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electricity, the cylinder A A' being in its natural state.

Let the balls s and s' be placed near the ends of the cylinder
A A', their centres being in line with its axis, as represented in
the figure. The positive electricity of s will now attract the
negative, and repel the positive constituent of the natural elec-
tricity of A A', so as to separate them, drawing the negative fluid
towards the end A, and repelling the positive fluid towards the
end A'. The negative electricity of s' will produce a like effect,
repelling the negative electricity of A A' towards A, and drawing
the positive towards A'.

Since the cylinder A A' is a conductor, and therefore the fluids
have freedom of motion on its surface, this decomposition will
take effect, and the half o A of the cylinder next to s will be
charged with negative, and the half o A' next to s' with positive
electricity.

That such is in fact the condition of A A' may be proved by
presenting a pith ball (1697) pendulum charged with positive
electricity to either half of the cylinder. When presented to
O A' it will be repelled, and when presented to o A it will be
attracted.

If the two balls s s' be gradually removed to increased but
equal distances from the ends A and A', the recomposition of the
fluids will gradually take place ; and when the balls are altogether

K 6



Jo



204 ELECTRICITY.

removed the cylinder A A' will recover its natural state, the
fluids which had been separated by the action of the balls being
completely recombined by their mutual attraction.

Let a metallic ring n',fig. 477., be supported on a rod or hook
of glass n, and let two pith balls b b' be'jSus-
C J pended from it by fine wires, so that when hanging

vertically they shall be in contact. Let a ball of
'/j^ 3 / " metal r, strongly charged with positive electricity,
be placed over the ring n' at a distance of eight
or ten inches above it. The presence of this ball
will immediately cause the pith balls to repel
v each other, and they will diverge to increased
\ distances the nearer the ball r is brought to the
Fig7477 . Tm S n '- If tn e ball r be gradually raised to
greater distances from the ring, the balls b b'
will gradually approach each other, and will fall to their position
of rest vertically under the ring when the ball r is altogether
removed.

If the charge of electricity of the balls s and s', fig. 476, or of
the ball r,fig. 477., be gradually diminished, the same effect will
be produced as when the distance is gradually increased ; and,
in like manner, the gradual increase of the charge of electricity
will have the same effect as the gradual diminution of the dis-
tance from the conductor on which the action takes place.

If the ring n', the balls b b', and the connecting wire be first
feebly charged with negative electricity, and then submitted to
the inductive action of the ball r charged with positive elec-
tricity, placed, as before, above the ring, the following effects will
ensue. When the ball r approaches the ring, the balls b b', which
previously diverged, will gradually collapse until they come into
contact. As the ball r is brought still nearer to n', they will
again diverge, and will diverge more and more, the nearer the
ball r is brought to the ring.

These various effects are easily and simply explicable by the
action of the electricity of the ball r on that of the ring. When
it approaches the ring, the positive electricity with which it is
charged decomposes the natural electricities of the ring, repel-
ling the positive fluid towards the balls. This fluid combining
with the negative fluid with which the balls are charged, neu-
tralizes it, and reduces them to their natural state: while this
effect is gradually produced, the balls b b' lose their divergence



INDUCTION. 205

and collapse. But when the ball r is brought still nearer to the
ring, a more abundant decomposition of the natural fluid is pro-
duced, and the positive fluid repelled towards the balls is more
than enough to neutralize the negative fluid with which they are
charged ; and the positive fluid prevailing, the balls again diverge
with positive electricity.

These effects are aided by the attraction exerted by the posi-
tive electricity of the ball r on the negative fluid with which
the balls b b' are previously charged.

If the electrified ball, instead of being placed above the ring,
be placed at an equal distance below the balls b b', a series of
effects will be produced in the contrary order, which the
student will find no difficulty in analysing and explaining.

If the ball r be charged with negative electricity, it will
produce the same effects when presented above the ring as
when, being charged with positive electricity, it is presented
below it.

In all cases whatever, the conductor whose electrical state
has been changed by the proximity of an electrified body returns
to its primitive electrical condition when the disturbing action
of such body is removed ; and this return is either instantaneous
or gradual, according as the removal of the disturbing body is
instantaneous or gradual.

1731. Effects of sudden inductive action. It appears, there-
fore, that sudden and violent changes in the electrical condition
of a conducting body may take place, without either imparting
to or abstracting from such body any portion of electricity.
The electricity with which it is invested before the inductive
action commences, and after such action ceases, is exactly the
same ; nevertheless, the decomposition and recomposition of the
constituent fluids, and their motion more or less sudden over it
and through its dimensions, are productive often of mechanical
effects of a very remarkable kind. This is especially the case
with imperfect conductors, which offer more or less resistance
to the reunion of the fluids.

1732. Example in the case of a frog. Let a frog be sus-
pended by a metallic wire which is connected with an insulated
conductor, and let a metallic ball, strongly charged with positive
electricity, be brought under without, however, touching it.
The effects of induction already described will ensue. The
positive fluid will be repelled from the frog towards the insulated



206 ELECTRICITY.

conductor, and the negative fluid will be attracted towards it,
so that the body of the frog will be negatively electrified ; but
this taking place gradually as the electrified ball approaches, is
attended with no sensible mechanical effect.

If the electrified ball, however, be suddenly discharged, by
connecting it with the ground by a conductor, an instantaneous
revulsion of the electric fluids will take place between the body
of the frog and the insulated conductor with which it is con-
nected ; the positive fluid rushing from the conductor, and the
negative fluid from the frog, to recombine in virtue of their
mutual attraction. This sudden movement of the fluids will
be attended by a convulsive motion of the limbs of the frog.

1733. Inductive shock ofjhe human body. If a person stand
close to a large conductor strongly charged with electricity, he
will be sensible of a shock when this conductor is suddenly
discharged. This shock is in like manner produced by the
sudden recomposition of the fluids in the body of the patient, by
the previous inductive action of the conductor.

1734. Development of electricity by induction. A conductor
may be charged with electricity by an electrified body, though
the latter shall not lose any of its own electricity or impart any
to the conductor so electrified. For this purpose, let the con-
ductor to be electrified be supported on a glass pillar so as to
insulate it, and let it then be connected with the ground by a
metallic chain or wire. If it be desired to charge it with
positive electricity, let a body strongly charged with negative
electricity be brought close to it without touching it. On the
principles already explained, the negative electricity of the con-
ductor will be repelled to the ground through the chain or wire ;
and the positive electricity will, on the other hand, be attracted
from the ground to the conductor. Let the chain or wire be
then removed, and, afterwards, let the electrified body by whose
inductive action the effect is produced be removed. The con-
ductor will remain charged with positive electricity.

It may in like manner be charged with negative electricity,
by the inductive action of a body charged with positive elec-
tricity.



ELECTRICAL MACHINES.



207



CHAP. IV.



ELECTRICAL MACHINES.

1735. Parts of electrical machines. An electrical machine is
an apparatus by means of which electricity is developed and
accumulated in a convenient manner for the purposes of experi-
ment.

All electrical machines consist of three principal parts, the
rubber, the body on whose surface the electric fluid is evolved,
and one or more insulated conductors, to which this electricity
is transferred, and on which it is accumulated.

The rubber is a cushion stuffed with hair, bearing on its
surface some substance, which by friction will evolve electricity.
The body on which this friction is produced is glass, so shaped
and mounted as to be easily and rapidly moved against the
rubber with a continuous motion. This object is attained by
giving the glass the form either of a cylinder revolving on its
geometrical axis, or of a circular plate revolving in its own
plane on its centre.

The conductors are bodies having a metallic surface and a
great variety of shapes, and always mounted on insulating
pillars, or suspended by insulating cords.

1736. The common cylindrical machine. A hollow cylinder
of glass A B, Jig. 478., is supported in bearings at c, and made to




Fig. 478.



Fig. 479.



revolve by means of the wheels c and D connected by a band, a
handle R being attached to the greater wheel. The cushion H,
represented separately in fig. 479., is mounted on a glass pillar,




208 ELECTRICITY.

and pressed with a regulated force against the cylinder by means
of springs fixed behind it. A chain K i,, fig. 478., connects the
cushion with the ground. A flap of black silk equal in width
to the cushion covers it, and is carried over the cylinder, ter-
minating above the middle of the cylinder on the opposite side.
The conductor is a cylinder of thin brass M N, the ends of
which are parts of spheres greater than hemispheres. It is
supported by a glass pillar o P. To the end of the conductor
next the cylinder is attached a row of points represented
separately in fig. 480 , which are presented close to the surface
of the cylinder, but without touching it.
The extent of this row of points corre-
sponds with that of the rubber.

As the efficient performance of the
machine depends in a great degree on the
good insulation of the several parts, and as glass is peculiarly
liable to collect moisture on its surface which would impair its
insulating virtue, it is usual to cover the insulating pillars of the
rubber and conductor, and all that part of the cylinder which
lies outside the cushion and silk flap, with a coating of resinous
varnish, which, while its insulating property is more perfect
than that of glass, offers less attraction to moisture.

To explain the operation of the machine, let us suppose that
the cylinder is made to revolve by the handle K. Positive
electricity is developed upon the cylinder, and negative elec-
tricity on the cushion. The latter passes by the conducting
chain to the ground. The former is carried round under the
flap, on the surface of the glass, until it arrives at the points
projecting from the conductor. There it acts by induction
(1729) on the natural electricity of the conductor, attracting
the negative electricity to the points and repelling the positive
fluid. The negative electricity issuing from the points com-
bines with and neutralizes the positive fluid diffused on the
cylinder, the surface of which, after it passes the points, is
therefore restored to its natural state, so that when it arrives
again at the cushion it is prepared to receive by friction a fresh
charge of the positive fluid.

It is apparent, therefore, that the effect produced by the
operation of this machine is a continuous decomposition of the
natural electricity of the conductors, and an abstraction from it
of just so much negative fluid as compensates for that which



ELECTRICAL MACHINES.



209



escapes by the cushion and chain KL to the earth. The con-
ductor is thus as it were drained of its negative electricity by a
stream of that fluid, which flowing constantly from the points
passes to the cylinder, and thence by the cushion and chain to
the earth. The conductor is therefore left surcharged with
positive electricity.

1737. Naime's cylinder machine. This apparatus, which is
adapted to produce at pleasure either positive or negative elec-
tricny, is similar to the last, but has a second conductor MF,
fig. 481., in connection with the cushion. When it is desired
to collect positive electricity, the conductor MF is put in con-
nection with the ground, and the machine acts as that described
above. When it is desired to collect negative electricity, the
conductor M" B is put in connection with the ground, and the
conductor MF is insulated. In this case a stream of positive
electricity flows continually from MF through the cushion to
the cylinder, and thence by the conductor JI'B to the 'ground,
leaving the conductor MF charged with negative electricity.





Fig. 481.

1738. Common plate machine. This apparatus consists of
a circular plate of glass AB, fig. 482., mounted as represented in
the figure. It is embraced between two pair of cushions at E
and E', a corresponding width of the glass being covered by
a silk sheathing extending to F', where the points of the
conductors are presented. The handle being turned in the
direction of the arrow, and the cushions being connected by
conducting chains with the ground, positive electricity is de-
veloped on the glass, and neutralized as in the cylinder machine,
by the negative electricity received by induction from the con-



210 ELECTRICITY.

ductors, which consist of a long narrow cylinder, bent into a
form to adapt it to the plate. It is represented at MN, a branch
MO being carried parallel to the plate and bent into the form
MOPQ, so that the part PQ shall be presented close to the plate
under the edge of the silk flap. A similar branch of the con-
ductor extends on the other side, terminating just above the
edge of the lower silk flap.

The principle of this machine is similar in all respects to
that of the common cylinder machine. With the same weight
and bulk, the extent of rubbing surface, and consequently the
evolution of electricity, is much greater than in the cylinder
machines.

1739. Armstrong's hydro-electrical machine. A new species
of electric machine has resulted from the accidental observation
of an electric shock produced by the contact of a jet of high
pressure steam issuing from a boiler at Newcastle-on-Tyne in
1840. Mr. Armstrong of that place took up the inquiry, and
succeeded in contriving a machine for the production and
accumulation of the electricity by the agency of steam. Pro-
fessor Faraday investigated the theory of the apparatus, and
showed that the origin of the electrical development was the
friction of minute aqueous particles produced by the partial
condensation of the steam against the surface of the jet from
which the steam issued.

The hydro-electric machine has since been constructed in
various forms and dimensions.

Let a cylindrical boiler a, fig. 483., whose length is about
twice its diameter, be mounted on glass legs v, so as to be in a
state of insulation.

f is the furnace door, the furnace being a tube within the
boiler.

s is the safety-valve.

h is the water-gauge, a glass tube indicating the level of the
water in the boiler.

r a regulating valve, by which the escape of steam from the
boiler may be controlled.

t a tube into which the steam rushes as it escapes from r.

e three or more jet pipes, through which the steam passes
from t, and from the extremities of which it issues in a
series of parallel jets.

d a condensing box, the lower half of Avhich contains water
at the common temperature.



ELECTRICAL MACHINES.



211



g the chimney.

g' an escape pipe for the vapour generated in the condensing

box d.

b the conductor which takes from the steam the electricity
which issues with it from the jet pipes e.

k the knob of the conductor from which the electricity may

be received and collected for the purpose of experiment.




Fig. 483.

The jet pipes e traverse the middle of the condensing box d,
above the surface of the water contained in it. Meshes of
cotton thread surround these tubes within the box, the ends of
which are immersed in the water. The water is drawn up by
the capillary action of these threads, so as to surround the tubes
with a moist coating, which by its low temperature produces a
slight condensation of the steam as it passes through that part
of the tube.

The fine aqueous particles thus produced within the tube are
carried forward with the steam, and on issuing through the jet
pipe rub against its sides. This friction decomposes the natural




212 ELECTRICITY.

electricity, the negative fluid remaining on the jet, and the
positive being carried out with the particles of water, and im-
parted by them to the conductor b.

It will be apparent that in this arrangement the interior
surface of the jet plays the part of the rubber of the ordinary
machine, and the particles of water that of the glass cylinder
or plate, the steam being the moving power which maintains
the friction.

In order to ensure the efficiency of the friction, the conduit
provided for the escape of the steam is not straight but angular.
A section of the jet pipe near its extremity
is represented in fig, 483 a. The steam
issuing from the box b encounters a plate
of metal m which intercepts its direct pas-
sage to the mouth of the jet. It is corn-
Fig. 483 a " P e M e d to turn downwards, pass under the
edge of this plate, and, rising behind it,
turn again into the escape pipe, which is a tube formed of par-
tidge wood enclosed with in the metal pipe n.

It is found that an apparatus thus constructed, the length of
the boiler being 32 inches and its diameter 16 inches, will
develope as much electricity in a given time as three common
plate machines, whose plates have a diameter of 40 inches, and
are worked at the rate of 60 revolutions per minute.

A machine on this principle, and on a great scale of magni-
tude, was erected by the Royal Polytechnic Institution of
London, the boiler of which was 78 inches long, and 42 inches
diameter. The maximum pressure of the steam at the com-
mencement of the operation was sometimes 90 Ibs. per sq. inch.
This, however, fell to 40 Ibs. or less. Sparks have been ob-
tained from the conductor at the distance of 22 inches.

1740. Appendages to electrical machines. To facilitate the
performance of experiments, various accessories are usually
provided with these machines.

1741. Insulating stools. Insulating stools, constructed of
strong, hard 'wood, well baked and dried, and supported on legs
of glass coated with resinous varnish, are useful when it is re-
quired to keep for any time any conducting body charged with
electricity. The body is placed on one of these stools while it
is being electrified.

Thus, two persons standing on two such stools, may be



ELECTRICAL MACHINES.



213



charged, one with positive, and the other with negative, elec-
tricity. If, when so charged, they touch each other, the con-
trary electricities will combine, and they will sustain a nervous
shock proportionate to the quantity of electricity with which
they were charged.

1742. Discharging rods. Since it is frequently necessary
to observe the effects of points and spheres, pieces such as
Jigs. 484, 485. are provided, to be inserted in holes in the con-
ductors ; also metallic balls, Jigs. 486, 487., attached to glass
handles for cases in which it is desired to apply a conductor to
an electrified body without allowing the electricity to pass to
the hand of the operator. With these rods the electricity may
be taken from a conductor gradually by small portions, the
ball taking by each contact only such a fraction of the whole
charge as corresponds to the ratio of the surface of the ball to
the surface of the conductor.

1743. Jointed dischargers. To establish a temporary con-
nection between two conductors, or between a conductor and
the ground, the jointed dischargers, Jigs. 488, 489., are useful.
The distance between the balls can be regulated at pleasure by
means of the joint or hinge by which the rods are united.




Fig. 484. Fig. 485. Fig. 486. Fig. 487.



Fig. 489.



1744. Universal discharger. The universal discharger, an
instrument of considerable convenience and utility in experi-
mental researches, is represented in Jig. 490. It consists of a
wooden table to which two glass pillars A and A' are attached.
At the summit of these pillars are fixed two brass joints capable
of revolving in a horizontal plane. To these joints are at-



214 ELECTRICITY.

tached brass rods cc', terminated by balls DD', and having glass
handles EE'. These rods play on joints at BB', by which they
can be moved in vertical planes.




Fig. 490.

The balls DD' are applied to a wooden table sustained on a
pillar capable of having its height adjusted by a screw T. On
the table is inlaid a long narrow strip of ivory extending in
the direction of the balls DD'. These balls DD' can be un-
screwed, and one or both may be replaced by forceps, by which
may be held any substance through which it is desired to
transmit the electric charge. One of the brass rods c is con-
nected by chain or a wire with the source of electricity, and
the other with the ground.

The electricity is transmitted by bringing the balls DD' with
the substance to be operated on between them, within such a
distance of each other as will cause the charge to pass from one
to the other through the introduced substance.



CHAP. V.

CONDENSER AND ELECTROPHOROUS.

1745. Reciprocal inductive effects of two conductors. If a
conductor A communicating with the ground be placed near
another conductor B insulated and charged with a certain
quantity of electricity E, a series of effects will ensue by the
reciprocal inductive power of the two conductors, the result of
which will be that the quantity of electricity with which B is
charged will be augmented in a certain proportion, depending
on the distance between the two conductors through which the



CONDENSER AND ELECTROPHOROUS. 215

inductive force acts. The less Ibis distance is the more energetic
the induction will be, and the greater the augmentation of the
charge of the conductor B.

To explain this, we are to consider that the electricity E,
acting on the natural electricity of A, repels a certain quantity
of the fluid of the same name to the earth, retaining on the
side of A next to B the fluid of the contrary name. This fluid
of a contrary name thus developed in A reacts upon the natural
electricity o,f B, and produces a decomposition in the same
manner, augmenting the charge E by the fluid of the same
name decomposed, and expelling the other fluid to the more
remote side of B. This increased fluid in B again acts upon
the natural electricity of A, producing a further decomposition ;
and this series of reciprocal inductive actions producing a suc-
cession of decompositions in the two conductors, and accumu-
lating a tide of contrary electricities on the sides of the con-
ductors which are presented towards each other, goes on through
an indefinite series of reciprocal actions, which, nevertheless,
are accomplished in an inappreciable interval of time ; so that,
although the phenomenon in a strict sense is physically pro-
gressive, it is practically instantaneous.

To obtain an arithmetical measure of the amount of the
augmentation of the electrical charge produced in this way, let
us suppose that a quantity of electricity on B, which we shall
take as the unit, is capable of decomposing on A a quantity
which we shall express by m, and which is necessarily less than
the unit, because nothing short of actual contact would enable
the electricity of B to decompose an equal quantity of the elec-
tricity of A.

If, then, the unit of positive electricity act from B upon A, it
will decompose the natural electricity, expelling a quantity of



Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 22 of 45)