_electric per se_, and _non-electric_, were at first made use of from an
erroneous idea that only those called electrics, contained the electric
matter _in their substance_, which was capable of being excited by
friction, and communicated by them to those called non-electrics, and
supposed to be destitute of it: for glass and other electrics, being
rubbed, discovered signs of having it, by snapping on the approach of a
finger or other conductor, and by attracting and repelling light bodies;
while other substances could not be made to produce any such effect. It
has however since been proved by experiments, that _both_ electrics and
non-electrics contain this matter in their substance; but that
non-electrics cannot be excited, owing to the fluid diffusing itself
through them as soon as collected. These terms are therefore improper,
and as the only difference is that some bodies will conduct electricity
and others will not, the terms _non-conductor_ and _conductor_ are those
which might generally be used with the most propriety in speaking on
this subject; though, in conformity with custom, we shall often use
_non-conductor_ and _electric_ as synonymous.
CHAP. II.
_Electric substances; with some of the phenomena attending their
excitation._
Those substances by which electrical phenomena may be produced, form the
subject which next demands our attention; but these are so numerous that
it would be vain to attempt to specify them all. Perhaps it may be
doubted, whether every material substance, with the exception only of
metals, water, and charcoal, may not be considered as an electric.
Some however exhibit particular phenomena more obviously than others;
and hence a number of catalogues have been formed, for shewing the
effects which arise when electrics are excited with different rubbers.
The specification which we esteem the most complete, was formed by the
ingenious Mr. Cavallo, and we shall give it in his own words.
“In the following table (says he) may be seen what electricity will be
excited in different bodies, when rubbed with different substances.
Smooth glass, for instance, will be found by this table to acquire a
positive electricity, when rubbed with any substance hitherto tried,
except the back of a cat: (by which I mean the skin of a cat while on
the animal alive:) rough glass, (viz. glass, the polish of which has
been destroyed by emery or otherwise) will be found to acquire the
positive electricity, when rubbed with dry oiled silk, sulphur &c. and
the negative when rubbed with woolen cloth, the hand &c. and so of the
rest.”
_Electrics._ │ _Qualities._ │ _Rubbers._
│ │Every substance with
“The back of a cat │Positive │ which it has hitherto
│ │ been tried.
───────────────────────┼───────────────────────┼───────────────────────
│ │Every substance
Smooth Glass │Positive │ hitherto tried,
│ │ except the back of a
│ │ cat.
───────────────────────┼───────────────────────┼───────────────────────
Rough Glass │Positive │Dry oiled silk,
│ │ sulphur, metals.
│ │Woolen cloth, quills,
„ │Negative │ wood, paper, sealing
│ │ wax, white wax, the
│ │ human hand.
───────────────────────┼───────────────────────┼───────────────────────
Tourmaline │Positive │Amber, or air blown
│ │ upon it.
„ │Negative │Diamond and the human
│ │ hand.
───────────────────────┼───────────────────────┼───────────────────────
White silk │Positive │Black silk, metals and
│ │ black cloth.
„ │Negative │Paper, hand, hare’s &
│ │ weasel’s skin.
───────────────────────┼───────────────────────┼───────────────────────
Black silk │Positive │Sealing wax.
│ │Hare’s, weasel’s and
„ │Negative │ ferret’s skin,
│ │ load-stone, brass,
│ │ iron, silver, hand.
───────────────────────┼───────────────────────┼───────────────────────
│ │Metals, silk,
Weasel’s skin │Positive │ load-stone, leather,
│ │ hand, paper, baked
│ │ wood.
„ │Negative │Other fine furs.
───────────────────────┼───────────────────────┼───────────────────────
Sealing wax │Positive │Metals.
│ │Hare’s, ferret’s and
„ │Negative │ weasel’s skin, hand,
│ │ leather, woolen
│ │ cloth, paper.
───────────────────────┼───────────────────────┼───────────────────────
Baked wood │Positive │Silk.
„ │Negative │Flannel.”
From the above table it appears, that the powers of electric substances
vary prodigiously from one another; and that, according to the different
rubbers made use of, we may sometimes produce one phenomenon and
sometimes another. Hence we have a foundation for classing electric
substances according to the various powers they occasionally exhibit;
which may be done in the following manner.
First. Those which exhibit a _strong_ and _permanent_ attractive and
repulsive power, of which the most remarkable is silk.
Second. Those which exhibit the electric light, attraction, repulsion,
and all the other phenomena of electricity in a very _vigorous_, though
_not durable_ manner; of these glass is eminently preferable to all
others.
Third. Those which exhibit electric appearances _for any length of
time_, and which communicate to conducting bodies, _the greatest
electric power_.—Of these, the substances called _negative electrics_,
such as sealing-wax, resinous substances, and resinous compounds, are
the best.
Fourth. Those which readily exhibit electrical phenomena by _heating_
and _cooling_.—Of these, the tourmaline[9] is the principal.
The best method of disturbing the electric fluid, that is of making it
pass from one body to another, is friction. This may be done either by
rubbing one electric with another, or with a conductor; but the
electricity is generally stronger in the latter case. Other methods for
causing electrics to shew electric appearances, are, melting, or pouring
a melted electric on another substance, heating and cooling, evaporating
or effervescing.
CHAP. III.
_Of Electrics and Conductors._
All bodies in nature are, with reference to this subject, divided into
two classes, _electrics_ and _conductors_.
It has been fully demonstrated by experiment, that no substance which is
a conductor can be excited so as to exhibit electrical phenomena: and in
the same manner it has been found, that no substance which can be
excited, is a conductor. But as we have already hinted, there is,
strictly speaking, no substance which is a _perfect_ conductor or
non-conductor; because, on the one hand, the electric fluid meets with
some resistance in its passage through the best conductors; and on the
other, it is in part transmitted through, or passes over the surface of,
most if not all electrics.
The two following lists contain as complete an enumeration of electrics
and conductors as the present state of knowledge, in regard to
electricity, permits us to make.
The substances are disposed in the order of their perfection; that is,
the best conductors and the best electrics are placed at the head of
their respective lists, and those of an inferior kind follow, somewhat
in the manner of a scale graduated downward. Perfect exactness however
is not to be here expected, because the subject forbids it, and some of
the specified articles are of classes of substances among which there
may be a sensible difference.
_Conductors or non-electrics._
Gold,
Silver,
Copper,
Platina,
Brass,
Iron,
Tin,
Mercury,
Lead,
Semi-metals.
Metallic ores.—Of which those are the best which contain the greatest
number of metallic parts and are nearest to a reguline state.
Charcoal, either of animal or vegetable substances—
Animal fluids,
Acids,
Saline substances,
Hot water,
Cold water,
Salt water,
All other liquids except oils,
Red hot glass,
Melted rosin,
Flame and the effluvia of flaming bodies,[10]
Ice and snow—but not below the temperature of 13° Fahrenheit.
Earthy and stony substances, of which the hardest are the worst.
Glass filled with boiling water,
Vapour or steam of boiling water,
Smoke.
All compounds which contain the above substances in different
proportions, are conductors in different degrees.
_Non-conductors or electrics._
Glass and all vitrifications; even those of metals.
All gems, of which the most transparent are generally the best.
All resinous substances and resinous compounds,
Amber,
Sulphur,
Baked wood—if not suffered to imbibe moisture.
All bituminous substances,
Wax,
Silk,
Cotton.
All dry animal excrescences; as feathers, hair, wool, horn, &c.
Paper,
White sugar and sugar candy,
Atmospheric air and other gasses,
Oils,
Dry and complete metallic oxyds,
The ashes of animal and vegetable substances,
All hard stones; of which the hardest are the best,
Powders not metallic.
Ice at and below the temperature of 13° of Fahrenheit’s thermometer.
According to Mr. Walsh’s and Mr. Morgan’s experiments, the Torricellian
vacuum ought to be placed at the head of this list; but the singular
nature of a vacuum, though a non-conductor, will hardly entitle it to
the name of an electric.
CHAP. IV.
_Of the electrical machine._
Having now explained the terms made use of in the study of electricity,
and noted some of the phenomena of different electric substances, and
the difference between electrics and conductors; we shall proceed to
describe the _electrical machine_ made use of for shewing experiments,
and for exhibiting other electric phenomena to the best advantage.
The principal parts of the machine are, _the electric_, _the rubber_,
the _moving engine_, and the _prime conductor_. We shall take notice of
each of these parts separately and then describe the whole machine
together.
Formerly different kinds of electrics were used; at present smooth glass
is preferred before all others, as most convenient, and because it will,
by itself, answer the purposes of several others. For when the machine
has an insulated rubber, which is easily prepared, the operator may
produce positive or negative electricity[11] at pleasure, without
changing the electric.
With respect to the forms of the glass, those commonly used are globes,
cylinders and plates. The most convenient size for a globe is from ten
to twelve inches in diameter. It should have two necks, centrally
opposite, which must be cemented[12] to strong caps, in order to adapt
them to a proper frame. Cylinders are also made with two necks. Their
common size is from six to seven inches in diameter, and from ten to
twelve inches in length; the glass generally used is the best flint.
It has long been questioned whether a coating[13] of some electric
substance, has any effect in increasing the power of an electric; but
now it seems pretty well determined, that if it does not increase the
power of a good one, it at least considerably improves a bad one.
The next thing to be considered is _the rubber_ which is to excite the
electric. This, as it is now made, consists of a cushion of buckskin,
stuffed with hair or flannel, and fastened to a piece of wood well
rounded at the edges; to this is glued a flap of Persian black silk,
which goes over nearly one half of the cylinder or globe. The rubber
should be supported by a small iron or brass spring, placed inside of
it, as is represented edgewise by R, figure 2, in the frontispiece. This
acts in a much more uniform and parallel manner than if it were placed
under the cylinder. It suits any inequalities that may be on the surface
of the glass, and by means of a screw may be made to press against the
cylinder as occasion requires. It should likewise be insulated in the
most perfect manner by glass, or by baked wood well varnished. But when
experiments are to be made which do not require or admit of insulation,
a communication must be made between the rubber and the earth, by a
chain or conductor.
To increase the effect of the rubber several substances have been used
with success, particularly whiting and pulverised chalk. But the best of
all is an amalgam of zinc and mercury.[14] This amalgam is to be used by
first applying a moderate quantity to the cushion; and afterwards by
spreading it on a separate piece of leather, and applying it
occasionally to the under part of the cylinder while turning. In this
method of using it, only a small quantity of amalgam is consumed, while
the glass is very strongly excited; and by degrees the whole rubber
contiguous to the cylinder is covered with amalgam, in the form of a
concave cake. It is with such a rubber that the cylinder is most
powerfully excited.
An ingenious friend has favoured us with the following explanation of
the manner in which electrics are excited, which to us is more
satisfactory than any other we have seen. “In order that electricity may
be accumulated in greater quantity in one body than in the surrounding
ones, it must be set in motion. This may be effected by the _rubbing of
electrics_; the _juxta-position_ of non-electrics of different
conducting powers; and by the _chemical action_ of many, if not all
bodies on each other. The rubber will act on the first principle, and
the more perfect the contact between it and the electric the greater
will be the effect. The chalk, whiting, amalgam &c. while they will, if
properly prepared, make the contact more perfect, will also be of
service on the second principle; and the amalgam will besides be of use
on the third. Mercury and zinc may be exposed separately to the air
without any alteration; but when combined they readily unite with the
oxygen of the atmosphere; especially when the surface of contact is
frequently renewed, and the temperature increased by friction.
“The glass, acquiring a different state of electricity from the rubber,
will, as each portion passes from under it, carry away and impart to the
prime conductor the excess which it has obtained; and this the more
certainly if the dissipation of the electricity be prevented, or the
accumulation increased, by a piece of silk connected with the
rubber.—The chain making the communication between the rubber and the
adjoining non-electrics will enable this process to go on; and perhaps
may also assist on the second principle.”
With respect to the engine which is to give motion to the electric, it
has been customary, simply to turn the globe or cylinder with a winch;
but this will not produce the greatest power of which the glass is
capable. To effect this it should be made to turn six or seven times in
a second, which is more than can conveniently be done with the winch
only; and therefore multiplying wheels are used with advantage.
The prime or first conductor is an insulated non-electric substance,
furnished with a number of points on the end towards the electric, in
order to collect the electricity from it. It is usually made
cylindrical, but whatever be its form it should always be perfectly free
from points or sharp edges, except the points toward the electric
already mentioned; and if holes are made in it, which on many accounts
are very convenient, they should be well rounded and perfectly
smooth.—The larger this conductor is, if not disproportionate to the
cylinder or globe, the stronger and more dense will be the electric
spark, which will proceed from it when touched by a blunt conductor.
There must however always be a certain proportion between the cylinder
or globe and the prime conductor, for if the former be small and the
latter large, the electricity will not be collected fast enough, to
preserve an accumulation of it in the prime conductor, because a portion
is always taken off by the air, in proportion to the surface presented
to it by the conductor.
We shall now give a short connected explanation of the whole machine, a
draft of which is exhibited in the frontispiece. AB and CD are two
pillars of baked wood well varnished, perpendicularly raised from the
top of the table EFGH—these serve to support the cylinder I, by the
axles of the caps KK; from one of these proceeds the long axle L, which
passes through a hole in the pillar CD, having the pulley M, fixed on
its square end. N is a multiplying wheel, around which the band or strap
O passes, and likewise around the pulley M.—The wheel N should be made
moveable with respect to the pulley M, to accommodate the stretching of
the band, or else the pulley should have a number of grooves of
different radii in its circumference.
The rubber R, is fastened to a pillar of glass, or baked wood P. The
pressure of the rubber may be augmented at pleasure, by means of a
sliding board and tightening screw.
The prime conductor is represented by Q. It is insulated by the glass
pillars SS, which support it. T represents the points which collect the
electricity from the cylinder.
Cylinders and globes made for electrical machines are not always to be
procured. Their place however, may be very well supplied by the large
show bottles of the apothecaries. When these are used, one of the caps,
instead of being concave (to receive the neck of the cylinder) must be
made convex—so as to fit the hollow in the bottom of the bottle.—It is
to be fastened with the cement used in the other machine.
The most powerful electrical machine ever constructed, was at Teyler’s
museum at Haarlem. It had, instead of the cylinder or globe as in the
common machines, two circular plates of glass, which were made to turn
upon the same horizontal axis. These plates were excited by eight
rubbers, which acted on their surfaces. In this machine the prime
conductor had branches which collected the electricity from between the
plates.
It is not necessary however in this form of the machine to have two
plates, the second being added only to increase the power. The plate
must be firmly fastened by its centre to an axis—so as to turn
vertically between two uprights of baked wood, as in the construction of
the cylindric machines; but in this case the uprights must be so close
together, as barely to leave room for a rubber on each side of the
plate. The rubbers may be made of the same form with that in the
cylindric machine—except that they must have a projection at the back,
to fit a niche cut in the uprights which support the plate. The power of
the machine will be increased by having four rubbers; two above and two
below the axis of the plate. The prime conductor is placed opposite one
of the ends of the axis, and is divided at the end towards the electric
into two branches or arms, which extend horizontally to the
circumference of the plate, each of which is furnished with points to
collect the electricity.
As plates are not always to be procured, a good substitute may be found
in a thick pane of glass or a piece of an old looking-glass. Mark with a
diamond or file a circle on the glass, of the size you intend for your
plate. Then putting the plate into warm water, after some time cut the
glass with a diamond in tangents. The more numerous the cuts, the nearer
the plate will be to a circle. A hole may be made in the centre for the
axis, by scratching with a diamond, and grinding with a rod of iron
(held between the hands) and emery.
CHAP. V.
_Of communicated electricity._
Having described the electrical machine, we are now to consider some of
the phenomena attending its operation. When the prime conductor receives
electricity from the cylinder, it is said to be _electrified by
communication_, and it then acts in every respect like the cylinder
itself, except that the latter, when touched by a conductor
communicating with the earth, gives a considerable number of sparks
before it is discharged; whereas the conductor discharges itself by a
single spark.
The cause of this difference is that the cylinder, being an electric,
cannot convey the electricity of all its surface to that part, to which
the conducting substance is applied; but the fluid accumulated in the
whole conductor, passing easily through its substance, is transmitted at
once to the point from which the discharge is made. Hence it appears
that the electricity discharged from an electrified conductor is more
powerful than that discharged from an electric—the conductor acquiring a
large quantity of electricity from an electric, by receiving it
gradually, spark after spark, and afterwards, when touched, discharging
it all at once.
The velocity of electricity is almost beyond conception. It is,
notwithstanding, in a small degree relative to the quantity put in
motion, and to the goodness of the conductor by which it is transmitted.
A large quantity of electricity passes through a good conductor with
such rapidity, that there is no perceptible difference in the time which
it takes to go one foot, or one thousand feet. A small quantity however
has been found to take a time barely perceptible, in passing through a
long and imperfect conductor. Experiments relative to this point will be
related hereafter.
CHAP. VI.
_Of the electric spark._
If a piece of metal be presented to an over-charged prime conductor, the
fluid passes with violence from the one to the other; an _electric
spark_, having the appearance of fire, is seen flashing between them,
and a snapping noise, like the cracking of a whip, is heard. If this
piece of metal be insulated, the prime conductor will be only partially
discharged, that is, the redundant electricity will be divided between
it and the piece of metal, nearly in proportion to their surfaces. This
electric spark has not only the appearance of fire, but, when large,
will actually set fire to a variety of easily inflammable substances;
such as cotton sprinkled with rosin, spirits of wine &c. This power of
exciting flame is not commonly believed to arise from any culinary heat
in the electric spark, because if the spark be small it will not excite
flame in substances the most inflammable. It acts probably by friction
on the same principle as the rubbing of sticks against each other
produces fire.
The electric spark, taken upon any part of a living animal, causes an
unpleasant sensation, which is more or less pungent and disagreeable, as
the spark is stronger or weaker, and the part more or less delicate.
There is a slight difference between the appearance of a spark taken
from a body positively electrified, and that from one negatively
electrified. The former, if not very long, appears straight and sharp;
the latter is generally ramified, or appears in a zig-zag line.
The noise which attends the spark, is caused by the sudden agitation
into which the air is thrown, by its passage through it.
CHAP. VII.
_Of the influence of pointed bodies on electricity, and some phenomena
attending their operation._
If an uninsulated conductor, which is broad, round and polished at the
end, be presented to the prime conductor, a short and dense spark,
accompanied with some noise, will be perceived; if the conductor be less
broad, the spark will be longer, less dense, and attended with less
noise; if the breadth be still more diminished, so that the conductor
may come under the denomination of a point, the electric matter will
pass to it, from the prime conductor, and through a greater space, with
a hissing noise, and in a continual stream; a still greater sharpness
will enable the electricity to pass over a yet more extended space, but
unaccompanied by noise, and only a small light will be seen upon the
point. The same result will arise if points of different acuteness be
affixed to the prime conductor, instead of the uninsulated one: but if
both be pointed, the electricity will be more readily discharged.
In all the above cases, the appearance of the electric matter at the
point, will indicate the kind of electricity from which it proceeds. A
large divergent cone indicates positive electricity; a small globular
light, that which is negative. Hence it is always easy to ascertain
whether an insulated conductor be electrified positively or negatively,
by presenting a point to it, as the light at the point is always
definitive of the contrary electricity in the conductor.
If a pointed conductor be electrified, either positively or negatively,
and the face be brought near the point during the electrization, a wind
will be felt blowing from the point, accompanied with a peculiar
sensation, commonly called _the spider’s web_. It is remarkable that the
current of air is always in the same direction, whether the point throws
off or receives electricity.