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found more favour also in England.

1701. Hypothesis of two fluids. According to the THEOUY
OF TWO FLUIDS, hodies in their natural or unelectrified state
are charged with a compound electric fluid consisting of two
constituent, called by some the VITREOUS and RESINOUS, and hy
others the POSITIVE and NEGATIVE, fluids. These fluids are each
self-repulsive, but are mutually attractive. When they pervade
a body in equal quantity, their mutual attractions, neutralizing
each other, keep them in repose, like equal weights suspended
from the arms of a balance. When either is in excess, the
body is positively or negatively electrified, as the case may be,
the attraction or repulsion of the surplus of the redundant fluid
being effective.

1702. Results of scientific research independent of these hy-
potheses. Since the language in which the phenomena of
electricity are described and explained must necessarily have
relation to these hypotheses, it has been necessary in the first
instance thus briefly to state them. It must, however, be re-
membered, that the question of the validity of these theories
does not affect the conclusions which will be deduced from ob-
servation, the proper use of hypotheses being limited to their
convenience in supplying a nomenclature to the science and
in grouping and classifying the phenomena.

1703. Hypothesis of two fluids preferred. The hypothesis
of two fluids supplying, on the whole, the most complete and
satisfactory explanation of the phenomena, is that which we
shall here generally adopt; but we shall retain the terms POSI-
TIVE and NEGATIVE electricity, which, though they are derived
originally from the theory of a single fluid, are generally adopted
by scientific writers who adhere to the other hypothesis.

1704. Explanation of the above effects produced by the pith
balls. We are then to consider that when the glass tube and
woollen cloth are submitted to mutual friction, their natural
electricities are decomposed, the positive fluid passing to the
glass, and the negative to the cloth. The glass thus becomes
surcharged with positive, and the cloth with negative, electricity.

The pith ball K,fig. 475., touched by the glass, receives the



ELECTRICAL ATTRACTIONS AND REPULSIONS. 193

positive fluid from it, and the pith ball B' touched by the cloth
receives the negative fluid from it. The ball B therefore be-
comes positively, and the ball B' negatively, electrified by
contact.

Since the contrary electricities are mutually attractive, the
balls B and B' in this case attract each other ; and, since like elec-
tricities are mutually repulsive, the glass rod repels the ball B,
and the cloth repels the ball B'.

17C5. Electricity developed by various bodies. A numerous
class of bodies, when submitted to friction, produce effects
similar to those described in the case of glass and woollen cloth.

If a stick of resin or sealing-wax be rubbed by a woollen
cloth, like effects will follow : but, in this case, the electricity
of the wax or resin will be contrary to that of the glass, as may
be rendered manifest by the pith balls. If B be electrified by
contact with the glass, and B' by contact with the resin or wax,
they will attract each other, exactly as they did when B' was
electrified by contact with the cloth rubbed upon the glass.

It appears, therefore, that while glass is positively, resin is
negatively, electrified by the friction of woollen cloth.

1706. Origin of the terms vitreous and resinous fluids. It
was this circumstance which gave the name of VITREOUS elec-
tricity to the POSITIVE, and RESINOUS electricity to the NEGATIVE
fluid. This nomenclature is, however, faulty ; inasmuch as
there are certain substances by the friction of which glass will
be negatively electrified, and others by which resin will be
positively electrified.

When a woollen cloth is rubbed on resin or wax which, as
has been stated, it electrifies negatively, it is itself electrified
positively ; since the natural fluid being decomposed by the
friction, and the negative element going to the resin, the posi-
tive element must be developed on the cloth.

Thus it appears that the woollen cloth may be electrified by
friction either positively or negatively, according as it is rubbed
upon resin or upon glass.

1707. No certain test to determine which of the bodies sub-
mitted to friction receives positive, and which negative, elec-
tricity. In general, when any two bodies are rubbed together,
electricity is developed, one of them being charged with the
positive, and the other with the negative, fluid. A great
number of experimental researches have from time to time been

n. K



194 ELECTRICITY.

undertaken, with a view to the discovery of the physical law
which determines the distribution of the constituent electric
fluids in such cases between the two bodies, so that it might in
all cases be certainly known which of the two would be posi-
tively, and which negatively, electrified. These inquiries,
however, have hitherto been attended with no clear or certain
general consequences.

It has been observed, that hardness of structure is generally
attended with a predisposition to receive positive electricity.
Thus, the diamond, submitted to friction with other stones or
with glass, becomes positively electrified. Sulphur, when
rubbed with amber, becomes negatively electrified, the amber
being consequently positive ; but if the amber be rubbed upon
glass or diamond, it will be negative.

It is also observed that when heat is developed by the friction
of two bodies, that which takes most heat is negatively, and
the other positively, electrified.

In short, the decomposition of the electricity and its distri-
bution between the rubbing bodies is governed by conditions
infinitely various and complicated.

An elevation of temperature will frequently predispose a
body to take negative, which would otherwise take positive,
electricity. An increase of polish of the surface produces a
predisposition for the positive fluid. The colour, the molecular
arrangement, the direction of the fibres in a textile substance,
the direction in which the friction takes place, the greater or
less pressure used in producing it, all affect more or less in par-
ticular cases the interchange of the fluids and the relative elec-
tricities of the bodies. Thus, a black silk ribbon rubbed on
one of white silk takes negative electricity. If two pieces of
the same ribbon be rubbed transversely, one being stationary
and the other moved upon it, the former takes positive, the
latter negative, electricity. ./Epinus found that "copper and
sulphur rubbed together, and two similar plates of glass, evolved
electricity, but that the interchange of the fluids was not always
the same. There are substances, disthene, for example, which,
when submitted to friction, develope positive electricity at some
parts, and negative at other parts of their surface, although
their structure and the state of the surface be perfectly uniform.

1 708. Classification of positive and negative substances.
Of all known substances, a cat's fur is the most susceptible of



ELECTRICAL ATTRACTIONS AND REPULSIONS. 195

positive, and probably sulphur of negative electricity. Between
these extreme substances others might be so arranged, that any
substance in the list being rubbed upon any other, that which
holds the higher place will be positively, and that which holds
the lower place negatively, electrified. Various lists of this
kind have been proposed, one of which is as follows :



1. Fur of a cat.

2. Polished glass.

3. WooUen cloth.

4. Feathers.

5. Wood.

Pfaff gives the following :

1. Fur of a cat.

2. Diamond.

3. Fur of a dog.

4. Tourmaline.

5. Glass.

6. Wool. '

7. Paper.

Ritter proposes the following :

1. Diamond.

2. Zinc.

3. Glass.

4. Copper.

5. Wool.



6. P



6. aper.

7. Silk.

8. Gum lac.

9. Eough glass.



8. White silk.

9. Black silk.

10. Sealing-wax.

11. Colophon.

12. Amber.

13. Sulphur.



6. Silver.

7. Black silk.

8. Grey silk.

9. Grey manganeseous earth.
10. Sulphur.



1 709. Method of producing electricity by glass and silk with
amalgam. Experience has proved that the most efficient
means of developing electricity in great quantity and intensity
is by the friction of glass upon a surface of silk or leather
smeared with an amalgam composed of tin, zinc, and mercury
mixed with some unctuous matter. Two parts of tin, three of
zinc, and four of mercury, answer very well. Let some fine
chalk be sprinkled on the surface of a wooden cup, into which
the mercury should be poured hot. Let the zinc and tin melted
together be then poured in, and the box being closed and well
shaken, the amalgam may be allowed to cool. It is then finely
pulverized in a mortar, and being mixed with unctuous matter
may be applied to the rubber.



196 ELECTRICITY.

CHAP. II.

CONDUCTION.

1710. Conducting power. Bodies differ from each other in a
striking manner in the freedom with which the electric fluid
moves upon them. If the electric fluid be imparted to a certain
portion of the surface of glass or wax, it will be confined strictly
to that portion of the surface which originally receives it, by
contact with the source of electricity ; but if it be in like manner
imparted to a portion of the surface of a metallic body, it will
instantaneously diffuse itself uniformly over the entire extent
of such metallic surface, exactly as water would spread itself
uniformly over a level surface on which it is poured.

1711. Conductors and non-conductors. The former class
of bodies, which do not give free motion to the electric fluid on
their surface, are called NON-CONDUCTORS; and the latter, on
which unlimited freedom of motion prevails, are called CON-
DUCTORS.

1712. Classification of conductors according to the degrees of
their conducting power. Of all bodies the most perfect conductors
are the metals. These bodies transmit electricity instantaneously,
and without any sensible obstruction, provided their dimensions
are not too small in relation to the quantity of electricity im-
parted to them.

The bodies named in the following series possess the con-
ducting power in different degrees in the order in which they
stand, the most perfect conductor being first, and the most
perfect non-conductor last in the list. The black line divides
the most imperfect conductors from the most imperfect non-
conductors, but it must be observed that the position of this line
is arbitrary, the exact relative position of many of the bodies
composing the series not being certainly ascertained. The
series, however, will be useful as indicating generally the bodies
which have the conducting and non-conducting property in a
greater or less degree.



All the metals.
Well-burnt charcoal.
Plumbago.
Concentrated acids.



Powdered charcoal.
Dilute acids.
Saline solutions.
Metallic ores.



CONDUCTION.



197



Animal fluids.

Sea-water.

Spring-water.

Rain-water.

Ice above 13 Fahrenheit.

Snow.

Living vegetables.

Living animals.

Flame.

Smoke,

Steam.

Salts soluble in water.

Rarefied air.

Vapoiir of alcohol.

Vapour of ether.

Moist earth and stones.

Powdered glass.

Flowers of sulphur.



Caoutchouc.

Camphor.

Some siliceous and argillaceous

stones.
Dry marble.
Porcelain.

Dry vegetable bodies.
Baked wood.
Dry gases and air.
Leather.
Parchment.
Dry paper.
Feathers.
Hair.
Wool.
Dyed silk.
Bleached silk,
Eaw silk.
Transparent gems.
Diamond.
Mica.

All vitrifactions.
Glass.
Jet.
Wax.
Sulphur.
Resins.
Amber.
Gum-lac.



Dry metallic oxides.

Oils, the heaviest the best.

Ashes of vegetable bodies.

Ashes of animal bodies.

Many transparent crystals, dry.

Ice below 13 Fahrenheit.

Phosphorus.

Lime.

Dry chalk.

Native carbonate of barytes.

Lycopodium.

1713. Insulators. Good non-conductors are also called
INSULATORS, because when any body suspended by a non-con-
ducting thread, or supported on a non-conducting pillar, is
charged with electricity, such charge will be retained, since it
cannot escape by the thread or pillar, which refuses a passage
to it in virtue of its non-conducting quality.

Thus, a globe of metal supported on a glass pillar or suspended
by a silken cord being charged with electricity, will retain the
charge ; whereas, if it were supported on a metallic pillar, or
suspended by a metallic wire, the electricity would pass away
by its free motion over the surface of the pillar or the wire.

1714. Insulating stools. Stools formed with glass legs are
called INSULATING STOOLS, because any body charged with
electricity and placed upon them will retain its electric charge.

1715. Electrics and non-electrics obsolete terms. Conducting
bodies were formerly called NON-ELECTRICS, and non-conducting
bodies were called ELECTRICS, from the supposition that the latter
were capable of being electrified by friction, but the former not.



198 ELECTRICITY.

The incapability of conductors to be electrified by friction
was, however, afterwards shown to be only apparent, and
accordingly the use of these terms has been discontinued. .

If a rod of metal be submitted to friction, the electricity
evolved is first diffused over its entire surface in consequence
of its conducting property, and thence it escapes by the hand
of the operator which holds it, and which, though not as per-
fect a conductor as the metal, is sufficiently so to carry off
the electricity, so as to leave no sensible trace of it on the
metal.

But if the metal rod be suspended by a dry silken thread
(which is a good non-conductor), or be supported on a pillar
of glass, and then be struck several times with the fur of a cat,
it will be found to be negatively electrified, the fur which
strikes it being positively electrified.

1716. Two persons reciprocally charged with contrary elec-
tricity when placed on insulating stools. In like manner, two
persons standing on insulating stools, if one strike the other two
or three times with the fur of a cat, he that strikes will have
his body positively, and he that is struck negatively, electrified,
as may be ascertained by the method already explained, of pre-
senting to them successively the pith ball B, fig. 474., previously
charged with positive electricity. It will be repelled by the
body of him that strikes, and attracted by that of him who is
struck.

But if the same experiment be made without placing the two
persons on insulating stools, the same effects will not ensue,
because, although the electricities are developed as before by
the action of the fur, it immediately escapes through the feet
to the ground.

1717. The atmosphere a non-conductor. Atmospheric air
must manifestly belong to the class of non-conductors, for if it
gave a free passage to electricity, the electrical effects excited
on the surface of any body surrounded with it would soon pass
away; and no electrical phenomena of a permanent nature
could be produced, unless the bodies were removed from the
contact of the air. It is found, however, that resin and glass,
when excited by friction, retain their electricity for a considerable
time.

1718. Rarefied air a conductor. Air, however, when rarefied,
loses in a great degree its non-conducting property; and an



CONDUCTION. 199

electrified body soon loses its electricity if placed in the
exhausted receiver of an air-pump.

The electric fluid may therefore be considered as forming
a coating upon the surface of electrified bodies, and as being
held upon them by the tension or pressure of the surrounding
air.

1719. Use of the silk string which suspends pith balls. In
the experiments described in (1697) et seq. with the pith balls,
the silken string by which they are suspended acts as an in-
sulator. The pith of elder being a conductor, the electric fluid
is diffused over the ball ; but the silk being a non-conductor, it
cannot escape. If the ball were suspended by a metallic wire
attached to a stand composed of any conducting matter, the
electricity would escape, and the effects described would not
ensue. But if the metallic wire were attached to a glass rod or
other non-conductor, the same effects would be produced. In
that case the electricity would be diffused over the wire as well
as over the ball.

1720. Water a conductor. Water, whether in the liquid
or vaporous form, is a conductor, though of an order greatly
inferior to the metals. This fact is of great importance in
electrical phenomena. The atmosphere contains suspended in
it always more or less aqueous vapour, the presence of which
impairs its non-conducting property. Hence, electrical experi-
ments always succeed best in cold and dry weather.

Hence it appears why the most perfect non-conductors lose
their virtue if their surface be moist, the electricity passing
by the conducting power of the moisture.

1721. Insulators must be kept dry. This circumstance also
shows why it is necessary to dry previously the bodies on which
it is desired to develope electricity by friction.

It will be apparent from what has been explained, that it
would be more correct to designate bodies as good and bad
conductors in various degrees, than as conductors and non-con-
ductors. There exists no body which, strictly speaking, is
either an absolute conductor or absolute non-conductor.

1722. No certain test to distinguish conductors from non-
conductors. No relation has been discovered between the
physical conditions which determine the conduction of light and
heat, and those which determine the conduction of electricity.
Electricity is transmitted, not like light and heat, through the



200 ELECTRICITY.

interior dimensions of bodies, but only on their surfaces. Glass,
which is an almost perfect conductor of light, is a non-conductor
of heat and electricity. Sealing-wax, which is a non-conductor
of electricity, is also a non-conductor of light and heat. The
metals, on the other hand, are conductors of heat and electricity,
but are non-conductors of light. Water is a conductor of elec-
tricity and light, but a non-conductor of heat.

1723. Conducting power variously affected by temperature.
The conducting power of bodies is affected in different ways
by their temperature. In the metals it is diminished by eleva-
tion of temperature ; but in all other bodies, and especially in
liquids, it is augmented. Some substances, which are non-con-
ductors in the solid state, become conductors when fused.
Sir H. Davy found that glass raised to a red heat became a
conductor ; and that sealing-wax, pitch, amber, shell-lac, sul-
phur, and wax became conductors when liquefied by heat.

The manner in which electricity is communicated from one
body to another depends on the conducting property of the
body imparting and the body receiving it.

1724. Effects produced by touching an electrified body by a
conductor which is not insulated. If the surface of a non-
conducting body, glass, for example, be charged with electricity,
and be touched over a certain space, as a square inch, by a con-
ducting body which is not insulated, the electricity which is
diffused on the surface of contact will pass away by the con-
ductor, but no other part of the electricity with which the body
is charged will escape. A patch of the surface corresponding
with the magnitude of the conductor will alone be stripped of
its electricity.

The non-conducting property of the body will prevent the
electricity which is diffused over the remainder of its surface
from flowing into the space thus drained of the fluid by the
conductor.

But if the body thus charged with electricity, and touched
by a conductor not insulated, be a conductor, the effects pro-
duced will be very different. In that case, the electricity which
covers the surface of contact will first pass off; but the moment
the surface of contact is thus drained of the fluid which
covered it, the fluid diffused on the surrounding surface will
flow in and likewise pass off, and thus all the fluid diffused over
the entire surface of the body will rush to the surface of con-



CONDUCTION. 210

tact and escape. These effects, though, strictly speaking, suc-
cessive, will be practically instantaneous ; the time which
elapses between the escape of the fluid which originally covered
the surface of contact, and that which rushes from the most
remote parts to the surface of contact, being inappreciable.

1725. Effect produced when the touching conductor is in-
sulated. If a conducting body which is insulated and charged
with electricity, be brought into contact with another conduct-
ing body, which is also insulated and in its natural state, the
electricity will diffuse itself over the surfaces of both conductors
in proportion to their relative magnitudes.

If s express the superficial magnitude of an insulated con-
ducting body, E the quantity of electricity with which it is
charged, and s' the superficial magnitude of the other insulated
conductor with which it is brought into contact, the charge E
will, after contact, be shared between the two conductors in the
ratio of s to s' ; so that

E x ,= the charge retained by s,

s -f- s

s'

E x ; >= the charge received by s'.
s-f-s

1726. Why the earth is called the common reservoir. If
the second conductor s' be the globe of the earth, s' will bear
a proportion to s which, practically speaking, is infinite ; and
consequently the quantity of electricity remaining on s, ex-
pressed by

EX

s + s"

will be nothing. Hence the body s loses its entire charge
when put in conducting communication with the ground.

An electrified body being a conductor, is therefore reduced
to its natural state when put into electric communication with
the ground, and the earth has been therefore called the common
reservoir, to which all electricity has a tendency to escape, and
to which it does in fact always escape, unless its passage is in-
tercepted by non-conductors.

1727. Electricity passes by preference on the best conductors.
If several different conductors be simultaneously placed in
contact with an insulated electrified conductor so as to form a

K 5



202 ELECTRICITY.

communication between it and the ground, the electricity will
always escape by the best conductor. Thus, if a metallic
chain or wire be held in the hand, one end touching the ground
and the other being brought into contact with the conductor,
no part of the electricity will pass into the hand, the chain
being a better conductor than the flesh of the hand. But if,
while one end of the chain touch the conductor, the other be
separated from the ground, then the electricity will pass into
the hand, and will be rendered sensible by a convulsive shock.



CHAP. III.

INDUCTION.

1728. Action of electricity at a distance. If a body A
charged with electricity of either kind be brought into prox-
imity with another body B in its natural state, the fluid with
which A is surcharged will act by attraction and repulsion on
the two constituents of the natural electricity of B, attracting
that of the contrary, and repelling that of the same kind. This
effect is precisely similar to that produced on the natural
magnetic fluid in a piece of iron when the pole of a magnet is
presented to it.

If the body B in this case be a non-conductor, the electric fluid
having no free mobility upon its surface, its decomposition will
be resisted, and the body B will continue in its natural state not-
withstanding the attraction and repulsion exercised by A on the
constituents of its natural electricity. But if B be a conductor,
the fluids having freedom of motion on its surface, the fluid
similar to that with which B is charged will be repelled to the
side most distant from B, and the contrary fluid will be attracted
to the side next to B. Between these regions a neutral line will
separate those parts of the body B over which the two opposite
fluids are respectively diffused.

1729. Induction defined. This action of an electrified body
exerted at a distance upon the electricity of another body is called
INDUCTION, and is evidently analogous to that which produces
similar phenomena in the magnetic bodies (1630). j



INDUCTION. 203

1730. Experimental exhibition of its effects. To render
it experimentally manifest, let s and s'ifig. 476., be two metallic




Fig. 476.

balls supported on glass pillars ; and let A A." be a metallic cylin-
der similarly mounted, whose length is ten or twelve times
its diameter, and whose ends are rounded into hemispheres.
Let s be strongly charged with positive, and s' with negative



Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 21 of 45)