Electricity
A public lecture delivered by George W. Benedict
The subject to
which I am about to call your attention is Electricity. We are all familiarly acquainted with the
many phenomena which owe their existence to the presence & action of this
extraordinary substance. We cannot
recollect the first time when our attention was excited by the glittering
lightning & our fears awakened by a knowledge of its fearful effects. We cannot recollect neither the first time in
our childhood we were amused by seeing sparks of light produced by stroking the
back of a cat in a winter’s evening.
Little should we be led to suspect that the object of our wonder &
terror was of the same nature precisely
with that which viewed & treated as a plaything. Yet such is the fact. Both these appearances are caused by a
peculiar substance called electricity - a substance that seems to exist in
everything, yet to be entirely invisible, except in some peculiar
circumstances. What those circumstances
are which present it to our view will appear hereafter. The truth it will be seen that we cannot be
certain that we ever do see the substance itself: for it may be that the light
which affects one’s sight is a thing wholly distinct from electricity itself
& only accompanies some of its operations. We are able however to make
ourselves acquainted with many peculiar rules to which this substance is
constantly subjected, & to show what it has to do with a vast many of the
operations of nature. A knowledge of
these rules of its action & connection with the rest of the world
constitute the Science of Electricity.
This science is in reality of rather modern date. To be sure the ancients knew the phenomena of
the lightning & many others depending on electricity as well as we do; but
they perceived not their connexion with each other or their relation to other
things, & were wholly unaware of their being subject to any particular laws
of action. The beginning of this science
is to be credited to Dr Wm Gilbert a philosphical physician who lived in
Colchester England, about the forepart of the sixteenth century. From that time the subject excited a high
interest in the minds of philosophers.
For a long period however no special benefit arose from their investigations,
except the gratification of a laudable curiosity. Indeed it must often be the case that no
practical good will arise from the
commencement of a new branch
of knowledge for some time. Men do not know enough about the subject to
make use of it. But as one great end of
all philosophical investigation is the benefit of man, the only course to be
pursued is, to persevere in making
additions to the stock of knowledge, being well assured that ultimately, good
will ensue. There is a remarkable difference
between the works of God
& those which denominate purely artificial. It often happens with
regard to the latter that the more we examine them the less valuable they
appear, & often they become after a while wholly uninteresting. With the works of God the case is directly
the reverse. We cannot find any of his
creations, which it will not be profitable & delightful for us to
study. They form a mine in which the
deeper we dig, the more rich & abundant is the ore. This remark applies with peculiar force to
the science of electricity. For a long
time , as I have already remarked, the gratification of curiosity was all that
the existing knowledge on this subject could produce. But now, while pleasure in a thousand fold
degree also springs from an examination of it, most signal benefits of a great
& positive kind are derived from the knowledge acquired.
The labors of many philosophers after the time of Dr
Gilbert were not lost; but before we find that any important good to man
directly resulted from a knowledge of Electricity we must come near to our own
days - to our own land, to the labours of one
our own countrymen Dr Benjamin Franklin.
It is indeed a fixed principle in the institutions of our country that
every man shall be estimated by his own merits, & that no one is entitled to expect honor or trust
because his ancestors were honorable & trustworthy. A regard to so important a principle as this
does not however interfere with a proper indulgence of national pride where we
can reckon among either our citizens now in the stage of action, or those who
long ago left the scenes of the present world, men of eminent genius &
worth. An indulgence of this feeling is
deemed justifiable in every nation under the whole heaven. So far as this feeling is proper it may be
then indulged by the people of the United States in the instance named. Saying nothing of the great advances which he
made in this science & of the opinions which he declared - which will be
more suitably treated of hereafter, we
will barely call to mind this one fact - that the first and greatest benefit
conferred on mankind, by a knowledge of Electricity, was through his
instrumentality. Let it be noticed that
the insertion of this brilliant gem in the diadem of honor which has been
awarded him by the united voice of Europe & America, was not caused by any
lucky accident. This proposal to disarm
the threatening thunder cloud - to
paralyze the hitherto invincible
lightning by what seemed to the unlearned as it were, the wand of a
magician, was the result of extended & ingenious experiment & profound
reasoning. Americans have no need of
memorials to aid their recollection of one whose labours appear so marked on
every stone in the foundation of their country’s liberties; but it must ever be
a gratifying thought to them that no
part of the civilized world is destitute of that token of his genius to which I
allude. It was one of those happy
inventions which, it is confidently believed, the ingenuity of future times can
never render unnecessary. So long as the
laws of nature remain unchanged, shall the lightning rod remind the people of
every nation where it is found, of the name and genius of its inventor.
From the time of Dr Franklin the science of
Electricity became widely extended - too
widely & rapidly extended for me to sketch its progress further in this
place. I shall refer as I proceed to the
prominent advances made in it & to the illustrious men who made them.
Attractive as
every branch of natural philosophy is found to be by everyone who enters upon
the study of it, the science of electricity possesses some peculiar
fascinations. The mysterious nature of
the subtle element itself - a thing which can neither be weighed nor measured,
yet whose presence & laws of
action can be so perfectly proved
beyond all question - its existence in every material thing that we know of -
its perfectly harmless and quiet deportment under all ordinary circumstances
& the fearful energy it can display when aroused to action - The perfect
control man is enabled to exact over its movements by knowing its relation to
other matter - the wonderful __erity with which it seems to move when unimpeded
& the ease with which its passage may be interrupted - the singularity and
often the great splendor of the experiments by which all these things are shown
& enforced - all there combined together, render it agreeable beyond
measure to persons of all ages & conditions.
To aid you in your recollection I will briefly set
forth the principal topics of the subject as I shall teat them. I would remark though, in a general point of
view, I shall proceed from the simplest to the more complicated parts of it
& endeavor to establish every principle in the plainest manner I can
use. I shall aim also to apply to
practical ends as I proceed such doctrines as may be proved.
The last thing to which I shall call your attention
will be some of the simplest electrical phenomena which occur & from them
show how some of the principal laws of electrical action are deduced from them.
The second general topic will be the different nethods
by which electrical indications can be produced.
The third principal division of the subject will treat
of some peculiar modes of electrical action & the consequent powers of
certain electrical instruments.
The fourth principal division of the subject will
treat of the effects of electricity on various substances wether unorganized,
vegetable, or animal, & its connexion with many of the natural phenomena
which we are acquainted with &the methods for preventing its hurtful &
securing its beneficial effects to society.
On some of these divisions I shall dwell longer than
others - in each of them I shall have occasion to present a great variety of
curious experiments, though by no means equally show. Many I do not hesitate to say will be very
beautiful & even splendid.
The first thing I call your attention to is the
exhibition of some of the simplest electrical phenomena. From these we will be able to infer some of
the simplest principles that lie at the foundation of what follows.
Expt a. Rub dry sealing wax with warm dry fur
or flannel. It attracts light
substances, & then sometimes repels them.
Expt b. Dry glass rubbed with similar
substances exhibits like effects.
We see such effects produced when in cool dry weather
we brush clothes of woolen or silk. The
light substances floating in the air seem to accumulate surprisingly, the more we brush. These effects are caused by
electricity, though in these cases we barely see any light. Wether all substances when rubbed would
exhibit like properties need not be considered here. More of that in a suitable place.
Expt c. Let any electrical machine in
action. The conductor will attract with
force any light substance brought towards it, & there repel it - as for
instance a light ball of cork - elder pith - a light ball or leaf of metal,
each being suspended by a clean silken string.
In these cases, the light substance will not only be
repelled from the machine, but if two that have been touched by it,
be made to approach each other, they will show a similar repulsive force; but
if we bring one towards anything which has not been similarly affected, it will
be attracted by it. We cannot draw any
safe conclusions of a general nature from these facts alone. Some others must be presented first. Of those we will take notice presently.
We observe also that it is not necessary in this case
where we use the machine to bring the light body into contact with the former
. When pretty near a bright flash is seen
extending from the machine to the attracted substance, which is then repelled.
Expt d. Touch one light pith ball to excited
wax & another to excited glass - the rubber being the same in both
cases. The two balls, though each will
be repelled by the substance it touched, will be attracted to each other. &
more forcibly than when only one had been touched.
The plain inference is that there is a difference in
the states of the wax and the glass or perhaps two kinds of electric
principles. Du Faye who first noticed
this peculiarity supposed that it indicated two kinds of electric
principles. The one apparent in excited glass he called vitreous
electricity;; the other (since
resinous bodies were generally
affected like the sealing wax) he called resinous electricity. Some or all of
the French writers retain these names still.
Without inquiring wether the observed phenomena indicate two kinds of
electrical substance - or merely two modifications of one, it will be plain
from the following experiment, that so far as the names are indicative of the
nature or origin of the electricity produced, they are inappropriate.
Expt e. Excite smooth glass with catskin, the
ball which touches it will attract one touched by glass excited by woolen or silk,
as if touched by the wax excited as above mentioned, & in fact will not
attract but repel the latter. The glass excited by catskin is in the same state
as the sealing wax.
Expt f. This conclusion is affirmed by exciting
the sealing wax with a piece of rough glass.
By similar trials it will appear that the wax is in the same state that
the smooth glass is when excited by woolen cloth or silk.
Though
then there be two kinds of electricity, one is not peculiar to vitreous nor the
other to resinous bodies, since both may be produced from either according to
the substance used to excite with. Dr
Franklin used to indicate there two states or kinds, the words positive &
negative, & these are the appellations used generally to the English &
American philosophers. The reasons for
these terms being selected , & those for which I prefer to retain them will
appear hereafter. It is much more
convenient to speak of these two states as being actually caused by two kinds
of electricity. It saves much
circ___tion & leads to no error, since it may be remarked once for all,
that we leave the question of there being actually two or barely one substance
differently modified, as yet unsettled. It can be considered to far better
advantage when we have proceeded a little farther. It being so understood we may conclude from
the experiment we have wrought, “that bodies
similarly electrified repel each
other , & bodies dissimilarly electrified attract each other.” These laws are fundamental in the science of
electricity.
11. On these
two laws are founded the construction of several small instruments used to
indicate the sensitive state of the electric principle & to measure its
intensity of action. They are called
electrometers or electroscopes - words which may be considered of equivalent
meaning.
a. A delicate
metallic needle on a pivot or a small pith-ball suspended by a silk string, are
very convenient for determining wether a body presented to them is electrified
or not.
b. A better
instrument is made by suspending parallel and contiguous, two small pith-balls
by silken threads. If both are touched
by an electrified substance, they will recede from each other.
c. The gold
leaf electrometer is more delicate still, being two strips of gold leaf
suspended to a metallic needle & ball in a glass tube, which recede when
the ball ( or cap if it be so) is
touched by an electrified body.
12. If the
balls of the electroscope be suspended by linen instead of silken threads, it
is not necessary to touch the balls
themselves, in order to make them diverge.
If the top of the threads or a metallic knob on which they are hung (the
knob being in contact with glass on sealing wax only) the same effect is
produced. This will not be the case if
the threads are of silk. From this it
seems that the electric principle passes readily along the linen threads but
not along the silken ones. The linen is
therefore called a conductor & the silk a non-conductor of
electricity. This very important
distinction was made D 1729 by Mr Stephen Gray a pensioner at the
Charter-House. He enriched the science
of Electricity with many important discoveries.
He was exciting a glass tube in the end of which there was a cork, by
which he observed a feather to be attracted.
This lead him to insert a wire in the end of the cork, & afterwards
to suspend to it a p__k thread of
several feet in length with a ball at the end, which still acquired the
property of attracting light bodies. He
then tried wires of great length, but still the electricity was communicated to
the farther end, till having extended his wire supported by silken threads to
nearly 800 feet in length, one of them broke.
He accidently supplied its place by a wire - when he found that he could
communicate no electricity to the suspended one, as before, but that on using a
silk string for the wire to suspend the other by, he could communicate it
readily. That the two substances
differed in the one conducting off & the other not conducting off the
electricity of the suspended wire was thus
manifest.
13. It is of
the highest consequence that this distinction be noticed & remembered.
It lies at the foundation off all practical electricity. Where it not for this difference in bodies we
should be wholly unable to exhibit any one of the properties of the electric
fluid. Various easy experiments show
this distinction.
14. Observe
that we are not able to tell what it is in the nature of any body which enables
it to transmit this peculiar principle, or prevents it from so doing. We learn wether a body is a conductor or
nonconductor only by experiment. Copious
tables of conductors & nonconductors
have been formed from the experiments of many persons. From these experiments it is ascertained also
that all conductors are not equally perfect & so of nonconductors. The following Table of Conductors is
arranged according to their excellence
as such, so far as has been satisfactorily determined.
Conductors All Metals
Pure Charcoal
Plumbago
Strong acids
Powdered charcoal
Diluted acids & Saline fluids
Metallic ores
Animal fluids
Water
- ice, snow
Green
vegetables
Flame
- smoke & steam
Most
Salts - Heated
Air
- vapor of alcohol
Most earth & Stones
Some
of these nearly equal in their conducting
powers & many of them vary under different circumstances of
temperature - humidity. The metals are
so much better than all the other substances that the latter are generally
called imperfect conductors.
15.
By calling another class of substances nonconductors of electricity, it
is not meant that the substances so called transmit absolutely none of the
electric principles - but they transmit it in small degrees & very slowly
indeed, whereas along conductors it passes with inconceivable celerity. The
practical results in most points are the same as if they did not conduct any at
any rate whatever. The following
substances are enumerated in the order of heir excellence as nonconductors.
Nonconductors
Gum Shellac
Amber
Resins
Sulphur
Wax
Glass
Vitrifications generally
Diamond
Transparent gems
Raw Silk
Bleached Silk
Dyed Silk
Wool, Hair, Feathers
Dry Paper, Parchment
Leather - Air, Dry Gasses
Baked Wood & dried vegetables generally
Porcelain
Dry marble, some stones
Camphor Caoutchow (?)
Dry Chalk, Lime, Phosphorous
Ice at -13 Fah.
Dry ashes
Oils, the heaviest the best.
Dry metallic oxides
Many of these vary as they
are more or less dry, water in any quantity diminishing their nonconducting
power.[1]
16.
It will be seen by a little consideration that the substances enumerated
as conductors are of a most dissimilar character in every other respect &
so the of the nonconductors. The common
quality which renders them capable or incapable of transmitting electricity
remains yet to be discovered. The
difference of conducting power & the variety of consequences thence arising
will be seen at every step in the prosecution of my subject, & I shall not
stay to treat of this any longer. We are
now prepared to consider wherein the attraction and repulsion of bodies in
which the electric principle has been rendered sensible is subject to certain
laws of action - especially so far as the variation of the force of attraction
& repulsion at different distances is concerned.
17. Nothing was
done by the early cultivators of electrical science to determine these laws. Conjectures where made that the attraction
& repulsion increased in intensity as the distance between the bodies so
acting diminished & in the inverse ratio of the cube & even higher
powers of that distance. Though
approximations to the truth had been made by some from experiment, the honor of
fully establishing it was acquired by Coulomb, a celebrated French
Philosopher. The Torsion balance a
simple and ingeniously contrived instrument, made & and its principles
demonstrated by Coulomb himself, enabled him to investigate this subject with
unparalleled & complete success. The
method persued by him will be readily understood by a recurrence to the
instrument. The silver wire which he
used was exceedingly small & being of considerable length was moved through
a small arc with great ease. The needle
was of pure gum (shel)lac having a minute pith ball on its end. The ball when at rest, just touched a fixed ball communicating by a metallic stem with another
on the outside of the glass tube containing the whole. To this a substance slightly electrified was
applied - the inner ball & the ball of the balance were electrified by
communication & consequently the ball on the balance receded. The number of degrees which it passed over
could be read off on the outside of the tube.
When it stopped its repulsive force was exactly counterbalanced by the
torsion of the wire. Now the principle
of the torsion balance is, that the number of degrees through which the wire is
twisted is in proportion to the force required
to effect it. If it is twisted through
twice an angle at one time & five times the same angle at another, then the
forces required to effect this are as two to five & so in all cases where
the torsion is not carried to great a degree as to destroy the elasticity of
the wire. Thus when the ball of the
balance was repelled 36 deg. The force
of repulsion was 36. By turning a button
on the end of the wire suspended, the ball was brought nearer, so as to stand
at 18 deg. It was observed that the button (& of course the wire) had been
turned through 144 deg. The repulsive
force at this point was then 144. By
turning the button 575 ˝ deg. The ball of the balance was made to stand at 8 ˝
. The repulsive force at that point was
then 575 ˝ . Thus at distances 36, 18, 8
˝ the repulsive forces were 36, 144, 575 ˝ or by expressing there ratios in as
small numbers as possible it appeared that the distances being represented by
the numbers 4, 2, 1, the repulsive forces would be 1, 4, 16 very nearly - for within
that limit (viz 36 deg.) The angles & their chords differ in no very
appreciable degree.
It was by a similar but reversed process that the attraction was found to be
in the same ratio to the distance. In
this case the ball of the balance was placed at a certain distance from the
fixed one which was then electrified.
The ball on the needle approached it.
The angle of torsion was noted.
The button was turned so as to place the ball at a different distance.
The variation in the degree of torsion is observed so.
18. No
electrometer will compare with the torsion balance of Coulomb for delicacy
& accuracy. Sensible as was the one
we have spoken of, he constructed one much more so. The suspending thread was a single fibre of
raw silk four inches in length. The
needle of gum lac(quer) an inch long & a bit of tinsel paper on its end did
not outweigh a half grain & a force so minute as a sixty thousandth part of
a grain was sufficient to twist it through 360 degrees.
In pursuing investigations in the science,
recourse must be often had to these instruments, but for a majority of
instances, the gold leaf & pith ball electrometers are equally well suited
& far more convenient.
19. We come now
to the methods by which the electric fluid may be rendered sensible & the instruments best adapted to this
& collateral purposes.
The first & principle method is friction
of different bodies. The friction of a
glass tube & stick of sealing wax by a woolen or silk cloth, or piece of
fur, has been already exhibited. It will
be recollected that electrical phenomena were first seen in the friction of
amber. Dr. Gilbert whose name has
already been mentioned discovered that many other substances produced the same
effect when rubbed - among these he enumerated Diamond - Sapphire - Opal -
Crystal - Glass, Sulphur - ____ - Sealing wax made of gum (Shel)lac - Rosin
& several others. For a long time no
other method was used for exciting than directly rubbing the excited substance
with a cloth held in the hand. It would
be useless to speak of all the imperfect contrivances resorted to successively
to produce the desired effect with the greatest facility. The electrical generally used of late though
differing in their fashions, may be reduced to two classes - the cylinder and
plate machines. The former originated
among the German Electricians, the latter was invented by Dr Higenhous(?) of
the Netherlands, but first made, it is said, by (Jesse) Ramisden (in 1768) an English philosopher
& mechanician. The improved forms of
them have been produced by the labors of artists & philosophers of amost
every nation in Christendom.
I shall not pretend to explain why friction
causes the electric fluid to become sensible, but barley describe the parts of
the machine & show the results of their action. Each kind of machine has its peculiar
advantages.
Plate Electrical Machine.
20. The best
amalgams to be put on the rubber is made according to Mr. Linger a
distinguished practical electrician, by mixing in a state of fusion 1 oz of
tin, 2 oz of lime & 6 oz of mercury - or 2 oz of tin, 4 oz of lime & 7
oz of mercury do equally well. A slight variation in the proportions is
uninjurious. The solid metals should be
melted & the mercury heated & then poured to them - the whole to be
well shaken till cold that the different metals may be well united. It must then be powdered finely & mixed into a stiff paste with lard &
smoothly applied to the cushion. Some of
it should pass off onto the silk flap.
21. Any
electrical machine must be kept clean or its effect will be diminished greatly.
The rubber needs special attention.
When the machine is in good condition &
the air dry ( if the temperature of the machine be a little above that of the
air, it will greatly prevent moisture from affixing itself to it) on turning
the plate several phenomena present themselves.
a. Luminous
streaks are seen on the plate where it is not covered with the silk.
b. Sparks pass
between the conductor or rubbers & the hand or any conducting substance
held in the hand, producing a peculiar sensation. From this we infer a material agency & as
the conductor exhibits the attractive & repulsive property in a striking
degree, we infer that these greater effects follow from a greater accumulation
of the electric principle.
c. From the
fact also that when we approach the hands to both conductor and rubber at the
same time a more intense action, or a larger spark is visible, a natural
influence or relation between the conductor & rubbers is indicated. So too on suspending a chain from the rubbers
to the ground or to some conducting substance communicating with the ground, we
see longer sparks than before. From
which it is evident that the earth generally is somehow related to this
peculiar principle & that the action is not wholly confined to the machine.
d. The action
will apparently cease entirely if we connect the rubber & conductor by a
wire; but on connecting them by a silken string strung with shot at very small
distances, or any equivalent instrument, sparks will be seen at the
intersections of the metallic circuit, & an apparent circulation of the
principle goes on. We infer that when
the wire was connected, the same circulation must go on, although not evident
to the senses. The necessity of keeping
the conductor & rubbers entirely asunder - having no conducting substance
communicating from one to the other will be plainly seen. It may be remembered that whenever a
conducting substance is connected with none but nonconducting ones, it is said
to be insulated. Thus a metallic
substance suspended by a silken string or placed on a small stand with a
standard of glass, sealing wax or the like is insulated. The prime conductor of the machine is
insulated. In the best constructed machines for purposes of investigation the
rubbers are insulated also.
22. Were not
the air a pretty good nonconductor, it would of course be impossible to
accumulate electricity or even to render it sensible on the prime
conductor. It must follow however that
were the air ever so good a nonconductor - even were it absolutely perfect as
such. Still the electricity would slowly
pass off by its means since every particle
of air in contact with the electrified conductor, becoming itself electrified
by communication would be repelled, and another would come in contact to be in its turn electrified
& repelled - each carrying off a small portion of the electric principle,
& so on till all should be dissipated.
If the air be moist, this will be so much the sooner accomplished. The rate of dissipation from a conductor
insulated as much as possible, by means of the air alone was determined also by
Coulomb. This was accomplished by aid of
the torsion balance. Applying an
electrified substance to the exterior knob, the pith ball on the needle would
of course be repelled & were the fixed conductor & the pith ball
absolutely insulated & the air not a moveable fluid, the needle would
remain stationary; but such was not the fact, for after the insulation had been
rendered so perfect as to produce no sensible error, except so far as the air
dissipated the electricity, the needle approached the pith ball slowly. By many observations at different angles of
torsion, it was found that for any one state of the air, the part of the
electric principle taken away by the air was in a constant ratio to the
repulsive force. If the repulsive force was greater, the
dissipation was proportionally greater
also. This ratio would be greater or
less as the air was more or less moist.
In ___ states of the atmosphere, it would be found to vary from 1/15
deg. To 1/100 deg. a minute
probably. It is worthy of notice too,
that the results were the same, whatever the substances were which were used as
vehicles & containers of the electric principle. This law of the dissipation of electricity
from the contact of the air is only strictly true on supposition that the
substances are of a globular shape, or nearly so. The effect of difference of shape will be
hereafter noticed.
23. Since all
substances conduct some, though as has been remarked, from the great slowness
with which electricity is conducted by one class, they are called non
conductors, when another class of substances from the immeasurable swiftness
with which it passed through then are called conductors, it becomes important
to know which are the best non conducting substances & if possible to
ascertain the rate of dissipation of the electric principle along such
supports. These points were also settled
by the same philosopher Coulomb and by
the same means. He found that when the
electrical action was very intense there would be a rapid dissipation along a
nonconducting support, but that when the action became very feeble, the
dissipation diminished and became ultimately nothing if the support were long
and small. He found that gum (shel)lac
is by far the best nonconductor known - that if a small cylinder of lac would
insulate a conductor perfectly (leaving the effect of the air out of the
question) when the electrical action was of a given intensity a silk thread of
the same size needs to be ten times as long to produce the same insulating
effect, & that for any one substance of a given size four times its length
was necessary to insulate perfectly if the electrical action was twice as
strong - nine times as long if the
electrical action was threefold - sixteen times as long if the action was
fourfold & so on. The lengths varying as the squares of the degree of electrical
intensity.
The importance of a knowledge of these
principles to every practical electrician is obvious. The more he increases the
size of his plate or cylinder - or generally the exciting power, the longer
& better must be his insulating supports or he will lose most of the
increased effects which he would otherwise obtain.
23.
[1]Heat also affects thee conducting power of bodies, singulary. Hot glass melted resins & wax are conductors. Hot air is not.