Magneto-Electricity—Volta-Electric Induction—Magneto-Electric
Induction—Identity in the Action of Electricity and
Magnetism—Description of a Magneto-Electric Apparatus and its
Effects—Identity of Magnetism and Electricity—The Submarine Telegraph.
FROM the law of action and reaction being equal and contrary, it might
be expected that, as electricity powerfully affects magnets, so,
conversely, magnetism ought to produce electrical phenomena. By proving
this very important fact from the following series of interesting and
ingenious experiments, Dr. Faraday has added another branch to the
science which he has named magneto-electricity. A great quantity of
copper wire was coiled in the form of a helix round one half of a ring
of soft iron, and connected with a galvanic battery; while a similar
helix connected with a galvanometer was wound round the other half of
the ring, but not touching the first helix. As soon as contact was made
with the battery, the needle of the galvanometer was deflected. But the
action was transitory; for, when the contact was continued, the needle
returned to its usual position, and was not affected by the continual
flow of the electricity through the wire connected with the battery. As
soon, however, as the contact was broken, the needle of the galvanometer
was again deflected, but in the contrary direction. Similar effects were
produced by an apparatus consisting of two helices of copper wire coiled
round a block of wood, instead of iron, from which Dr. Faraday infers
that the electric current passing from the battery through one wire
induces a similar current through the other wire, but only at the
instant of contact, and that a momentary current is induced in a
contrary direction when the passage of the electricity is suddenly
interrupted. These brief currents or waves of electricity were found to
be capable of magnetizing needles, of passing through a small extent of
fluid, and, when charcoal points were interposed in the current of the
induced helix, a minute spark was perceived as often as the contacts
were made or broken, but neither chemical action nor any other electric
effects were obtained. A deviation of the needle of the galvanometer
took place when common magnets were employed instead of the Voltaic
current; so that the magnetic and electric forces are identical in their
effects in this experiment. Again, when a helix formed of 220 feet of
copper wire, into which a cylinder of soft iron was introduced, was
placed between the north and south poles of two bar magnets, and
connected with the galvanometer by means of wires from each extremity,
as often as the magnets were brought into contact with the iron cylinder
it became magnetic by induction, and produced a deflection in the needle
of the galvanometer. On continuing the contact the needle resumed its
natural position, and, when the contact was broken, deflection took
place in the opposite direction; when the magnetic contacts were
reversed, the deflection was reversed also. With strong magnets, so
powerful was the action, that the needle of the galvanometer whirled
round several times successively; and similar effects were produced by
the mere approximation or removal of the helix to the poles of the
magnets. Thus it was proved that magnets produce the very same effects
on the galvanometer that electricity does. Though at that time no
chemical decomposition was effected by these momentary currents which
emanate from the magnets, they agitated the limbs of a frog; and Dr.
Faraday justly observes, that “an agent which is conducted along
metallic wires in the manner described, which, whilst so passing,
possesses the peculiar magnetic actions and force of a current of
electricity, which can agitate and convulse the limbs of a frog, and
which finally can produce a spark by its discharge through charcoal, can
only be electricity.” Soon after he completely established the identity
of the two powers by producing the spark, heating metallic wires, and
accomplishing chemical decomposition. Hence it appears that electrical
currents are evolved by magnets, which produce the same phenomena with
the electrical currents from the Voltaic battery: they, however, differ
materially in this respect—that time is required for the exercise of the
magnetico-electric induction, whereas Volta-electric induction is
instantaneous.
Thus the effect of induction or the influence of the spiral wire in
increasing the electric and magnetic power is very great indeed, and to
that we are indebted for the electric telegraph, for Voltaic electricity
alone is too feeble to overcome the resistance of a long wire.
Electric currents, whatever their tension may be, produce the phenomena
of induction; these again induce other currents in bodies capable of
induction, and so on indefinitely; the first and second flow in the same
direction, the others alternately opposite and direct. They all give the
shock and can decompose water, but with Volta-electric currents the
elevation of temperature as well as their physiological and magnetic
effects are produced by instantaneous actions, which only depend upon
the quantity and tension of the current, and by no means on its
duration, for induced currents only exist for a moment when the circuit
of the battery is broken. The most energetic physiological effects are
produced by a small quantity of electricity moving with great velocity.
The apparatus first employed by Dr. Faraday is in effect a battery,
where the agent is the magnetic instead of the Voltaic force, or, in
other words, electricity, and is thus constructed:—
A very powerful horseshoe magnet, formed of twelve steel plates in close
approximation, is placed in a horizontal position. An armature,
consisting of a bar of the purest soft iron, has each of its ends bent
at right angles, so that the faces of those ends may be brought directly
opposite and close to the poles of the magnet when required. Ten copper
wires—covered with silk, in order to insulate them—are wound round one
half of the bar of soft iron, as a compound helix: ten other wires, also
insulated, are wound round the other half of the bar. The extremities of
the first set of wires are in metallic connexion with a circular disc,
which dips into a cup of mercury, while the ends of the other ten wires
in the opposite direction are soldered to a projecting screw-piece,
which carries a slip of copper with two opposite points. The steel
magnet is stationary; but when the armature, together with its
appendages, is made to rotate vertically, the edge of the disc always
remains immersed in the mercury, while the points of the copper slip
alternately dip in it and rise above it. By the ordinary laws of
induction, the armature becomes a temporary magnet while its bent ends
are opposite the poles of the steel magnet, and ceases to be magnetic
when they are at right angles to them. It imparts its temporary
magnetism to the helices which concentrate it; and, while one set
conveys a current to the disc, the other set conducts the opposite
current to the copper slip. As the edge of the revolving disc is always
immersed in the mercury, one set of wires is constantly maintained in
contact with it, and the circuit is only completed when a point of the
copper slip dips in the mercury also; but the circuit is broken the
moment that point rises above it. Thus, by the rotation of the armature,
the circuit is alternately broken and renewed; and as it is only at
these moments that electric action is manifested, a brilliant spark
takes place every time the copper point leaves the surface of the
mercury. Platinum wire is ignited, shocks smart enough to be
disagreeable are given, and water is decomposed with astonishing
rapidity, by the same means; which proves, beyond a doubt, the identity
of the magnetic and electric agencies, and places Dr. Faraday, whose
experiments established the principle, in the first rank of experimental
philosophers.
A magneto-electric machine has been recently constructed by Mr. Henley,
of enormous power. It consists of two permanent magnets, from which the
induction is obtained; each of these is formed of thirty horseshoe steel
magnets, two feet and a half long, and from four to five inches broad,
and each is surrounded by a coil of wire six miles long, coated with
silk to insulate the coils. A shock from these wires would be
instantaneous death. This apparatus will ultimately be employed to send
a stream of electricity through long submarine and subterraneous wires;
but a Volta-electric machine has hitherto been used, in which the
electricity is generated by a galvanic battery instead of magnets.
Induction, or the effect of the spiral wires in augmenting the power of
Voltaic electricity, is admirably illustrated in the Atlantic telegraph.
Wires that are to convey electricity under ground, or through water,
must be defended from injury and insulated to prevent the lateral escape
of the electricity. For that purpose the cable that is laid at the
bottom of the Atlantic, from near Valentia in Ireland to Trinity Bay in
Newfoundland, is formed of seven fine copper wires which convey the
electricity, bound together by a coating of gutta percha, over which
there are layers of cloth dipped in pitch, and then the whole is covered
by steel wires twisted together in strands and twined round in long
close spirals, forming a cord or cable not more than an inch and a
quarter in diameter, and 2100 miles long. The use of the gutta percha is
to insulate the wires; the other coatings are merely for protection.
The Voltaic battery which generates the electricity consists of 40
cells, the plates of which are alternately of zinc and platinized
silver, each about nine inches square, the exciting fluid being dilute
sulphuric acid. Although the force developed by this battery is so great
that a piece of iron three inches long and three eighths of an inch in
diameter placed in contact with the poles may be consumed in a few
minutes, it is absolutely incapable of sending a current of electricity
through wires 2500 miles long, on account of their resistance, without
the aid of Dr. Faraday’s inductive action. It is only the primary agent
for inducing a current of sufficient strength.
To accomplish that, many thousand yards of fine copper wire coated with
silk are wound round a hollow soft iron cylinder; the whole is then
coated by gutta percha, and the end of the wire is joined to the wires
in the cable so as to form a continuous line from Valentia to
Newfoundland. A second copper wire, shorter but thicker than the
preceding, and also insulated by a coating of silk, is wound round the
cylinder above the gutta percha: when the ends of this thick wire are
brought into contact with the poles of the battery, currents of
electricity flow through it, between pole and pole, and in their passage
temporarily convert the hollow iron cylinder into a powerful
electro-magnet, which by its reaction induces a current of electricity
in the fine wire of sufficient power to cross the Atlantic. The
efficiency of the electric telegraph depends upon the power we possess
of breaking and renewing the current at pleasure, since by that means
distinct and successive signals are made from station to station. In the
Atlantic cable positive and negative electricity are transmitted
alternately; the electricity is sent to America from alternate poles,
and the current returns again through the water, which completes the
circuit.
The passage of electricity through a cable or telegraphic wire in air is
sensibly instantaneous; that through a cable, whether extended in water
or under ground, requires time on account of lateral induction through
the gutta percha; for the electricity, in passing through the wires,
induces the opposite electricity on the surface of the water or moist
earth in contact with the cable, and in that respect it is precisely
like a Leyden jar, the gutta percha representing the glass. As the power
of induction is proportional to the tension of the electricity, and as
the tension is continually diminished by the resistance of the wires,
the induction is continually diminished and requires a longer time.
Electricity took two seconds to pass through a cable 768 miles long,
laid under ground from London to Manchester, and back again twice; while
in air it was all but instantaneous, because the inductive capacity of
air is very much less than that of water or moist earth. In the
experiment with the cable under ground it took two-thirds of a second to
overcome the resistance of the wires, and then the velocity of the
electricity was 1000 miles in a second, and it was the same whatever the
intensity of the electricity.
It has already been mentioned that the efficiency of the electric
telegraph depends upon the breaking and renewing the current of
electricity by means of which a succession of waves of electricity are
sent through the conducting wires. Now it has been ascertained that
three electric waves may travel simultaneously through the wires of the
Atlantic telegraph with sufficient intervals between them to record the
indications they are intended to convey; that is, three signals can be
intelligibly and practically transmitted in two seconds.
The original design, structure, and difficulty of depositing the cable
are only equalled by the talent and perseverance with which it has been
done. The 5th of August, 1858, will be memorable for the accomplishment
of the boldest enterprise that ever was undertaken by man, and which is
only the beginning of a vast submarine communication that will
ultimately encircle the globe. It has been granted to British genius
thus to annihilate time and space, in order to connect all mankind into
one great family for their moral and religious advancement; and,
whatever may be the fate of the British Islands in the course of ages,
to their energetic race the glory will remain of having been the chief
instruments in the hands of Providence for the civilization of the
world—a civilization which will extend with the development of their
numerous colonies into great independent Christian states, like those of
the Union in North America. The thunderbolt snatched from heaven by
Franklin now passes through the depths of the Atlantic as a messenger of
peace between the kindred nations.[16]
In telegraphs on land the intensity of the battery or magnets is
increased by induction on the same principle. It is by intensity that
the electric current is enabled to pass through the wires, and that is
augmented by increasing the number of coils round the cylinder: however,
it is only advantageous when the distance between the stations is great,
for then the resistance in the additional coils bears a small proportion
to the resistance offered by very long wires, but a very great
proportion to that opposed in very short ones. The nice adjustment for
each case has been determined by the experiments of eminent
electricians, and all the arrangements have been brought to great
perfection in this wonderful triumph of science, which is due to Volta,
who called into existence the fiery stream, and to Faraday, who has
given it the energy of the lightning.
When the length of the wire in the helices of an electro-magnet is very
great, it offers increasing resistance to the passage of the
electricity, so that the cessation of magnetism is not instantaneous
when the contact with the Voltaic battery is broken. To remedy that
defect an instrument has been invented which instantaneously deprives
the apparatus of the remaining electricity. A great length of fine wire
gives the severest shocks, while a shorter and thicker wire gives the
longest sparks and ignites the greatest quantity of platinum wire.
Ruhmkorff’s electro-inductive apparatus has either been improved, or new
machines constructed, by Messrs. Grove, Gassiot, and Joule, of intense
energy. Indeed, so great is the energy of electro-induction, that hopes
were entertained of its superseding steam as a motive power. For the
current of electricity from an electro-magnet can be made to flow in
opposite directions, so as to produce alternate attractions and
repulsions, and consequently a continued motion, which might be applied
as a motive force to machinery. However, Mr. Joule has proved that the
power developed by one pound of coal in combustion is to that produced
by one pound of zinc consumed in Mr. Grove’s powerful electro-magnetic
apparatus as nine to one, so that, even if zinc were as cheap as coal,
and a Voltaic battery as easily kept in order as an engine-furnace,
electricity will not supersede steam as a motive power.
A current of electricity traversing a conductor gives out a quantity of
heat determined by fixed laws, the amount of which is invariable as long
as the machine to which it is applied remains at rest; but the instant
the machine is set in motion a reaction takes place in the intensity of
the current, causing a diminution in the quantity of heat, because the
heat that disappears is converted into the mechanical force exerted by
the engine.
Mr. Joule’s experiments prove that, whenever a current of electricity is
generated by a magneto-electric machine, the quantity of heat evolved by
that current has a constant relation to the power required to work the
machine; and on the other hand, whenever an engine is worked by a
Voltaic battery, that the power developed is at the expense of the
calorific force of the battery for a given consumption of zinc, the
mechanical effect produced having a fixed relation to the heat lost in
the Voltaic current. The obvious conclusion Mr. Joule draws from these
experiments is, that heat and mechanical power are convertible into one
another, and it becomes evident, therefore, that heat is either the vis
viva or living force of ponderable particles, or a state of attraction
and repulsion capable of generating vis viva (N. 222).