William Allen Miller.

Elements of chemistry: online

. (page 28 of 43)
Online LibraryWilliam Allen MillerElements of chemistry: → online text (page 28 of 43)
Font size
QR-code for this ebook


as developed by friction :

Soften a little sealing-wax in the flame of a candle, and draw
it out into a thread eight or ten inches long, and of the thickness



FIG. 131.



of a stout knitting pin. Attach to
one end of it a disk of paper about
an inch square, as represented in
fig. 131 ; suspend this rod and disk
by means of a paper stirrup and a
few fibres of unspun silk from a glass
rod fixed horizontally to some con-
venient support. Now rub a stick of
sealing-wax with a bit of dry flannel,
and bring it near the paper disk : the
disk will at first be strongly attracted,
and will then be as strongly driven
away. Whilst it is in this condition
of repulsion by the wax, bring towards
it a warm glass tube that has been
rubbed with a dry silk handkerchief;

the disk will be immediately attracted, and in an instant afterwards
it will again be repelled, but it will now be found to be attracted by




274 TWO KINDS OF ELECTRICITY.

the wax. It is therefore evident, that by the friction of the glass
and of the wax, two similar but opposite powers are developed. A
body which has been electrified or charged with electricity from the
wax is repelled by the wax ; but it is attracted by the excited
glass, and vice versa. In order to distinguish these two opposite
powers from each other that power which is obtained from the
glass has been termed vitreous or positive electricity ; that from
the wax resinous or negative electricity.

Let us suppose that the paper disk has been charged by
means of the glass tube, so that it is repelled on attempting to
bring the glass near it ; this state will be retained by the disk for
many minutes. This contrivance forms, in fact, an electroscope, for
it furnishes a means of ascertaining whether a body be electrified
or not, and even of indicating the kind of electricity. Suppose
that a body suspected to be electrified is brought near the disk,
which is in a state repulsive of the glass tube ; if repulsion occur
between the disk and the body which is being tested for elec-
tricity, it is at once obvious that the substance is electrified; and
moreover, that it is vitreously electrified, since it produces an effect
similar to that which would be exhibited by an excited glass tube.
The phenomena of attraction and repulsion may be further
exemplified by the following experiments : Suspend two
132. strawg ^ separately, by a fibre of silk, each to a glass
rod (fig. 132); bring an excited stick of sealing-wax
towards each; each will be first attracted and then
repelled : whilst thus repulsive to the wax, bring the
one near to the other ; they will recede from each other
as they did from the wax. If both straws be excited by
glass, they will in like manner repel each other ; but if
one be excited by the glass and the other by the wax
they will attract each other. Hence we learn, that
bodies similarly electrified repel, those differently elec-
trified attract each other.

Proceeding a step further, it will be found that whenever two
bodies are rubbed together, both kinds of electricity are liberated,
but so long as the two bodies remain in contact no sign of the
presence of either electricity appears ; on separating them, both
are found to be electrified one vitreously, the other resinously :
for example, stretch a piece of dry silk over a brass plate, and
rub it upon a glass plate ; so long as the two bodies are in con-
tact, the quantities of each kind of electricity set free are precisely
sufficient to neutralize each other, but as soon as the plates are



INSULATORS AND CONDUCTORS. 275

separated the glass will repel the disk (fig. 131), while the silk
will attract it.

(195) Insulators and Conductors. Bodies that have been thus
electrically excited, return to their neutral condition when touched
by other substances, but with degrees of rapidity depending on
the kind of body which touches them. A rod of sealing-wax or
of shell lac may, for example, be held in contact with any electrified
body without sensibly lessening the charge ; but the momentary
touch of a metallic wire, or of the hand, is sufficient to remove all
indications of electric excitement : it is therefore clear that there
are some bodies which readily allow the passage of electricity, and
these are termed conductors ; whilst there are others which do not
easily allow its passage, and these are called insulators. There is,
however, no absolute line of distinction between these two classes of
bodies ; there is no such thing as perfect insulation, or perfect con-
duction, for the two classes of bodies pass gradually one into the other.

In the following table each substance enumerated is superior
in insulating power to all those which follow it. The nearer the
substance is to the bottom of the table, the better, on the contrary,
is its conducting power :



Insulators.

Dry Gases and Dry Steam.

Shell Lac.

Sulphur.

Amber.

Resins.

Gutta Percha.

Diamond.

Silk.

Dry Fur.

Glass.

Ice.



Spermaceti.

Turpentine and Volatile Oils.

Fixed Oils.

String and Vegetable Fibres.

Moist Animal Substances.

Water.

Flame.

Melted Salts.

Plumbago.

Charcoal.

All the Metals.

Conductors.



Any object is spoken of as being electrically insulated when it
is supported by means of some badly-conducting substance which
prevents the free escape of the electricity. The presence of mois-
ture deposited from the air upon the surface even of the best insu-
lator, converts it for the time into a conductor, and is one of the
most annoying impediments to the success of electrical experi-
ments, as the power is carried off as fast as it is accumulated.
Glass is especially liable to this inconvenience, but by varnishing
it when practicable, and keeping it thoroughly warm, the difficulty

T2



276



ELECTROSCOPES.



FIG. 133.




is diminished. By due precautions instruments may be con-
structed which, in dry air, will preserve a charge for several hours.
The most perfect insulators still allow electric power to traverse
them, although by a process different from conduction, and hence
they are termed Dielectrics. Thus, if one side of a plate of glass be
electrified by rubbing it with a piece of silk, the opposite face also
acquires the power of attracting particles of bran or other light
objects.

(196) Electroscopes. Various instruments have been devised
for detecting feeble charges of electricity. One of the most con-
venient of these is the gold leaf electroscope
(fig. 133), which is sensible to extremely small
charges. It consists of a pair of gold leaves sus-
pended from the lower extremity of a metallic
wire w r hich terminates above in a brass plate.
The wire is insulated by passing through a
varnished glass tube, packed with silk, and the
whole is surrounded and supported by a glass
case. The approach of an excited body instantly
causes the divergence of the leaves. If a glass
tube be rubbed with a dry handkerchief and
touched with a small disk of paper insulated
by attaching it to a rod of sealing wax, as directed in preparing the

electroscope (fig. 131), a small vitreous
charge will be received by the paper, and if
carried by it to the cap of the electroscope,
the leaves will diverge permanently with
vitreous electricity. The approach of the
glass rod would cause the leaves to diverge
further, whilst that of a stick of excited
wax would cause them to collapse.

An instrument (fig. 134,) called a
torsion electrometer was devised by Coulomb
for accurately measuring minute differences
in the amount of electrical force. The
force which he opposed to that of elec-
tricity was the resistance to twisting
which is offered by an elastic thread. A
fibre of silk, a fine silver wire, or a thread
of glass, have been used for the purpose of measuring the angle
of torsion, this angle in perfectly elastic bodies being exactly pro-
portioned to the force applied.

By means of a long glass thread, fastened above to a pin, P



FIG. 134.




277

(carrying an index which traverses the graduated plate B), a needle
of shell lac is suspended freely in the glass case A. This needle is
terminated at one end by a gilt ball, b, at the other by a paper
disk which serves to check its oscillations. In the glass cover
of the instrument is a small aperture through which another gilt
ball, a (the carrier), also suspended by shell lac, can be introduced
and withdrawn. In order to equalize the induction, two narrow
strips of tinfoil, c and d, connected with the earth, and having a
narrow interval between them, are pasted upon the inside of the
glass cylinder, one a little above and tbe other a little below the
level of the balls; a graduated circle is pasted on the glass for
reading off the angular deviation of the needle. When the instru-
ment is to be used, the carrier ball is adjusted so that after it has
been removed it can with certainty be replaced in the same posi-
tion as at first ; the ball upon the needle is adjusted by turning
the pin until, without any twist upon the thread, it shall just touch
the carrier, its centre being at the zero of the scale, and the position
of the index on the upper graduated plate, B, is noted. The carrier
ball, 0, is next made to touch the object the electricity of which
is to be measured : it takes oif a quantity proportioned to the
amount accumulated on the spot. The ball a is immediately
replaced in the instrument ; it divides its charge with the ball b
on the needle, and repulsion ensues. The thread which supports
the needle is then twisted until the centre of the ball b is, by the
force of torsion, brought back towards the carrier, , to some deter-
minate angle (say 30) marked on the graduation of the glass case ;
suppose the number of degrees through which it has been necessary
to twist the thread to be 160; i6o + 3O, or 190 will represent the
repulsive force. To compare this amount with any other quantity,
the balls must be discharged, and the experiment repeated under
the new conditions, noting the number of degrees of torsion
required to make the needle stand at 30 as before : the amount
of the force is directly proportionate to the torsion angle in the two
cases. Suppose in a second experiment that the thread sustain
a twist of 1 80 before the ball b is brought back to the angle
of 30; the force will now be 180 + 30 or 2JO, and the relative
electrical repulsions in the two experiments will be as 190 : 210.

It was long imagined that non-conductors only were capable
of excitement by friction, and hence they were termed electrics ;
all bodies, however, if proper care be taken to insulate them,
exhibit this phenomenon. If, for example, a piece of brass tube
insulated by a glass handle be rubbed upon fur, it receives a
charge, as may be shown by bringing it near the disk of the elec-



278 ELECTRICAL HYPOTHESES.

troscope (fig. 131). Even two dissimilar metals, after being brought
into contact with each other may, with proper precautions, be
made to show signs of electric excitement on being separated (225).
The friction of glass against metal spread over silk is attended by
a more powerful development of electricity than when silk alone is
used ; and an amalgam consisting of I part of tin, 2 of zinc, and 6
of mercury, rubbed to fine powder and mixed with a little lard, is
found to be highly effectual in exalting the force which is developed.
The same substance, however, does not always manifest the same
electrical condition when rubbed : glass when rubbed upon silk
becomes vitreously excited ; but if rubbed on the fur of a cat it
exhibits resinous electricity. The amount of friction necessary
to produce electric excitement is exceedingly small; the mere
drawing of a handkerchief across the top of the electroscope (fig. 133),
or even across the clothes of a person insulated by standing on a
cake of resin, or on a stool with glass legs, provided he touch the
cap of the instrument, is sufficient to cause divergence of the leaves.
The simple act of drawing off silk stockings, or a flannel waistcoat,
or the combing of the hair in frosty weather, frequently occasions
the snapping and crackling noise due to the electric spark ; and the
stroking of the fur of a cat at such a season is known to produce
similar effects.

(197) Electrical Hypotheses. These various phenomena have
been accounted for by two principal hypotheses.

One of these, commonly known as the ' theory of one fluid/
is due to Franklin. Electricity, upon this view, is supposed to be
a subtle imponderable fluid, of which all bodies possess a definite
share in their natural or unexcited state. By friction, or otherwise,
this normal state is disturbed. If the body rubbed receive more
than its due share, it acquires vitreous electricity, or, in the terms
of Franklin, becomes electrified positively, or + ; whilst at the same
time the quantity of electricity in the rubber which becomes
resinously charged is supposed to be diminished, and thus the
rubber acquires a negative or state. Franklin supposed the
particles of the electric fluid to be highly self-repulsive, and to be
powerfully attractive of the particles of matter.

The other hypothesis, the ' theory of two fluids/ was origi-
nally proposed by Dufay. According to this view there are two
electric fluids, the vitreous and the resinous, equal in amount but
opposite in tendency; when associated together in equal quantity
they neutralize each other perfectly : a portion of this compound
fluid pervades all substances in their unexcited state. By friction
the compound fluid is decomposed ; the rubber acquires an excess






ELECTRICAL HYPOTHESES. 279

of one fluid, say the resinous, and thus becomes resiiiously excited ;
the body rubbed takes up the corresponding excess of vitreous
electricity, and becomes excited vitreously to an equal extent.
Upon this view the particles of each fluid are self-repulsive, but
powerfully attract those of the opposite kind.

The language of either theory may be employed in order to
distinguish the two kinds of electricity : the term vitreous or posi-
tive may be used indifferently for one kind, and resinous or nega-
tive for the other kind, provided it be borne in mind that positive
and negative are mere distinguishing terms : negative electricity
being as real a force as the positive.

It is manifest that one or other of these hypotheses must be
false, yet either will serve to connect the facts together. The
supposition of an electric fluid is, notwithstanding, gradually being
abandoned. The supposition of a gravitative fluid might, with
nearly as much propriety/be insisted on to explain the phenomena
of gravitation, or a cohesive fluid to account for those of cohesion.

Electricity is now regarded a compound force, remarkable for
the peculiar form of action and reaction which it exhibits. This
kind of action and reaction follows the same law of equality and
opposition in its manifestations as that which is exhibited more
obviously in the phenomena of mechanics. Whenever vitreous
electricity is manifested at one point, a corresponding amount of
resinous electricity is invariably developed in its vicinity, reacting
against it, and thus enabling its presence to be recognised, although
this reacting force may not be immediately perceptible.

The phenomena of vitreous and resinous electricity may be
rudely but not inaptly illustrated by those
of elasticity exhibited by an ordinary spring,
as shown at s, fig. 135. The spring in its
unstretched state may represent the body
in its unelectrified condition ; it then displays
nothing of the peculiar power that it possesses.
The spring cannot be stretched from one
extremity only; but if fixed at one end, as
by hooking it to the pin, p, a weight, w, may
be applied to the other end, and it will seem
to be stretched by one force only. In
reality, however, it is not so; for by substi-
tuting at v a weight equal in amount to that

at w, instead of the fixed point p, the strain upon the spring
remains unaltered, but a reaction, equal in amount to the original
action of the weight w, is instantly rendered evident.




280 ELECTRICAL INDUCTION.

So it is with electricity ; cases not ^infrequently occur where
only one kind of electricity seems to be present, but a careful
examination will always detect an equal amount of the opposite
kind. This essential character of action and reaction in the elec-
trical force will be more clearly manifested in the following
remarks and experiments.

(198) Electrical Induction. In the preceding cases the elec-
tricity has been excited by friction and communicated to other
bodies by contact. An insulated charged body, however, exerts
a remarkable action upon other bodies in its neighbourhood.
Long before contact occurs, the mere approach of an excited
glass tube towards the electroscope causes divergence of the
leaves, and on removing the glass, if it have not been allowed to
touch the cap of the instrument, all signs of disturbance cease.

The following mode of performing the experiment will afford
a means of examining this action o'f an electrified substance
upon objects at a distance :

Place two cylinders of wood,

FlG - J 3 6 - or of metal, each supported on a

varnished stem of glass, so as to
touch each other end to end (fig.
136, i); from the outer extremity
of each suspend a couple of pith
balls by a cotton thread, and bring
the excited glass tube near one end
of the arrangement as shown at 2.
Electric disturbance will be shown
by the repulsion of both pairs of
balls. Separate the two cylinders
without touching the conducting portion, and then remove the glass
tube; the balls will still continue to diverge (3). But let the
glass be again brought near ; the balls on the cylinder originally
nearest the glass will collapse, showing this cylinder to be
resinously excited, while the same excited glass will cause the
balls on the further cylinder to diverge from the presence of
vitreous electricity. Again, remove the glass altogether, and
bring the two cylinders into contact; a spark may sometimes
be seen to pass between them, and both pairs of balls will
immediately collapse and continue at rest. The entire amount
of force existing upon the two cylinders taken together remains
the same throughout the whole period of the experiment, but
its distribution is altered, as is shown by the position of the
signs + and . The experiment may be explained in the




ELECTRICAL INDUCTION.



281



following manner : Suppose the two cylinders to be in the
neutral state (No. i) ; on bringing the excited glass tube near to
them, the resinous, or negative electricity, appears to be drawn
towards the end of the cylinder nearest to the glass, as in No. 2,
whilst the disengaged positive electricity causes the balls on
both cylinders to diverge : the moment the glass is removed, the
negative electricity redistributes itself as in No. i, and the balls
collapse ; but if the two cylinders be separated before the glass is
removed, and if the excited glass be then withdrawn, the results
will be such as are represented in No. 3, in which the negative
electricity on one of the cylinders is more than sufficient to
neutralize the positive, and hence the balls diverge negatively;
while on the other it is less than sufficient for the positive, conse-
quently the balls diverge with positive electricity. On causing the
two cylinders to approach each other when in this state, the two
forces will neutralize each other, and if of sufficient power, the
reunion will be attended with a slight spark.

This action at a distance of one electrified body upon others in
its neighbourhood is termed electrical induction. It is a principle
of very extensive application, and indeed it furnishes a key to the
explanation of the greater number of electrical phenomena.

An instance of electrical induction is afforded in the action of
the gold leaf electroscope. Let i (fig. 137) represent the instru-

Fm. 137.




2 2

ment in a neutral state. As soon as an excited glass tube, G, is
caused to approach the cap of the electroscope, the leaves will
diverge, as at 2. Whilst the glass tube is still near the instru-
ment, let the cap of the electroscope be touched with the hand,
so as to uninsulate it for a moment, as at 3, by placing it in com-
munication with the earth through the body, which acts the part
of a conductor ; the leaves will collapse, and the instrument will
seem to be quiescent : now remove the finger from the cap, and
then take away the glass tube G ; instantly the leaves diverge, and
the electroscope is permanently charged, in consequence of a
change in the distribution of the electricity, as represented at 4.
But its charge is not positive like that of the glass, but negative ;



282



FARADAY S THEORY OF INDUCTION.



for, if the glass be again brought near, the leaves will collapse,
while a stick of excited wax will make them open out further.
These effects arise from electrical induction, and the process which
takes place is believed to be the following. The approach of the
tube in the first instance causes the negative electricity to accu-
mulate in the cap, as at 2, where it is retained by a species of
attraction. The leaves therefore diverge with a corresponding
quantity of positive electricity thus set free ; things being in this
state, a touch is sufficient to neutralize the excess of positive elec-
tricity, as seen in 3, and the instrument appears quiescent. Remove
the glass tube, however, and the negative electricity that had been
accumulated on the surface of the cap spreads over the whole instru-
ment, and the leaves diverge with negative electricity (4).

In all these cases the excited body itself, neither loses nor
gains electricity by the process just described. The mode in
which this transfer of force from a distance is effected still re-
mains to be considered.

(199) Faraday's Theory of Induction. We owe to Faraday
a theory of these effects which has been thus concisely summed
up by Sir W. S. Harris (Rudimentary Electricity, first edition,
pp. 33 and 34). Mr. Faraday ' conceives electrical induction
to depend on a physical action between contiguous particles,
which never takes place at a distance without operating
through the molecules of intervening non-conducting matter.
In these intermediate particles, a separation of the oppo-
site electricities takes place, and they become disposed in an
alternate series or succession of positive or negative points or
poles : this he terms a polarization of the particles,
' * 3 and in this way the force is transferred to a dis-
tance. Thus, if in fig. 138, p represent a positively charged
body, and abed intermediate particles of air, or other
Q Q Q a non-conducting matter, then the action of P is transferred
fe Q& to a distant body, N, by the separation and electrical
Q 9 Qo polarization of these particles, indicated by the series
6 Q Q c2 of black and white hemispheres. Now, if the particles

can maintain this state, then insulation obtains; but if
B the forces communicate or discharge one into the

other, then we have an equalization or combination
of the respective and opposite electricities throughout the whole
series, including p and N.' . . . . 'He assumes that all particles
of matter are more or less conductors ; that in their quiescent state
they are not arranged in a polarized form, but become so by the
influence of contiguous and charged particles. They then assume



DISTRIBUTION OF ELECTRIC CHARGE.



283



a forced state, and tend to return, by a powerful tension, to their
original normal position ; that being more or less conductors the
particles charge either bodily or by polarity ; that contiguous
particles can communicate their forces more or less readily one
to the other. When less readily, the polarized state rises higher,
and insulation is the result ; when more readily conduction is the

consequence/ f Induction of the ordinary kind is the

action of a charged body upon insulating matter, or matter the
particles of which communicate the electrical forces to each other
in an extremely minute degree ; the charged body producing in
it an equal amount of the opposite force, and this it does by
polarizing the particles' (fig. 138).

(200) Distribution of Electric Charge. Bodies susceptible of
this polarization are termed dielectrics ; and whether they be solid,
liquid, or aeriform, the electric force is transmitted through them
freely. A pane of glass interposed between the excited tube and the
cap of the electroscope will in no sensible manner affect the diver-



Online LibraryWilliam Allen MillerElements of chemistry: → online text (page 28 of 43)