Dionysius Lardner.

Hand-book of natural philosophy and astronomy (Volume 2) online

. (page 30 of 45)
Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 30 of 45)
Font size
QR-code for this ebook

of tinfoil, with one end pointed and the other broad, are laid
in two watch-glasses which contain water, in such a manner,
that the pointed part lies in one glass and the broad part in the
other. After some time, they develope at their poles a feeble
electricity, which they retain for several days, the metal pole
being positive in the dry pile, and the pointed end of the zinc
in the moist one.

1899. Piles of a single metal. Piles of a single metal have
been constructed by causing one surface to be exposed to a
chemical action different from the other. This may be effected
by rendering one surface smooth and the other rough. A pile
of this kind has been made with sixty or eighty plates of zinc
of four square inches surface. These are fixed in a wooden
trough parallel to each other, their polished faces looking the
same way, and an open space of the tenth to the twentieth of
an inch being left between them, these spaces being merely
occupied by atmospheric air. If one extremity of this apparatus
be put in communication with the ground, the other pole will
sensibjy affect an electroscope.


In this case, the electromotive action takes place between
the air and the metal.

1900. Hitter's secondary piles. The secondary piles, some-
times called HITTER'S PILES, consist of alternate layers of homo-
geneous metal plates, between which some moist conducting
substance is interposed. When they stand alone, no electro-
motive force is developed ; but, if they be allowed to continue
for a certain time in connexion with the poles of a battery,
and then disconnected, positive electricity will be found to
be accumulated at that end which was connected with the
positive pole, and negative electricity at the other end ; and
this polar condition will continue for a certain time, which will
be greater the less the electrical tension imparted. This phe-
nomenon has not been satisfactorily explained, but would seem
to arise from the low conducting power of the strata of liquid
interposed between the plates.



1901. The voltaic current. The voltaic pile differs from the
electrical machine inasmuch as it has the power of constantly
reproducing whatever electricity may be drawn from it by con-
ductors placed in connexion with its poles, without any manipu-
lation, or the intervention of any agency external to the pole
itself. So prompt is the action of this generating power, that
the positive and negative fluids pass from the respective poles
through such conductors in a continuous and unvarying stream,
as a liquid would move through pipes issuing from a reser-
voir. The pile may indeed be regarded as a reservoir of the
electric fluids, with a provision by which it constantly replenishes


If two metallic wires be connected at one end with the poles
r and ff,fig. 558., of the pile, and at the other with any con-
ductor o, through which it is required to transmit the electri-
city evolved in the pile, the positive fluid will pass from p


along the wire to o, and the negative fluid in like manner from
N to o. The positive fluid will therefore form a stream or

Fig. 558.

current from p through o to N, and the negative fluid a con-
trary current from N through o to P.

It might be expected that the combination of the two
opposite fluids in equal quantity would reduce the wire to its
natural state ; and this would, in fact, be the case, if the fluids
were in repose upon the wire, which may be proved by de-
taching at the same moment the ends of the wires from the poles
p and N. The wires and the conductor o will, in that case,
show no indication of electrical excitement. If the wire be
detached only from the negative pole N, it will be found, as well
as the conductor o, to be charged with positive electricity ; and
if it be detached from the positive pole P, they will be charged
with negative electricity, the electricity in each case being in
repose. But when both ends of the wire are in connexion
with the poles P and N, the fluids, being in motion in contrary
directions along the wire and intermediate conductors, impart
to these qualities which show that they are not in the natural
or unelectrified state, but which have nothing in common with
the qualities which belong to bodies charged with the electric
fluid in repose. Thus, the wire or conductor will neither
attract nor repel pith balls, nor produce any electroscopic
effects. They will, however, produce a great variety of other
phenomena, which we shall presently notice.

The state of the electricities in thus passing between the
poles of the piles through a metallic wire or other conductor
exterior to the pile, is called a VOLTAIC CURRENT.

1902. Direction of the current. Although, according to
what has been stated, this current consists of two streams
flowing in contrary directions, it receives its denomination ex-
clusively from the positive fluid ; and, accordingly, the DIUEC-


TIOX OF THE CURRENT is always from the positive pole through
the wire or other conductor to the negative pole.

It is necessary, however, to observe, that in passing through
the pile itself from element to element, it moves from the
negative pole to the positive pole. The direction and course of
the current is indicated \\\fig. 558. by the arrows.

1903. Poles of the pile, how distinguished. In designat-
ing the poles of the pile, much confusion and obscurity, and
consequent difficulty to students, has arisen from identifying
the poles of the pile with the extreme plates of metal com-
posing it. In the piles first constructed by Volta, the last plate
at the positive end was zinc, and the last at the negative end
copper ; and such an arrangement was often retained at more
recent periods. Hence the positive pole was called the zinc
pole, and the negative pole the copper pole. The extreme
plates being afterwards dispensed with, the final plate at the
positive end became copper, and that at the negative end zinc ;
and, consequently, the positive pole was then the copper pole,
and the negative pole the zinc pole.

This confusion, however, may be avoided, by observing that
the poles, positive or negative, are not dependent on the plates
or cylinders of metal with which the pole may terminate, but
on the direction of the electromotive forces of its elements. In
general, in a pile composed of zinc and copper elements, the
zinc plates of each pair all look towards the positive pole, and
the copper plates towards the negative pole. It must, how-
ever, be observed, that, in the ordinary arrangement, one ele-
ment of a pair is placed at each extremity of the pile, and con-
stitutes its pole, the pair being only completed when the poles
are united by the conducting wire. Thus, the pole to which
the copper elements look, terminates in a zinc plate in contact
with the exciting liquid, but r.ot with the adjacent copper
plate ; and the pole to which the zinc plates look terminates
in like manner with a copper plate in contact with the ex-
citing liquid, but not with the adjacent zinc plate. The single
extreme plates of zinc and copper thus forming the poles of the
pile being connected by the conducting wire, form, in fact, a
pair through which the current passes exactly as it passes
through any other pair in the series.

1904. Voltaic circuit. When the poles are thus connected
by the conducting wire, the VOLTAIC CIRCUIT is said to be



complete, and the current continually flows, as well through the
pile as through the conducting wire. In this state the pile
constantly evolves electricity at its electromotive surfaces, to
feed and sustain the current ; but if the voltaic circuit be not
completed by establishing a continuous conductor between pole
and pole, then the electricity will not be in motion, no current
will flow ; but the wire or other conductor which is in con-
nexion with the positive pole will be charged with positive, and
that in connexion with the negative pole will be charged with
negative electricity, of a certain feeble tension, and in a state
of repose. Since, in such case, the electricity with which the
pile is charged has no other escape than by the contact of the
surrounding atmosphere, the electromotive force is in very
feeble operation, having only to make good that quantity which
is dissipated by the air. The moment, however, the voltaic
circuit is completed, the pile enters into active operation, and
generates the fluid necessary to sustain the current.

These are points which it is most necessary that the student
should thoroughly study and comprehend ; otherwise, he will
find himself involved in great obscurity and perplexity as he
attempts to proceed.

1905. Case in which the earth completes the circuit. If the
conducting wires connected with the poles p and N, instead of
being connected with the conductor Q,fig. 558., be connected with
the ground, the earth itself \vi\[ take the place and play the part
of the conductor o in relation to the current. The positive fluid
will in that case flow by the wire P E, fig. 559., and the

Fig. 559.

negative fluid by the wire N E to the earth E ; and the two fluids
will be transmitted through the earth E E in contrary directions,


exactly in the same manner as through the conductor O. In
this case, therefore, the voltaic circuit is completed by the
earth itself.

1906. Methods of connecting the poles with the earth. In
all cases, in completing the circuit, it is necessary to ensure
perfect contact wherever two different conductors are united.
"We have already explained the application of mercurial cups
and metallic clamps for this purpose, where the conductors to
be connected are wires or strips of metal. When the earth is
used to complete the circuit, these are inapplicable. To ensure
the unobstructed flow of the current in this case, the wire is
soldered to a large plate of metal, having a surface of several
square feet, which is buried in the moist ground, or, still better,
immersed in a well or other reservoir of water.

In cities, where there are extensive systems of metallic pipes
buried for the conveyance of water or gas, the wires proceeding
from the poles P and N may be connected with these.

There is no practical limit to the distance over which a
voltaic current may in this manner be carried, the circuit being
still completed by the earth. Thus, if while the pile PN, Jig.
559., is at London, the wire P E is carried to Paris or Vienna
(being insulated throughout its entire course), and is put in
communication with the ground at the latter place, the current
will return to London through the earth E E as surely and as
promptly as if the points E E were only a foot asunder.

1907. Various denomination of currents. Voltaic currents
which pass along wires are variously designated, according to
the form given to the conducting wire. Thus they are RECTI-
LINEAR CURRENTS when the wire is straight ; INDEFINITE
CURRENTS when it is unlimited in length ; CLOSED CURRENTS
when the wire is bent so as to surround or inclose a space ;
CIRCULAR or SPIRAL CURRENTS when the wire has these

1908. The electric Jluid forming Ihe current not necessarily
in motion. Although the nomenclature which has been adopted
to express these phenomena implies that the electric fluid has a
motion of translation along the conductor similar to the motion
of liquid in a pipe, it must not be understood that the existence
of such motion of the electric fluid is necessarily assumed, or
that its non-existence, if proved, could disturb the reasoning


or shake the conclusions which form the basis of this branch
of physics. Whether an actual motion of translation of the
electric fluid along the conductor exist or not, it is certain that
the effect which would attend such a motion is propagated along
the conductor ; and this is all that is essential to the reasoning.
It has been already stated, that the most probable hypothesis
which has been advanced for the explanation of the phenomena
rejects the motion of translation, and supposes the effect to be
produced by a series of decompositions and recompositions of
the natural electricity of the conductor (1826).

1909. Method of coating the conducting wires. When the
wires by which the current is conducted are liable to touch
other conductors, by which the electricity may be diverted from
its course, they require to be coated with some non-conducting
substance, under and protected by which the current passes.
Wires wrapped with silk or linen thread may be used in sucli
cases, and they will be rendered still more efficient if they are
coated with a varnish of gum-lac.

When the wires are immersed in water, they may be pro-
tected by enclosing them in caoutchouc or gutta percha.

If they are carried through the air, it is not necessary to
surround them with any coating, the tension of the voltaic
electricity being so feeble, that the pressure of the air and its
non-conducting quality are sufficient for its insulation.

1910. Supports of conducting wire. When the wire is
carried through the air to such distances as would render its
weight too great for its strength, it requires to be supported at
convenient intervals upon insulating props. Rollers of por-
celain or glass, attached to posts of wood, are used for this
purpose in the case of telegraphic wires.

1911. Ampere's reotrope to reverse the current. In experi-
mental inquiries respecting the effects of currents, it is frequently
necessary to reverse the direction of a current, and sometimes
to do so suddenly, and many times in rapid succession. An
apparatus for accomplishing this, contrived by Ampere, and
which has since undergone various modifications, has been
denominated a commutator, but may be more appropriately
named a REOTROPE, the Greek words pt'oc (reos) signifying a
current, and rpoTrof (tropos}, a turn.

Let two grooves rr, fig. 560., about half an inch in width



and depth, be cut in a board, and
between them let four small cavi-
ties v, t, v', t' be formed. Let these
cavities be connected diagonally in
pairs by strips of copper II' and
mm', having at the place where
they cross each other a piece of
cloth or other non-conducting sub-
stance between them, so as to pre-
vent the electricity from passing
from one to the other. Let the
grooves r and r', and the four cavi-
ties, be masticated on their surfaces with resin, so as to render
them non-conductors.

These grooves and cavities being filled with mercury, let the
apparatus represented in jig. 561. be placed upon the board.
A horizontal axis a a' moves in two holes oo' made in the up-
right pieces pp'. It carries four rectangular pieces of metal

e, c', d, d', so adapted, that
when they are pressed
downwards one leg of
each will dip into the mer-
cury in the groove, and
the other into the adjacent
cavity. The arms uniting
the rectangular metallic

pieces are of varnished wood, and are therefore non-conductors.
When this apparatus is in the position represented in the
figure, it will connect the groove r with the cavity v, and the
groove r' with the cavity t. When the ends dd! are depressed,
and therefore cc' elevated, it will connect the groove r with the
cavity t', and the groove r' with the cavity t.

The conductor which proceeds from the positive pole of the
pile is immersed in the mercury in r, and that which comes
from the negative pole is immersed in the mercury in r'. Two
strips of copper bb' connect the mercury in the cavities t' and
v', with the wire ww f which carries the current.

The apparatus being arranged as represented in^z^. 560., the
current will pass from the pile to the mercury in r; thence to
v by the conductor c ; thence to v' by the diagonal strip of
metal IV ; thence to w by the metal b f , and will pass along the


wire as indicated by the arrows to b ; thence it will pass to the
mercury in t; thence by the diagonal strip m' in to t! ; thence
by the conductor c' to the mercury in the groove r' ; and thence,
in fine, to the negative pole of the pile.

If the ends dd' be depressed, and the ends cc' elevated, the
course of the current may be traced in like manner, as
follows : from r to t r ; thence by b to w; thence along the
conducting wire in a direction contrary to that of the arrows
to b' ; thence to v; thence to r; and thence to the negative
pole of the pile.

1912. PohFs reotrope. Various forms have more recently
been given to reotropes, one of the most convenient of which
is that of Pob.1, in which the use of mercury is dispensed with.
Four small copper columns A, B, c, D, Jig. 562., about inch
diameter, are set in a square board, and connected diagonally,
A with D, and B with c, by two bands of
copper, which intersect without contact.

ft tkjrf^57Ll/ 7] These pillars correspond to the four cavi-
ff ~

tiefl v, t/, t, t' in Ampere's reotrope. An
horizontal axis crosses the apparatus
similar to Ampere's; the ends of which
are copper, and the centre wood or ivory.
On each of the copper ends a bow a c, b d of copper rests, so
formed, that when depressed on the one side or the other, it
falls into contact with the copper pillars A, B, c, D. Two
metallic bands connect the pillars A and B with clamps or
binding screws p and m, to which the ends of the wire carrying
the current are attached. The ends of the horizontal axis are
attached to conductors which proceed from the poles of the pile.
The course of the current may be traced exactly as in the
reotrope of Ampere.

The arrangement and mode of operation of the metallic bows,

by depressing one end or the other of \\hich the direction

of the current is charged, is represented in

Wn^ Jig. 563., where ac is the bow, A and c the

\r\ A T\.^\r\ c two copper pillars with which it falls into con-

Fi- 563 tact n ^ ie ne S '^ e r ^ 1G ot ^ er ' ant * P tne

binding screw connected with the wire which
carries the current.

1913. Electrodes. The designation of POLES being usually
limited to the extreme elements of the pile, and the ne-



cessity often arising of indicating a sort of secondary pole,
more or less remote from the pile by which the current enters
and leaves certain conductors, Dr. Faraday has proposed the
use of the term ELECTRODES to express these. Thus in the
reotrope of Ampere, the electrodes would be the mercury in
the grooves r' r', fig. 560. In the reotrope of Pohl, the electrodes
would be the ends of the horizontal axis p and M.

This term electrode has reference, however, more especially to
the chemical properties of the current, as will appear hereafter.

1914. Floating supports for conducting wire. It happens
frequently in experimental researches respecting the effects of
forces affecting voltaic currents or developed by them, that the
wire upon which the current passes requires to be supported
or suspended in such a manner as to be capable of changing its
position or direction in accordance with the action of such
forces. This object is sometimes attained by attaching the
wire, together with a small vessel containing zinc and copper
plates immersed in dilute acid, to a cork float, and placing the
whole apparatus on water or other liquid, on which it will be
capable of floating and assuming any position or direction which
the forces acting upon it may have a tendency to give to it.

1915. Ampere's apparatus for supporting movable currents.
A more convenient and generally useful apparatus for this
purpose, however, is that contrived by Ampere ; which consists
of two vertical copper rods v v', Jig. 564., fixed in a wooden
stage TT', the upper parts being bent at right angles and termi-
nated in two mercurial cups yy', one below the other in the
same vertical line. The horizontal parts are rolled with silk
or coated with gum-lac, to prevent the electricity passing from

Fig. 565.


one to the other. Two small cavities rr' filled with mercury,
being connected with the poles of a battery, become the elec-
trodes of the apparatus. These may be connected at pleasure
with two mercurial cups ss', which are in metallic communi-
cation with the rods v v'. The reotrope may be applied to this
apparatus, so as to reverse the connexions when required.

The wire which conducts the current is so formed at its ex-
tremities as to rest on. two points in the cupsyy', and to balance
itself so as to be capable of revolving freely round the vertical
line passing through yy' as an axis.

A wire thus arranged is represented in fig. 565., having its
ends resting in the cups yy', the current passing from the cup
y' through the wire, and returning to the cupy. If the reotrope
be reversed, it will pass from y through the wire and return




1916. Mutual action of magnets and currents. When a

voltaic current is placed near a magnetic needle, certain mo-
tions are imparted to the needle or to the conductor of the
current, or to both, which indicate the action of forces exerted
by the current on the poles of the needle, and reciprocally by
the poles of the needle on the current. Other experimental
tests show that the magnets and currents affect each other in
various ways ; that the presence of a current increases or
diminishes the magnetic intensity, imparts or effaces magnetic
polarity, produces temporary magnetism where the coercive
force is feeble or evanescent, or permanent polarity where it
is strong ; that magnets reciprocally affect the intensity and
direction of currents, and produce or arrest them.

1917. Electro-magnetism. The body of these and like
phenomena, and the exposition of the laws which govern them,
constitute that branch of electrical science which has been



To render clearly intelligible the effects of the mutual action
of a voltaic current and a magnet, it will be necessary to con-
sider separately the forces exerted between the current and
each of the magnetic poles ; for the motions which ensue, and
the forces actually manifested, are the resultants of the sepa-
rate actions of the two poles.

1918. Direction of the mutual forces exerted by a rectilinear
current and the pole of a magnet Tosimplify the explana-
tion, we shall, in the first instance, consider only the case of
rectilinear currents.

Let c c', Jig. 566., represent the
C wire along which a voltaic current

passes, directed from c to c 7 , as in-
dicated by the arrows. Let N N 7 be
a straight line which is parallel to
the current c C 7 , and which passes
through the magnetic pole. We
shall call this the line of direction
of the magnetic pole. Let a plane
\X b e imagined to pass through these
lines c c' and N N', and let a line A B
be drawn in this plane at right
angles to c c' and N N'.

The force exerted by the current
upon the magnetic pole, and re-
ciprocally the force exerted by the
magnetic pole upon the current,
; will have a direction at right angles

Fi". 566. * the plane passing through the

direction c c' of the current, and
the line of direction N N' of the magnetic pole.

Thus the line of direction N N' will be impelled by a force in
the direction of the line L R, and the current C c' by a force in
the direction of the line I/ R' ; these lines L R and L' R' being
understood to be drawn at right angles to the plane passing
through c c' and N N'.

But it is necessary to show in which direction on the lines
L R and L 7 R 7 these forces respectively act.

This direction will depend on and vary with the name of
the magnetic pole and the direction of the current on the line


It' we suppose the magnetic pole to be an austral or north
pole, and the current to descend on the line c c', as indicated
by the arrows, let an observer be imagined to stand with his
person in the direction c c' of the current, looking towards
N N', and the current consequently passing from his head to his
feet. In such case the direction of the force impressed by the
current on the line N N' will be directed to the right of such
observer, that is, from A towards R.

If the observer stand in the direction of the line of direction
N N' of the magnetic pole, looking towards the current c c', the
force impressed by the magnetic pole upon the current will, as

Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 30 of 45)