Faraday, in 1830, and this discovery, and that of magneto-electricity,
may be ranked among the most memorable of his many brilliant
contributions to electric science. Let two wires be stretched parallel
and very near to each other, but not in contact. Let the extremities of
one wire, which we shall term A, be connected with a galvanometer (page
415), so that the existence of any current through the wire may be
instantly indicated. Let the two extremities of the other wire, B, be
put into connection with the poles of a battery. The moment the
connection is complete, and the battery current _begins_ to rush through
B, a deflection of the galvanometer needle will be observed, indicating
a current of very short duration through A in the opposite direction to
the battery current through B. This induced current, which is called the
_secondary_ current, does not continue to flow through A: it occurs
merely at the time the _primary_ or battery current is established; and
though the latter continues to flow through the wire, B, no further
effect is produced in the other wire. When, however, the battery
connection is broken, and the primary current ceases to flow, at that
instant there is set up in the wire, A, another momentary secondary
current, but this one is in the _same_ direction as the battery current.
This is termed the _direct secondary_ current, in opposition to the
former, which is called the _inverse_ current.
These effects are much more powerful when, instead of lengths of
straight wire, or single circles of wires, we use two coils of wire, one
of which, namely, that which conveys the primary currents, is placed in
the axis of the other. It must be distinctly understood that the
secondary currents are of momentary duration only; they are not produced
at all while the battery _is flowing_, but only at the time of its
commencement and cessation. If, however, we make the primary coil so
that it can be slid in and out of the axis of the other, then while the
primary current is continuously flowing, we can produce secondary
currents in the other coil, by causing the coils to approach or recede
from each other. As we bring the coils near each other, and slide the
primary into the secondary, the current in the latter is _inverse_; when
the one coil is receding from the other, it is _direct_. These
mechanical actions are not produced without expenditure of force, for
the approaching coils repel each other and the receding coils attract
each other. The setting up of the battery current in the primary coil
when placed within the other is equivalent to bringing it, with the
current flowing, from an immense distance in an extremely small time.
Similarly, when the battery current is broken, it is equivalent to an
instantaneous recession. The effects, therefore, are proportionately
powerful. It is found, also, and this we shall presently refer to more
fully, that when, instead of the primary coil, a magnet is similarly
moved into, or removed from, the axis of the secondary coil, currents in
opposite directions are set up in the latter without any battery being
used at all. The direction of these currents is the same as would be
produced by a primary current that would form, in a piece of iron placed
in the axis of the coil, an electro-magnet with poles similarly situated
to those of the magnet so introduced or withdrawn. Hence, by placing a
bar of soft iron in the axis of the primary coil, the secondary currents
will be produced with increased force. When a long secondary coil,
having the turns of its wire well insulated from each other, surrounds a
primary coil provided with a core of soft iron, or still better, with a
bundle of annealed iron wires, a series of powerful discharges, like
those of a Leyden jar, may be obtained between the terminals of the
secondary coil, when the battery contact is made and broken in rapid
succession.
[Illustration:
FIG. 267.—_Ruhmkorff’s Coil._
]
Such induction coils have been very carefully and skilfully constructed
by Ruhmkorff, and are therefore often called “Ruhmkorff’s Coils.” One of
these is represented in Fig. 267. A B is the coil, and the apparatus is
provided with what is termed a _condenser_, which consists of layers of
tin-foil placed between sheets of thick paper, and alternately connected
so that one set communicates with one extremity of the primary coil, and
the other with the other. This condenser is conveniently contained in
the wooden base of the instrument. Its introduction has greatly
increased the intensity of the secondary current, and sparks of 18 in.
or 20 in. in length have been obtained in the place of very short ones.
It should be stated that of the two secondary currents, only one has
sufficient intensity to traverse the secondary circuit when there is any
break in its continuity. This is the _direct secondary current_, or that
which is produced on breaking the primary circuit. The reason is that
the commencing current in the primary circuit induces in the spires of
its own coil an inverse current, and the battery current therefore
attains its full strength gradually, but still in a very short time;
while, on the cessation of the battery current, the same induction sends
a wave of electricity through the primary coil in the same direction,
and then the current ceases abruptly. Consequently, in the latter case,
the induced electricity of the secondary coil is set in motion in much
less time, and therefore possesses much greater intensity.
The magnetism of the iron core is usually made use of to break and make
the current, by the attraction of a piece of iron attached to a spring,
which, by moving towards the end of the core, separates from a point in
connection with the battery, and, the current no longer flowing, the
magnetism ceases, and the spring again brings back the iron and renews
the contact.
[Illustration:
FIG. 268.—_Discharge through Rarefied Air._
]
By means of such coils many surprising effects have been produced.
Perhaps one of the most beautiful experiments in the whole range of
physical science is made by causing the discharges of the secondary coil
to take place through an exhausted vessel in the manner represented in
Fig. 268. A beautiful light fills the interior of the vessel, and the
terminals appear to glow with a strange radiance—one being surrounded
with a kind of blue halo and another with a red. On reversing the
direction of the currents, which is done by the little apparatus at the
right-hand end of the coil in Fig. 267, the blue and the red radiance
change places. Beautiful flashes of light may also be made to appear in
the vessel, having the most marked resemblance to the streamers of the
Aurora Borealis. When, instead of vessels almost free from common air,
we repeat the experiment with tubes containing an extremely small
residue of some other gas, such as hydrogen, carbonic acid, &c., the
colour of the light and other appearances change Geissler’s tubes have
already been spoken of in connection with the spectroscope; but,
independently of that, the various beautiful appearances which such
tubes have been made to present, by the introduction of fluorescent
substances and other devices, render the induction coil an instrument of
the highest interest to the scientific amateur. Then there are striking
physiological and other effects which the coil is capable of producing.
For instance, we are able by its instrumentality to produce from
atmospheric air unlimited quantities of that singular modification of
oxygen which is called _ozone_. The electricity of the coil has been
used for firing mines, torpedoes and cannons, and for lighting the
gas-burners of large buildings.
[Illustration:
FIG. 268_a_.—_Large Induction Coil at the old Polytechnic Institution,
London._
]
The late Mr. Apps, who was well known as a skilful constructor of
scientific apparatus, devoted much attention to improving the induction
coil, and he made a very large one for the Polytechnic Institution in
Regent Street, London, which Institution was at that time the home of
popular science, under the direction of Mr. Pepper. This coil is
represented in Fig. 268_a_, surrounded by the somewhat scenic
accessories which were then supposed to be required for making science
attractive to the multitude. Externally, the coil appeared as a
cylinder, nearly 5 feet long and 20 inches in diameter. From each end
projected smaller cylinders. All these and also the two upright pillars
upon which the apparatus was supported were covered with ebonite. The
large cylinder contained the primary coil, which was made of copper wire
one-tenth of an inch in diameter and 3,770 yards long, covered with
cotton thread, and making about 6,000 turns round the central core. This
primary coil was inclosed in an ebonite tube ½-inch thick, and outside
of the tube, occupying 4 feet 2 inches of its length, was the secondary
coil, containing 150 miles of silk covered wire, ·015 inch diameter, and
very carefully arranged for insulation, so as to resist the tension of
the electricity when the coil was in action. The condenser contained 750
square feet of tin-foil, and 40 Bunsen cells supplied the current for
the primary coil. The power of this instrument was very great, for it
would give a spark through the air of more than two feet in length, and
the discharge could perforate a certain thickness of glass. It would
charge a battery of Leyden jars having 40 square feet of tin-foil by
only three breaks of contact in the primary circuit, so that the
discharge would deflagrate considerable lengths of wire. The appearance
of the spark, with this, as with other large induction coils, may be
described as a thick line of light, surrounded by a reddish halo of less
brilliancy, and this halo, unlike the line of the spark, had a sensible
duration. The reddish glow might be blown aside by a current of air when
a series of discharges was taking place, and partly separated from the
denser looking line of light. The latter is no doubt formed by intensely
heated particles of the metals between which the discharge takes place,
while the former is probably due to the incandescence of the oxygen and
nitrogen gases in the air. The disc shown in our illustration behind the
coil was for carrying six Geissler tubes, to display the pretty
experiment of the various colours of the luminous discharge in different
attenuated gases. When the coil was first mounted it was provided with
an ordinary contact-breaker, but as the strong sparks were found to very
soon destroy the contact points, a contact-breaker was substituted on
Foucault’s plan. In this, the contacts are made by a platinum tipped
wire dipping into mercury, that occupies the bottom of a strong glass
vessel and forms part of the circuit. The vessel is filled with alcohol,
which is a non-conductor, and it is therefore in the midst of this
liquid that the contacts are made and broken. This apparatus is shown in
the illustration, on the table at the left. A favourite experiment at
the Polytechnic was to connect one of the discharging wires of the coil
with the back of a large looking-glass, and bring the other wire to the
front. In this case the sparks assumed a peculiar appearance, for they
became thin and wiry-looking, and divided into many branches. They were
very bright, and the noise of the discharges, was crackling and quite
different from that produced by the blow of the flaming sparks taken
through the air. Their appearance is represented in Fig. 269. The
effects in this experiment were probably due to the spark taking a path
on the surface of the glass determined by points of moisture or other
inequalities.
[Illustration:
FIG. 269.—_Spark on the Looking-glass._
]
Ruhmkorff’s coil has been of great advantage to the electrician, for it
supplies a stream of _high tension_ electricity like that of the common
machine, but more readily and conveniently. M. Ruhmkorff was the first
person to obtain the great prize of £2,000, which the late Emperor of
the French (Napoleon III.) directed, in 1852, should be awarded every
five years for the most useful application of the voltaic battery. But
no award had been made until 1864, when the inventor of the induction
coil was properly considered worthy of it. This invention was the means
of bringing into notice a new range of interesting phenomena, especially
those attending the discharge passed through highly exhausted vessels.
Investigations into the circumstances which modify the appearances, and
especially into the nature of the _stratified discharge_ in which the
vessels are filled with bands or flakes of light separated by dark
intervals, have long engaged the attention of some of our ablest
physicists. Remarkable results were obtained by Mr. Crookes with very
highly exhausted vessels. These showed not only beautiful fluorescent
luminous effects, but in them the discharge could produce mechanical
actions, and Mr. Crookes was led to regard it as a stream of radiant
matter.
_MAGNETO-ELECTRICITY._
When it had been shown that an electric current was capable of evoking
magnetism, it seemed reasonable to expect that the reverse operation of
obtaining electric currents by means of magnets should be possible.
Faraday succeeded in solving this interesting problem in November, 1831,
and one of his earliest, simplest, and most convincing experiments for
the demonstration of the production of electricity by a magnet is
represented in Fig. 270. A B is a strong horse-shoe magnet, C is a
cylinder of soft iron, round which a few feet of silk-covered copper
wire are wound; one end of the wire terminates in a little copper disc,
and the other end is bent, as shown at D, so that it is in contact with
the disc, but pressing so lightly against it that any abrupt movement of
the bar causes the point of the wire and the disc to separate. When the
bar is allowed to fall upon the poles of the magnet, the separation
occurs, and again when it is suddenly pulled off; and on each occasion a
very small but brilliant spark is observed where the contact of the wire
and disc is broken. It was in allusion to this experiment that a
contributor to “Blackwood’s Magazine” wrote:
Around the magnet, Faraday
Is sure that Volta’s lightnings play;
But how to draw them from the wire?
He took a lesson from the heart;
‘Tis when we meet, ‘tis when we part,
Breaks forth the electric fire.
[Illustration:
FIG. 270.—_Magneto-electric Spark._
]
[Illustration:
FIG. 271.—_A Magnet producing a Current._
]
[Illustration:
FIG. 272.—_Clarke’s Magneto-electric Machine._
]
If a coil of fine insulated wire be passed many times round a hollow
cylinder, open at the ends, and the extremities of the wire connected
with a galvanometer at some distance, then if into the axis of the coil,
A B, Fig. 271, a steel magnet be suddenly introduced, an immediate
deflection of the needle takes place; but after a few oscillations it
returns to its former position. When the magnet is quickly withdrawn,
the needle receives a momentary impulse in the opposite direction. The
magnetization and demagnetization of the iron core in the induction coil
would, therefore, of itself cause the induced currents already
described, for these actions are equivalent to sudden insertion and
withdrawal of a magnet. If we suppose C, in Fig. 271, to represent, not
a magnet, but a piece of soft iron—the reader will remember that this
soft iron can be, as often as required, magnetized and demagnetized by
simply bringing near one end of it the pole of a permanent magnet (see
page 484). Upon this principle many ingenious machines have been
constructed for producing electric currents by the relative motions of
magnets and of soft iron cores surrounded by wires. Clarke’s machine is
shown in Fig. 272. A is a powerful steel magnet fixed to the upright. A
brass spindle passing between the poles can be made to rotate very
rapidly by the multiplying-wheel, E, on which a handle is fixed. There
are two short cylinders of soft iron parallel to the spindle, united
together by the transverse piece of iron, D, which turns with the
spindle. Each bar is surrounded by a great length of insulated copper
wire, and the ends of the wires are so connected with springs which
press against a portion of the spindle, which is here partly formed of a
non-conducting material, that the currents generated in the coils,
although in different directions as they approach a pole and recede from
it, are nevertheless made to flow in one direction in the external
circuit. R R in the figure represent two brass handles, which are
grasped by a person wishing to experience the shocks the machine can
give when the wheel is turned. When the terminals of the coil are
provided with insulating handles and connected with pointed pencils of
charcoal, the electric light can readily be produced by expenditure of
mechanical effort in turning the handle. The arrangement of the points
for this purpose is shown in Fig. 273, and we shall presently see what
advantage has been drawn from this experiment on a great scale as a
source of light.
[Illustration:
FIG. 273.—_Magneto-electric Light._
]
It will be observed that during the revolution of the armatures, as the
wire-covered iron cores are termed, there are two maximum and two
minimum points at which the currents are strongest and weakest. These
variations may be lessened by increasing the number of armatures and of
magnets, and Mr. Holmes arranged a machine with eighty-eight coils and
sixty-six magnets, and the connections were so contrived that the
currents always flowed in the same direction in the external circuit.
This machine required 1¼ horse-power to drive it when the currents were
flowing, but much less when the circuit was interrupted, and it was
designed for, and successfully applied to, the production of the
electric light for lighthouse illumination. Instead of steel magnets
which gradually lose their strength, it is obvious that electro-magnets
might be employed, but this source of electricity is costly,
troublesome, and inconstant. Mr. Wilde hit upon the idea of using a
small magneto-electric machine with permanent steel magnets, to generate
the current for exciting a larger electro-magnet, and the current from
this produced a still more powerful electro-magnet, from which a
magneto-electric current could be collected and applied. The same idea
was subsequently applied in other forms, as by shunting off a portion of
the current produced from the mere residual magnetism of an
electro-magnet, to pass through its own coils and evoke a stronger
magnetism, which again reacts by producing a more powerful current, and
so on continually; the limit being dependent only on the mechanical
force employed, and on the power of the wires to convey the electricity,
for they become very hot, and, unless artificially cooled, the
insulating material would be destroyed. The armatures used in Wilde’s,
Ladd’s, and other machines of this kind, are quite different in
arrangement from those of Clarke’s machine, and are far superior. They
are formed of a long bar of soft iron, of a section like this, Ꮋ, and
the wire is wound longitudinally between the flanges from end to end of
the bar, up one side and down the other. This armature rotates about its
longitudinal axis between the pairs of the poles of a file of horse-shoe
magnets, either permanent, or electro-magnets excited by the
magneto-electric currents. In this case opposite poles are induced along
the edges of the bar, and these poles are reversed at each half-turn.
The intensity of the induced currents increases with the velocity with
which the armature is made to revolve up to a certain point; but because
the magnetization of the soft iron requires a sensible time to be
effected, and the poles are reversed at every half-turn, it is found
that a speed increasing beyond the limit is attended by decrease of the
intensity of the current. The intensity in such machines has, therefore,
a definite limit. But in a modification of the magneto-electric machine,
which has quite recently been invented by M. Gramme, the limit is vastly
extended by the ingenious disposition of the iron core and armatures,
and his machines appear to solve the problem of the cheap production of
steady and powerful electric currents, so that electricity will soon be
applied in processes of manufacture where the cost of electrical power
has hitherto placed it out of the question. We shall now endeavour to
explain the principle on which the Gramme machine depends, and describe
some forms in which it is constructed.
_THE GRAMME MAGNETO-ELECTRIC MACHINE._
[Illustration:
FIG. 274.
]
[Illustration:
FIG. 275.—_Gramme Machine for the Laboratory or Lecture Table._
]
Let X, Fig. 274, be a coil of covered wire; then while a bar magnet, B
A, is advancing towards it and passing through it, as at M, a current
will flow through the coil and along a wire connecting its ends, _s s_.
The current will change its direction as the centre of the magnet is
leaving the coil to advance in the direction, B A. If A A´ be a bar of
soft iron, with the coil fixed upon it, we can still excite currents in
the coil by magnetizing the bar inductively. If the pole of a permanent
magnet be carried along from A´ to M in a direction parallel to the bar,
but not touching it, the part of the bar immediately opposite will be a
pole of opposite name, and the advance of this induced pole towards M
will be attended with a current in the coil, and its recession by an
opposite current. It need hardly be mentioned that the same result is
attained if the magnetic pole is stationary, and the bar with the coil
upon it moved in proximity to it. Now imagine that the bar is bent into
a ring, the ends, A A´, being united. If the ring be made to turn round
its centre in its own plane, and near a magnetic pole, it is plain that
when the coil is approaching this pole a current will be produced in it,
and when it is receding, an opposite current. Let the number of coils be
increased, and each coil in turn will be the seat of a current, or of
the electrical state which tends to produce a current. In Fig. 275 the
reader may see how this disposition is realized. The figure shows a form
of the Gramme Machine adapted for the lecture-table or laboratory. A M´
B M is the soft iron ring, covered with a series of separate coils
placed radially, O is a compound horse-shoe steel magnet, S its south
pole, N its north pole, each pole being armed with a block of soft iron
hollowed into the segment of a circle and almost completely embracing
the circle of coils. The magnetism of each pole is strongly developed in
the interior faces of these armatures. The inductive action tends to
produce two equal and opposite currents, which, like the currents of two
similar voltaic batteries joined by their like poles, neutralize each
other in the connected coils, but flow together through an external
circuit. Fig. 276 will make clear the manner in which the coils, B B,
are placed on the ring, A. The length of wire in each coil is the same,
and the extremities are attached to strips of copper, R R, which are
fixed on the spindle of the machine. The two ends of each wire are
connected with two consecutive strips, while the coils are insulated
from each other, and thus each coil, like the element of a battery,
contributes to the aggregate current. The currents are drawn off, as it
were, from these axial conductors at two opposite points of the ring, by
springs very lightly touching them on each side of the spindle, as may
be seen in Fig. 275. In Fig. 277 is another arrangement of the table
apparatus with the magnet vertical, and formed according to the new plan
suggested by M. Jamin, who finds the best magnets are made by tying
together thin strips of steel.
[Illustration:
FIG. 276.—_Insulated Coils surrounding an Annulus of Iron Wires._
]
[Illustration:
FIG. 277.—_Hand Gramme Machine, with Jamin’s Magnet._
]