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the current continues to pass without interruption on A, the
needle of the reoscope will remain at rest, showing that no cur-
rent passes on B. But if the contact of A with either pole of
the pile be suddenly broken, so as to stop the current, the
needle of the reoscope will be deflected for a moment in the
direction which indicates a current similar in direction to that
which passed on A, and which has just been suspended ; but this
deflection will only be momentary. The needle will imme-
diately recover its position of rest, indicating that the cause of
the disturbance has ceased.

If the extremity of A be then again placed suddenly in con-
tact with the pile, so as to re-establish the current on A, the
needle of the reoscope will again be deflected, but in the other
direction, showing that the current produced on B is in the con-
trary direction to that which passes on A, and, as before, the
disturbance will only be momentary, the needle returning im-
mediately to its position of rest.

These momentary currents are therefore ascribed to the
inductive action of the current A upon the natural electricity of
the wire B, decomposing it and causing for a moment the positive
fluid to move in one direction, and the negative in the other.
It is to the sudden presence and the sudden absence of the
current A, that the phenomena must be ascribed, and not to any
action depending on the commencement of the passage of the
current on A, or on its discontinuance, because the same effects
are produced by the approach and withdrawal of A while it
carries the current, as by the transmission and discontinuance
of the current upon it.

1976. Experimental illustration. The most convenient form
of apparatus for the experimental exhibition of these moment-
ary currents of induction, consists of two wires wrapped with
silk, which are coiled round a cylinder or roller of wood or metal,
as represented in fig. 621. The ends are separated in leaving
the roller, so that those of one wire may be carried to the
pile, and those of the other to the reoscope. The effect of



ELECTRO-MAGNETIC INDUCTION.



345




the inductive action is augmented in proportion to the length
of the wires brought into prox-
imity, other things being the
same. It is found that the
wire B, which receives the in-
ductive action, should be much
finer and longer than that, A,
which bears the primary cur-
Flg> 621- rent. Thus, for example, while

J50 feet of wire No. 18. were used for A, 2000 feet of No. 26.
were used for B.

The effect of the induction is greatly augmented by intro-
ducing a cylinder of soft iron, or, still better, a bundle of soft
iron wires, into the core of the roller. The current on A
renders this mass of soft iron magnetic, and it reacts by in-
duction on the wires conducting the currents.

1977. Momentary currents produced by magnetic induction.
Since, as has been shown, a magnetic bar and an heliacal
current are interchangeable, it may naturally be inferred that
if an heliacal current produces by induction momentary currents
upon an heliacal wire placed in proximity with it, a magnet
must produce a like effect. Experiment has accordingly con-
firmed this inference.

1978. Experimental illustrations. Let the extremities of a

covered wire coiled on a roller, fig, 622., be
connected with a reoscope, and let the
pole of a magnet AB be suddenly inserted
in the core of the coil. A momentary
deflection of the needles will be produced,
similar to that which would attend the
sudden approach of the end of an heliacal
current having the properties of the mag-
netic pole which is presented to the coiL
Thus the boreal pole will produce the same
deflection as the front, and the austral
pole as the back of an heliacal current.
In like manner, the sudden removal of a magnetic pole from
proximity with the heliacal wire will produce a momentary
current on the wire, similar to that which would be produced
by the sudden removal of an heliacal current having like
magnetic properties.

Q 5




Fig. 622.




346 VOLTAIC ELECTRICITY.

The sudden presence and absence of a magnetic pole within
the coil of wire on which it is desired to produce the induced
current may be caused more conveniently and efficiently by
means of the effects of magnetic induction on soft iron. The
manner of applying this principle to the production of the
induced current is as follows :

Let a b,fig. 623., be a powerful horse-shoe magnet, over which
is placed a similar shoe of soft iron, round
which the conducting wire is coiled in the
usual manner, the direction of the coils
being reversed in passing from one leg of
the horse-shoe to the other, so that the
current in passing on each leg may have
its front presented in opposite directions.
The extremities of the wire are connected
with those of a reoscope at a sufficient
distance from the magnet to prevent its
indications from being disturbed by the
influence of the magnet.

If the poles ab of the magnet be suddenly brought near the
ends of the legs of the horse-shoe men, the needle of the reo-
scope will indicate the existence of a momentary current on the
coil of wire, the direction of which will be opposite to that which
would characterize the magnetic polarity imparted by induction
to the horse-shoe men. If the magnet ab be then suddenly
removed, so as to deprive the horse-shoe men of its magnetism,
the reoscope will again indicate the existence of a momentary
current, the direction of which will now, however, be that
which characterizes the polarity imparted to the horse-shoe
men.

It appears, therefore, as might be expected, that the sudden
decomposition and recomposition of the magnetic fluids in the
soft iron contained within the coil has the same effect as the
sudden approach and removal of a magnet.

1979. Inductive effects produced by a permanent magnet
revolving under an electro-magnet. If the magnet ab were
mounted so as to revolve upon a vertical axis passing through
the centre of its bend, and therefore midway between its legs,
its poles might be made to come alternately under the ends of
the horse-shoe men, the horse-shoe men being stationary.
During each revolution of the magnet ab, the polarity imparted
by magnetic induction to the horse- shoe would be reversed.



ELECTRO-MAGNETIC INDUCTION. 347

"When the austral pole a passes under m, and therefore the
boreal pole under n, m would acquire boreal and n austral
polarity. After making half a revolution b would come under
m, and a under n, and m would acquire by induction austral
and n boreal polarity. The momentary currents produced in
the coils .of wire would suffer corresponding changes of direction
consequent as well on the commencement as on the cessation of
each polarity, austral and boreal.

To trace these vicissitudes of the inductive current produced
upon the wire, it must be considered that the commencement of
austral polarity in the leg m, and that of boreal polarity in the
leg n, give the same direction to the momentary inductive
current, inasmuch as the wire is coiled on the legs in contrary
directions. In the same manner it follows that the commence-
ment of boreal polarity in m, and of austral polarity in n, pro-
duce the same inductive current.

The same may be said of the direction of the inductive
currents consequent on the cessation of austral and boreal
polarity in each of the legs. The cessation of austral polarity
in m, and of boreal polarity in n, or the cessation of boreal
polarity in m, and of austral polarity in n, produce the same
inductive current. It will also follow, from the effects of the
current and the reversion of the coils in passing from one leg
to the other, that the inductive current produced by the cessa-
tion of either polarity on one leg of men will have the same
direction as that produced by the commencement of the same
polarity in the other.

If the magnet ab were made to revolve under men, it would
therefore follow that during each revolution four momentary
currents would be produced in the wire, two in one direction
during one semi-revolution, and two in the contrary direction
during the other semi-revolution. In the intervals between
these momentary currents the wire would be in its natural
state.

It has been stated that if the extremities of the wire were not
in metallic contact with each other, or with a continuous system
of conductors, these inductive currents would not be produced.
This condition supplies the means of producing in the wire an
intermitting inductive current constantly in the same direction.
To accomplish this, it will be only necessary to contrive means
to break the contact of either extremity of the coil with the



348 VOLTAIC ELECTRICITY.

intermediate conductor during the same half of each successive
revolution of the magnet. By this expedient the contact may
be maintained during the half revolution in which the com-
mencement of austral polarity in the leg m, and of boreal in the
leg n, and the cessation of boreal polarity in the leg ?, and of
austral in the leg n, respectively take place. All these changes
produce momentary currents having a common direction. The
contact being broken during the other semi-revolution, in which
the commencement of boreal polarity in m, and of austral in M,
and the cessation of austral polarity in m, and of boreal in n,
respectively take place, the contrary currents which would
otherwise attend these changes will not be produced.

1980. Use of a contact breaker. If it be desired to reverse
the direction of the intermitting current, it will be only neces-
sary to contrive a contact breaker which will admit of such an
adjustment that the contact may be maintained at pleasure,
during either semi -revolution of the magnet a b, while it is
broken during the other.

1981. Magneto-electric machines. Such are the principles
on which is founded the construction of magneto-electric
machines, one form of which is represented in jig. 624. The
purpose of this apparatus is to produce by magnetic induction
an intermitting current constantly in the same direction, and
to contrive means by which the intervals of intermission shall
succeed each other so rapidly that the current shall have prac-
tically all the effects of a current absolutely continuous.

A powerful compound horse-shoe magnet A is firmly attached
by bolts and screws upon an horizontal bed, beyond the edge of
which its poles a and b extend. Under these is fixed an electro-
magnet XY, with its legs vertical, and mounted so as to revolve
upon a vertical axis. The covered wire is coiled in great
quantity on the legs XY, the direction of the coils being re-
versed in passing from one leg to the other ; so that if a voltaic
current were transmitted upon it, the ends x and Y would ac-
quire opposite polarities.

The axis upon which this electro-magnet revolves has upon
it a small grooved wheel f, which is connected by an endless
cord or band n, with a large wheel R driven by a handle m.
The relative diameters of the wheels K and f is such that an
extremely rapid rotation can be imparted to XY by the hand
applied at m.



ELECTRO-MAGNETIC INDUCTION.



349



The two extremities of the wire proceeding from the legs x
and Y are pressed by springs against the surfaces of two rollers,
c and d, fixed upon the axis of the electro-magnet. These




Fig. 624.

rollers themselves are in metallic connexion with a pair of
handles P and N, to which the current evolved in the wire of
the electro-magnet XT will thus be conducted.

If the electro-magnet XY be now put in rotation by the
handle m, the handles P and N being connected by any con-
tinuous conductor, a system of intermitting and alternately
contrary currents will be produced in the wire and in the con-
ductor by which the handles P and N are connected. But if the
rollers c and d are so contrived that the contact of the ends of
the wire with them shall be only maintained during a semi-
revolution in which the intermitting currents have a common
direction, then the current transmitted through the conductor
connecting the handles P and N will be intermitting, but not



350 VOLTAIC ELECTRICITY.

contrary ; and by increasing the velocity of rotation of the
electro-magnet XY, the intervals of intermission may be made
to succeed each other with indefinite celerity, and the current
will thus acquire all the character of a continuous current.

The contrivances by which the rollers c and d are made to
break the contact, and re-establish it with the necessary regu-
larity and certainty, are various. They may be formed as
excentrics, so as to approach to and recede from the ends of the
wire as they revolve, touching them and retiring from them at
the proper moments. Or, being circular, they may consist
alternately of conducting and non-conducting materials. Thus
one half of the surface of such roller may be metal, while the
other is wood, horn, or ivory. When the end of the wire
touches the latter the current is suspended, when it touches the
former it is maintained.

1982. Effects of this machine its medical use. All the
usual effects of voltaic currents may be produced with this
apparatus. If the handles P and N be held in the hands, the
arms and body become the conductor through which the cur-
rent passes from P to N. If XY be made to revolve, shocks
are felt, which become insupportable when the motion of XY
acquires a certain rapidity.

If it be desired to give local shocks to certain parts of the
body, the hands of the operator, protected by non-conducting
gloves, direct the knobs at the ends of the handles to the parts
of the body between which it is desired to produce the voltaic
shock.

1983. Inductive effects of the successive convolutions of the
same helix. The inductive effect produced by the commence-
ment or cessation of a current upon a wire, forming part of a
closed circuit placed near and parallel to it, would lead to the
inference that some effect may be produced by one coil of an
heliacal current upon another at the moment when such cur-
rent commences or ceases. At the moment when the current
commences, it might be expected that the inductive action of
one coil upon another, having a tendency to produce a moment-
ary current in a contrary direction, would mitigate the initial
intensity of the actual current, and that at the moment the cur-
rent is suspended the same inductive action, having a tendency
to produce a momentary current in the same direction, would,
on the contrary, have a tendency to augment the intensity of
the actual current.



INFLUENCE OF TERRESTRIAL MAGNETISM. 351

The phenomena developed when the contact of a closed
circuit is made or broken, are in remarkable accordance with
these anticipations.

If the wires which connect the poles of an ordinary pile, con-
sisting of a dozen pairs, be separated or brought together, a very-
feeble spark will be visible, and no sensible change in the in-
tensity of this spark will be produced when the length of the
wire compassing the circuit is augmented so much as to amount
to 150 or 200 yards. If this wire be folded or coiled in any
manner, so long as the parts composing the folds or coils are
distant from each other by a quarter of an inch or more, no
change of intensity will be observed. But if the wire be coiled
round a roller or bobbin, so that the successive convolutions
may be only separated from each other by the thickness of the
silk which covers them, a very remarkable effect will ensue.
The spark produced when the extremities of the wire are
brought together will still be faint ; but that which is manifest
when, after having been in contact, they are suddenly separated,
will have an incomparably greater length, and a tenfold or even
a hundredfold splendour. The shock produced, if the ends of
the wire be held in the hands when the contact is broken, has
also a great intensity.

1984. Effects of momentary inductive currents produced
upon revolving metallic discs : researches of Arago, Herschel,
Babbage, and Faraday. It was first ascertained by Arago
that if a circular disc of metal revolve round its centre in its
own plane under a magnetic needle, the needle will be deflected
from the magnetic meridian, and the extent of its deflection will
be augmented with the velocity of rotation of the disc. By in-
creasing gradually that velocity, the needle will at length be
turned to a direction at right angles to the magnetic meridian.
If the velocity of rotation be still more increased, the needle
will receive a motion of continuous rotation round its centre in
the same direction as that of the disc.

That this fact does not proceed from any mechanical action
of the disc upon the intervening stratum of air, is proved by
the fact that it is produced in exactly the same manner where a
screen of thin paper is interposed between the needle and the
disc.

Sir John Herschel and Mr. Babbage made a series of ex-
periments to determine the relative power of discs composed of



352



VOLTAIC ELECTRICITY.



different metals to produce this phenomenon. Taking the
action of copper, which is the most intense, as the unit, the fol-
lowing are the relative forces determined for discs of other
metals:



Copper
Zinc -
Tin -



- 1-00

- 0-93

- 0-46



Lead -
Antimony -
Bismuth



- 0-25

- 0-09

- 0-O2



Professor Barlow ascertained that iron and steel act more
energetically than the other metals. The force of silver is con-
siderable, that of gold very feeble. Mercury holds a place
between antimony and bismuth.

Herschel and Babbage found that if a slit were made in the
direction of a radius of the disc it lost a great part of its force ;
but that when the edges of such a slit were soldered together
with any other metal, even with bismuth, which itself has a
very feeble force, the disc recovered nearly all its force.

The motion of rotation of the needle is an effect which would
result from a force impressed upon it parallel to the plane of
the disc and at right angles to its radii. It was also ascertained,
however, that the disc exercises on the needle forces parallel to
its own plane in the direction of its radii, and also perpendicular
to its plane.

A magnetic needle, mounted in the manner of a dipping-
needle, so as to play on a horizontal axis in a vertical plane,
was placed over the revolving disc, so that the plane of its play
passed through the centre of the disc. The pole of the needle
which was presented downwards was attracted to or repelled from
the centre of the disc according to its distance from that point.
Placed immediately over the centre, no effect, either of at-
traction or repulsion was manifested. As it was moved from
the centre along a radius, attraction to the centre was mani-
fested. This attraction was diminished rapidly as the distance
from the centre was increased, and, at a certain point, it became
nothing, the pole of the needle resting in its natural position.
Beyond this distance repulsion was manifested, which was con-
tinued even beyond the limits of the disc. These phenomena
indicate the action of a force directed parallel to the plane of
the disc and in the direction of its radii.

A magnetic needle was suspended vertically by one of its
extremities, and, being attached to the arm of a very sensitive
balance, was accurately counterpoised. It was then placed sue-



INFLUENCE OF TERRESTRIAL MAGNETISM. 353

cessively over different parts of the disc, and was found to be
every where repulsed, whichever pole was presented downwards.
These phenomena indicate the action of a repulsive force di-
rected at right angles to the plane of the disc.

All these phenomena have been explained with great clearness
and felicity by Dr. Faraday, by the momentary inductive
currents produced upon the disc by the action of the poles of
the magnet, and the reaction of those currents on the moveable
poles themselves. By the principles which have been ex-
plained (1977), it will be apparent that upon the parts of the
disc which are approaching either pole of the magnet, moment-
ary currents will be produced in directions contrary to those
which would prevail upon an electro-magnetic helix substituted
for the magnet, and having a similar polarity ; while upon
the parts receding from the pole, momentary currents will be
produced, having the same direction.

These currents will attract or repel the poles of the magnet
according to the principles explained and illustrated in (1977);
and thus all the motions, and all the attractions and repulsions
described above, will be easily understood.



CHAP. VII.

INFLUENCE OF TERRESTRIAL MAGNETISM ON VOLTAIC CURRENTS.

1985. Direction of the earth's magnetic attraction. The laws
which regulate the reciprocal action of magnets and currents in
general being understood, the investigation of the effects pro-
duced by the earth's magnetism on voltaic currents becomes
easy, being nothing more than the application of these laws to
a particular case. It has been shown that the magnetism of
the earth is such, that in the northern hemisphere the austral
pole of a magnet freely suspended is attracted in the direction
of a line drawn in the plane of the magnetic meridian, and in-
clined below the horizon at an angle which increases gradually
in going from the magnetic equator, where it is nothing, to the
magnetic pole, where it is 90. In this part of Europe the
direction of the lower pole of the dipping-needle, and therefore



354 VOLTAIC ELECTRICITY.

of the magnetic attraction of the earth, is that of a line drawn
in the magnetic meridian at an angle of about 70 below the
horizon, and therefore at an angle of about 20, with a vertical
line presented downwards.

1986. In this part of the earth it corresponds to that of the
boreal pole of an artificial magnet. Now, since the magnetism
of the earth in this part of the globe attracts the austral pole of
the needle, it must be similar to that of the boreal or southern
pole of an artificial magnet (1656). To determine, therefore,
its effects upon currents, it will be sufficient to consider it as a
southern magnetic pole, placed below the horizon in the direc-
tion of the dipping-needle, at a distance so great that the
directions in which it acts on all parts of the same current are
practically parallel.

1987. Direction of the force impressed by it upon a current.
To ascertain the direction, therefore, of the force impressed
by terrestrial magnetism on a current, let a line be imagined to
be drawn from any point in the current parallel to the dipping-
needle, and let a plane be imagined to pass through this line
and the current. According to what has been explained of the
reciprocal action of magnets and currents, it will follow that
the direction of the force impressed on the current will be that
of a line drawn through the same point of the current perpen-
dicular to this plane.

Let cc', fig. 625., be the line of
direction of the current, and draw
OP parallel to the direction of the
dip. Let LOR be a line drawn
through o, at right angles to the
plane passing through OP and cc".
This line will be the direction of
the force impressed by the mag-
netism of the earth on the current
Fig. 625. cc'. If the current pass from c to

c', this force will be directed from o towards L, since the effect
produced is that of a southern magnetic pole placed in the line
OP. If the current pass from c' to c, the direction of the force
impressed on it will be from o towards R (1918).

It follows, therefore, that the force which acts upon the
current is always in a plane perpendicular to the dipping-
needle. This plane intersects the horizontal plane in a line




INFLUENCE OF TERRESTRIAL MAGNETISM. 355

directed to the magnetic east and west, and therefore perpen-
dicular to the magnetic meridian ; and it intersects the plane
of the magnetic meridian in a line directed north and south,
making, in this part of the earth, an angle with the horizon of
20 elevation towards the north, and depression towards the
south.

1988. Effect of terrestrial magnetism on a vertical current.
If the current be vertical, the plane passing through its direc-
tion and that of the dipping-needle will be the magnetic meri-
dian. The force impressed upon the current will therefore be at
right angles to the plane of the magnetic meridian, and directed
eastward when the current descends, and westward when it
ascends.

1989. Effect upon a horizontal current directed north and
south. If the current be horizontal, and in the plane of the
magnetic meridian, and therefore directed in the line of the



Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 34 of 45)