small
interruption in the circuit. Similar effects may also be produced, by
means of a large battery weakly charged and furnished with Lane’s
electrometer.
The third and last fish that we shall mention, is the Silurus or Silurus
electricus, a genus in Ichthyology belonging to the order of Pisces
Abdominales.—The body of this is long, smooth, and without scales, being
rather large and flattened towards the lower part. The eyes are of the
middle size and covered by the skin which envelopes all the head. Each
of the jaws is furnished with a great number of small teeth. About the
mouth it has six filamentous appendices, two from the upper, and four
from the under lip. The colour of the body is greyish, with a few dark
spots towards the tail.
With regard to its electrical properties very little is known, enough
however to entitle it to the name of electricus.
CHAP. XVII.
_The influence of electricity on vegetables._
With regard to this part of our subject there has been considerable
controversy between philosophers, some of them asserting that
electricity is unfavourable, and others that it is advantageous to
vegetation. It was asserted by the Abbè Bertholon, in his book entitled
Electricitè des meteores, that plants situated near a metallic conductor
increased considerably in consequence of their situation. And, on the
other hand, Giardini says that plants growing near such conductors are
generally unhealthy, and produce very little fruit, but upon removing
the conductor the plants become luxuriant and fruitful.
The Abbè Bertholon in endeavouring to establish his opinion, constructed
what he called an _electro vegetometer_ by means of which the
electricity of the atmosphere may be collected in abundance. “This
apparatus (says he) having been raised with care in the midst of a
garden, the happiest effects were perceived, viz. different plants,
herbs and fruits, in greater forwardness than usual, more multiplied and
of better quality.” These facts are analogous to an observation that I
have often made, viz. that plants grow best, and are more vigorous near
thunder rods, where their situation favours their developement. They
likewise serve to explain why vegetation is so vigorous in lofty
forests, and where the trees raise their heads far from the surface of
the earth, so that they seek as it were the electric fluid at a far
greater height than plants less elevated: while the sharp extremity of
their leaves, boughs and branches serve as so may points granted them,
by the munificent hand of nature, to draw down from the atmosphere that
electric fluid which is so powerful an agent in forwarding vegetation,
and in promoting the different functions of plants.—Such are the theory
and experiments of the Abbè, but Doctor Ingenhaus, in two letters to Mr.
Molitor, published in the _journal de physique_ for 1786–88 has shewn
the fallacy of the theory, by exposing the insufficiency of the
experiments upon which it was established.
We shall translate a few passages from the Doctor’s letter, which will
shew us his opinion and the result of his experiments.
I have frequently made experiments of this kind by exposing plants to a
weak degree of electricity, and at other times to a considerable
quantity, without ever being able to observe that plants under its
influence prospered more than those which were not electrified at all.
It even appeared to me more than once, that those which had been
electrified were a little less thrifty than those which were not
electrified.
In another place he says, Not being content with these experiments, I
have made others infinitely more conclusive, by strewing seeds of
mustard and cresses, over the largest plates of delf that I could
procure, covering them with brown paper, and sprinkling them continually
with a sufficiency of water. Each of these plates was covered with more
than a thousand seeds; I kept them electrified night and day, according
to the method which Mr. Schewankhard directed, in a letter quoted by Mr.
Elermann, but which I shall not repeat in this place, lest I should
swell this memoir: the vegetation of these little shrubberies was always
more or less precarious, in proportion to the greater or less quantity
of light that they received; the electricity really contributing nothing
to advance the growth: thus the controversy stands, we leave the reader
to form his own opinion.
That some plants are more affected than others by electricity is an
unquestionable fact. It is however not true as some have affirmed, that
the contractions of the mimosa or sensitive plant, are attributable to
this cause. The plant is equally affected when touched either by a
conductor or an electric.
CHAP. XVIII.
_Medical electricity._
Electricity has one advantage over other medical applications, in as
much as it may be applied to the healthy, as well as the diseased part
of the body, without proving prejudicial, and because it requires rather
a nice application, than a perfect knowledge of the complaint. In a
number of cases it has unquestionably proved salutary.
When electricity was first used in removing bodily complaints, it was
done only by means of the Leyden phial pretty highly charged; but this
mode of administering it, was strenuously opposed by Mr. Lovet, who was
a celebrated electrical practitioner, and in an essay called _Subtil
medium proved_, asserts that electricity should be used in small sparks,
by which mode of treatment he affirms he scarcely ever failed curing or
at least relieving his patients.
The apparatus for medical electricity in addition to the machine
described in chapter IV, is an _insulating stool_. This stool is made in
the common way, only that the feet must be of glass, the upper or wooden
part, should be about three feet square, so that a chair or bench may
conveniently stand upon it; care must be taken to leave no sharp edges
about the stool. For a representation of one, see plate letter W.