ELECTRO-MAGNETIC MACHINES.
The experiments already described in illustration of some of the
phenomena of electro-magnetism are of such a simple nature that they may
be comprehended without difficulty; but it is not such an easy task to
appreciate the curious fact of an invisible power producing motion. It
has already been explained that a copper or other metallic wire
conveying a current of electricity becomes for the time endowed with a
magnetic power, and if held above, or below, or close to, a suspended
magnetized steel needle, affects it in a very marked degree, causing it
to move to the right or left, according to the _direction_ of the
electric current; and in order to form some notion of the condition of a
metallic wire whilst the electricity is passing through it, the annexed
diagrams may be referred to. (Figs. 201, 202.)
[Illustration: Fig. 201. Portion of a square copper conductor, in which
A B represents the direction of the electricity, and the small arrows, C
C C C, the magnetic current or whirl at right angles to the electrical
current, and exercising a tangential action.]
[Illustration: Fig. 202. A round conducting wire, in which the
electrical current is flowing in the direction of the large dart A B,
and the small arrows indicate the direction of the magnetic force.]
Dr. Roget says: "The magnetic force which emanates from the electrical
conducting wire is entirely different in its mode of operation from all
other forces in nature with which we are acquainted. It does not act in
a direction parallel to that of the current which is passing along the
wire, nor in any plane passing through that direction. It is evidently
exerted in a plane perpendicular to the wire, but still it has no
tendency to move the poles of the magnet in a right or radial line,
either directly towards, or directly from, the wire, as in every other
case of attractive or repulsive agency. The peculiarity of its action is
that it produces motion in a circular direction _all round_ the
wire--that is, in a direction at right angles to the radius, or in the
direction of the tangent to a circle described round the wire in a plane
perpendicular to it; hence the electro-magnetic force exerts a
tangential action, or that which Dr. Wollaston called a vertiginous or
whirling motion."
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Dr. Faraday concluded that there is no real attraction or repulsion
between the wire and either pole of a magnet, the action which imitates
these effects being of a compound nature; and he also inferred that the
wire ought to revolve round a magnetic pole of a bar magnet, and a
magnetic pole round a wire, if proper means could be devised for giving
effect to these tendencies, and for isolating the operations of a single
pole. For the first idea of electro-magnetic rotation the world is
indebted to Dr. Wollaston; but Dr. Faraday, with his usual ingenuity,
was the first who carried out the theory practically. The rotation of a
wire (conveying a current of voltaic electricity) round one of the poles
of a magnet is well displayed with the simple contrivance devised by
him. (Fig. 203.)
[Illustration: Fig. 203. N. A small bar magnet cemented into a
wine-glass, the north pole being at N. A is a moveable wire looped over
the hook, which is the positive (+) pole of the battery; the free
extremity rotates round the pole of the magnet when the current of
electricity passes. The dotted line represents the level of the mercury
which the glass contains. The electricity passes in at A, and out at the
wire B, as shown by the arrows. C is connected with the negative, and D
with the positive, pole of the battery.]
By a careful observation of the complex action of an electrified wire
upon a magnetic needle, Dr. Faraday was enabled to analyse the phenomena
with his usual penetration and exhaustive ability, and he found, as
Daniell relates,--
"That if the electrified wire is placed in a perpendicular position,
and made to approach towards one pole of the needle, the pole will
not be simply attracted or repelled, but will make an effort to pass
off on one side in a direction dependent upon the attractive or
repulsive power of the pole; but if the wire be continually made to
approach the centre of motion by either the one or the other side of
the needle, the tendency to move in the former direction will first
diminish, then become null, and ultimately the motion will be
reversed, and the needle will principally endeavour to pass in the
opposite direction. The opposite extremity of the needle will
present similar phenomena in the opposite direction; hence Dr.
Faraday drew the conclusion that the direction of the forces was
_tangential_ to the circumference of the wire, that the pole of the
needle is drawn by one force, not in the direction of a radius to
its centre, but in that of a line touching its circumference, and
that it is repelled by the other force in the opposite direction. In
this manner the northern force acted all round the wire in one
direction, and the southern in the opposite one. Each pole of the
needle, in short, appeared to have a tendency to revolve round the
wire in a direction opposite to the other, and, consequently, the
wire round the poles. Each pole has the power of acting upon the
wire by itself, and not as connected with the opposite pole, and the
_apparent attractions_ and _repulsions_ are merely _exhibitions_ of
the _revolving motions_ in different parts of their circles."
[Page 213]
The same fact illustrated at Fig. 203, is also demonstrated in a still
more striking manner by means of wire bent into a rectangular form, and
so arranged that whilst the current of electricity passes, it is free to
move in a circle; and when the poles of a magnet are brought towards the
electrified wire, it may be attracted or repelled at pleasure, and in
fact becomes a magnetic indicator, and places itself (if carefully
suspended) at right angles to the magnetic meridian. (Fig. 204.)
[Illustration: Fig. 204. A A A A. The rectangular wire covered with silk
and varnished, one end of which being pointed, rests on the little cup
B, connected with a covered wire passing down the centre of the brass
support to the binding screw C let into ivory. D. The other extremity of
the rectangular wire; this being covered and varnished, is not in
metallic contact with the end B, but is likewise pointed, and dips into
the mercury contained in the large cup E E. The upper and lower cups do
not touch, and are separated by ivory, marked by the shaded portion, and
the cup E E is in metallic communication with the brass pillar, and is
connected with the negative pole of the battery at F, whilst C is
connected with the positive pole of the battery, and the electricity
circulates round the wire in the direction of the arrows. When a bar
magnet, N, is brought towards the wire, the latter is immediately set in
motion, and by alternately presenting the opposite poles of the magnet,
the rectangular wire rotates freely round the cup B.]
These curious movements of a magnetized needle, and rotations of wires
and magnets, brought about by the agency of an active current of
electricity, have induced Sir David Brewster to advance his admirable
theory, which supposes the affection of the mariner's compass needle,
and all other suspended pieces of steel, to be due to the agency of
_electrical currents_ continually _circulating_ around the globe; and
Mr. Barlow contrived the following experiment in illustration of
Brewster's theory. A wooden globe, sixteen inches in diameter, was made
hollow, for the purpose of reducing its weight, and while still in the
lathe, grooves one-eighth of an inch deep and broad were cut to
represent an equator, and parallels of latitude at every four and a half
degrees each way from the equator to the poles. A groove of double depth
was also cut like a meridian from pole to pole, but only half round. The
grooves were cut to receive the copper wire covered with silk, and the
laying on was commenced by taking the middle of a length of ninety feet
of wire one-sixteenth of an inch in diameter, which was applied to the
equatorial groove so as to meet in the transverse meridian; it was then
made to pass round this parallel, returned again along the meridian to
the next parallel, and then passed round this again, and so on, till the
wire was thus led in continuation from pole [Page 214] to pole. The
length of wire still remaining at each pole was returned from each pole
along the meridian groove to the equator, and at this point, each wire
being fastened down with small staples, the wires from the remaining
five feet were bound together near their common extremity, when they
opened to form separate connexions for the poles of a voltaic battery.
When the battery was connected, and magnetic needles placed in different
positions, they behaved precisely as they would do on the surface of the
earth, the induction set up by the electrified wire being a perfect
imitation of that which exists on the globe.
The opposite effect to that already described--viz., the rotation of one
pole of a magnet round the electrified wire, was also arranged by
Faraday in the following manner. (Fig. 205.)
[Illustration: Fig. 205. N S. A little magnet floating in mercury
contained in the glass A A; the north pole is allowed to float above the
surface of the quicksilver, and the south pole is attached to the wire
passing through the bottom of the glass vessel. The electricity passes
in at B, and taking the course indicated by the arrows travels through
the glass of quicksilver to the other pole of the battery at C. Directly
contact is made with the battery, the little magnet rotates round the
electrified wire, W. The dotted line shows the level of the mercury in
glass.]
In the examination of the magnetic phenomena obtained from wires
transmitting a current of electricity, it should be borne in mind that
any conducting medium which forms part of a closed circuit--_i.e._, any
conductor, such as charcoal, saline fluids, acidulated water, which form
a link in the endless chain required for the path of the
electricity,--will cause a magnetic needle placed near it to deviate
from its natural position.
These positions of the electrified wire and the magnetic needle are of
course almost unlimited, and in order to assist the memory with respect
to the fixed laws that govern these relative movements, Monsieur Ampère
has suggested a most useful mechanical aid, and he says:--"Let the
observer regard himself as the conductor, and suppose a positive
electric current to pass from his head towards his feet, in a direction
parallel to a magnet; then its north pole in front of him will move to
his right side, and its south pole to his left.
"The plane in which the magnet moves is always parallel to the plane in
which the observer supposes himself to be placed. If the plane of his
[Page 215] chest is horizontal, the plane of the magnet's motion will be
horizontal, but if he lie on either side of the horizontally-suspended
magnet, his face being towards it, the plane of his chest will be
vertical, and the magnet will tend to move in a vertical plane."
This very lucid comparison will be seen to apply perfectly to the
direction of the rotations in Figs. 203 and 205.
The whole of this apparatus is made in the most elegant and finished
manner by Messrs. Elliott, of 30, Strand; and by a modification of the
latter arrangement (Fig. 206), the opposite rotations of the opposite
poles of the magnets round the electrified wire, are shown in the most
instructive manner. The apparatus (Fig. 206) was devised by the late Mr.
Francis Watkins, and consists of two flat bar magnets doubly bent in the
middle, and having agate cups fixed at the under part of the bend (by
which they are supported) upon upright pointed wires, the latter being
fixed upright on the wooden base of the apparatus, and the magnets turn
round them as upon an axis.
[Illustration: Fig. 206. A. Wire conveying the current of electricity. B
B. The magnets balanced on points rotating round the wires.]
Two circular boxwood cisterns, to contain quicksilver, are supported
upon the stage or shelf above the base. A bent pointed wire is directed
into the cup of each magnet, the ends of which dip into the mercury
contained in the boxwood circular troughs on the stage. By using a
battery to each magnet, and taking care that the currents of electricity
flow precisely alike, they will then rotate in opposite directions.
Directly after the ingenious experiments of Faraday became known, a
great number of electro-magnetic engine models were constructed, and
many thought that the time was fast approaching when steam would be
superseded by electricity; and really, to see the pretty
electro-magnetic models work with such amazing rapidity, it might be
supposed that if they were constructed on a larger scale, a great amount
of hard work could be obtained from them. This idea, however, has been
proved to be a fallacy, for reasons that will be presently explained.
The figure on p. 216 displays two of these engines, one of which
represents the rotation of electro-magnets within four _fixed steel
magnets_, and the other the rotation of steel magnets by the _fixed
electro-magnets_. The latter (No. 2) moves with such great velocity,
that unless the strength of the battery is carefully adjusted, the
connexions are soon destroyed. (Fig. 207.)
[Page 216]
[Illustration: Fig. 207.--No. 1 consists of vertical permanent steel
magnets and horizontal soft-iron electro-magnets which rotate.
No. 2 consists of two fixed soft-iron electro-magnets, and four bent
permanent steel magnets, which rotate, in both cases of course, only
when connected with the battery.]
Considering the prodigious power or _pull_ of a soft-iron
electro-magnet, and its capability of supporting considerable weight,
the most reasonable expectations of success might be entertained with
machines acting by the direct pull. It was, however, discovered that
they soon became inefficient, from the circumstance that the repeated
blows received by the iron so altered its character, that it eventually
assumed the quality of steel, and had a tendency to retain a certain
amount of permanent magnetism, and thus to interfere with the principle
of making and unmaking a magnet. It was this fact that induced Professor
Jacobi, of St. Petersburg, after a large expenditure of money, to
abandon arrangements of this kind, and to employ such as would at once
produce a rotatory motion. The engine thus arranged was tried upon a
tolerably large scale on the Neva, and by it a boat containing ten or
twelve people was propelled at the rate of three miles an hour.
Various engines have been constructed by Watkins, Botta, Jacobi,
Armstrong, Page, Hjorth; the engine made by the latter (Hjorth) excited
much attention in 1851-52, and consisted of an electro-magnetic piston
drawn within or repelled from an electro-magnetic cylinder; and by this
motion it was thought that a much greater length of stroke could be
secured than by the revolving wheels or discs, but the loss of power
(not only in this engine, but in others) through space is very great,
and the lifting power of any magnet is greatly reduced and [Page 217]
altered at the smallest possible distance from its poles. This loss of
power is therefore a great obstacle in the way of the useful application
of electro-magnetic force, and can be appreciated even with the little
models, all of which may be stopped with the slightest friction,
although they may be moving at the time with great velocity.
In the second place, supposing the reduced force exerted by the two
magnets, a few lines apart, was considered available for driving
machinery, the moment the magnets begin to move in front of one another
there is again a great loss of power, and as the speed increases, there
is curiously a corresponding diminution of available mechanical power, a
falling-off in the _duty_ of the engine as the rotations become more
rapid. In the third place, the cost of the voltaic battery, as compared
with the consumption of coal in the steam-engine, is very startling, and
extremely unfavourable to electro-magnetic engines.
Mr. J. P. Joule found that the economical duty of an electro-magnetic
engine at a given velocity and for a given resistance of the battery is
proportioned to the mean intensity of the several pairs of the battery.
With his apparatus, every pound of zinc consumed in a Grove's battery
produced a mechanical force (friction included) equal to raise a weight
of 331,400 pounds to the height of one foot, when the revolving magnets
were moving at the velocity of eight feet per second. Now, the _duty_ of
the best Cornish steam-engine is about one million five hundred thousand
pounds raised to the height of one foot by the combustion of each pound
of coal, or nearly five times the extreme _duty_ that could be obtained
from an electro-magnetic engine by the consumption of one pound of zinc.
This comparison is therefore so very unfavourable, that the idea of a
successful application of electricity as an _economic_ source of power,
is almost, if not entirely abandoned.
By instituting a comparison between the different means of producing
power, it has been shown that for every shilling expended there might be
raised by
Pounds.
Manual power 600,000 one foot high in a day.
Horse 3,600,000 " "
Steam 56,000,000 " "
Electro-magnetism 900,000 " "
A powerful magnet has been compared to a steam-engine with an enormous
piston but with an exceedingly short stroke. Although motive power
cannot be produced from electricity and applied successfully to
commercial purposes, like the steam-engine, yet the achievements of the
electric telegraph as an application of a small motive power must not be
lost sight of, whilst the fall of the ball at Deal and other places, by
which the chronometers of the mercantile navy are regulated, as also the
means of regulating the time at the General Post Office and various
railway stations, are all useful applications of the power which fails
to compete in other ways with steam.
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