PIONEERS IN ELECTROMAGNETIC THEORY
The Danish physicist, Hans Christian Örsted, professor of natural
philosophy at the University of Copenhagen, showed us, more than a
century ago, that a magnetic needle can be deflected by an electric
current. He had been led by theoretical considerations to assume that
there must be a correlation between electric and magnetic forces. While
yet a young man, Örsted endeavored by persevering experimentation to
prove the correctness of his theory. While he did not expect a parallel
action of the two forces, he was firmly convinced that magnetism and
electricity were inseparable twins.
He noted that both heat and light radiated from a conductor when heated
to incandescence. He also assumed that magnetic forces are radiated
from a conductor traversed by electricity.
In 1820, while lecturing before his class, he became convinced that
the apparatus he was then using could be made to demonstrate the
correctness of his views. He asked his pupils to accompany him to his
laboratory, where, as he predicted, a slight deflection of the magnetic
needle, turned at right angles to the electric current, was shown
when placed close to the copper wire. Some months afterwards, with
a stronger current (made up of twenty cells), he obtained much more
intense effects. Investigating these in detail, he found that they met
all the requirements of his theory. So, on July 21, 1820, he sent out
to the scientific world his now famous circular, “_Experimenta circa
effectum conflictus electrici in acum magneticum_” (Experiments on
the effect of the electrical conflict in the magnetic needle).
Örsted showed, furthermore, how changes in the position of the magnetic
needle occurred with variation of the position of the conductor
(copper wire) in regard to it. He demonstrated also that the magnetic
effect was not weakened by insulators—that it would penetrate various
materials, whether these were conductors of electric currents or not.
He showed that the magnetic field created by the electric current does
not have any influence on a needle of non-magnetic material—i. e.,
brass, glass, etc. It is, in fact, chiefly in the fact that it cannot
be insulated that magnetism differs from electricity. It will freely
pass through air, stone, mica, glass, clay, brick, or any insulating
material.
It is well worthy of especial mention that Örsted employed the term
“_conflictus_” to designate the electric current, many decades
before the origin of the electron theory of matter. For, on modern
theories of electricity, it is the movement to and fro of electric
particles (electrons) through the conductor, and their impact
(“_conflictus_”) that produces what we call electrical phenomena.
Örsted’s fundamental discovery of the mutual effects between
electricity and magnetism led to further discoveries which made
possible the construction of telegraph and telephone instruments,
since these depend on the fact that _an electric current can produce
magnetism, and that magnetism can produce an electric current_.
If we wind around an iron bar a number of turns of insulated wire,
and an electric current is allowed to pass through the coil, the bar
becomes a strong electromagnet. But it remains a magnet only as long
as the current is passing. Now, the magnetic effects obtained with
the electromagnet are identical with those obtained from a permanent
magnet—such as the familiar horseshoe magnet, commonly seen on the
flywheel of the Ford automobile, or in the ordinary telephone generator
for calling up “Central”. In the case of a telegraph instrument, it is
important that the iron is a temporary magnet. On the other hand, a
permanent magnet is an essential part of every Bell telephone receiver.
This permanency is secured by employing a bar of steel instead of a
piece of iron—a temporary magnet.
The power produced from a dynamo—or electric generator—depends
upon the fact that when a magnet is put into a coil of wire, only
a momentary current of electricity passes through the wire, in one
direction. If the magnet is withdrawn, a current starts in the opposite
direction. Copper wire coiled about an iron core forms the “armature”
of the dynamo. The rotating coils are said to “cut the magnetic
field.” On this principle of electricity, intense electric currents
are produced, furnishing the “power” for the electric motors in
electric cars, elevators, musical instruments, etc., and for electric
lights—incandescent and arc.
Dynamos may contain either permanent magnets or electromagnets. They
produce the magnetic field in which the “armature” or conductor—the
coils of wire wound around the iron core—rotates. A machine with
permanent magnets is usually termed a _magneto_, and is never
made in large sizes. The current for the electromagnets may be
derived wholly from an outside source, or part of the current which
it generates may be used for that purpose. The current generated in
the armature winding is alternating, but may be rectified to a direct
current by a _commuter_ if desired; otherwise it is conveyed to
the line circuit by _collector_ or slip rings and brushes.
We owe much of our knowledge of magnetism and electricity to Michael
Faraday (1791-1867), who brilliantly covered the whole field of these
sciences. Faraday was distinguished alike as a chemist and as an
experimenter in electricity and magnetism.
Örsted had shown that magnetism could be produced by a current of
electricity, but it remained for Faraday to produce current electricity
by a magnetic “field of force”, thus laying the foundation for those
modern industries which derived motive force for their machinery from
the gigantic dynamos of our “power houses”.
But I must here introduce a few facts concerning the contributions
to electric theory and practice of the great French mathematician
and physicist, André Marie Ampère (1775-1836). His discoveries in
electrodynamics aided greatly in laying a broad foundation for this
science. Very notable was the influence exercised by Ampère on the
development of electrodynamics. And it was he who first clearly
established the fact that magnetic action is a peculiar form of
electromotive action, and that, in phenomena of this class, “action and
reaction are equal and opposite.”
From these considerations it was natural for him to suppose that
magnetism might be made to produce electricity, as it had already been
shown that electricity might be made to imitate all the effects of
magnetism. Numerous attempts were made to effect this predicted result,
but for some years all such efforts proved to be fruitless.
Meanwhile the French physicist and astronomer, François Arago
(1785-1853), was also conducting experiments with the object of
producing electricity by magnetism. One of his experiments actually
involved the effect sought, but it was not clearly recognized. Arago
observed that the rapid revolution of a conducting plate in the
neighborhood of a magnet gave rise to a force acting on the magnet. But
it was not recognized by either Arago or other physicists of the day
that the forces involved were electric currents—produced by the rapidly
revolving conducting plate.
Faraday, in 1831, after several years of preoccupation with other
problems, returned to his task of discovering electrodynamical
induction, begun in 1825. After a number of fruitless efforts, he was
finally rewarded with success, but not in the form which had been
anticipated. It was observed that at the precise time of making or
breaking the contact which closed the galvanic circuit, a momentary
effect was induced in a neighboring wire, which, however, disappeared
instantly.[9]
Faraday then discovered that a similar effect could be induced
merely by moving the wire nearer to or farther away from the closed
circuit—instead of suddenly making or breaking the contact of the
“inducing circuit”. Later he found that the effects were increased by
the proximity of soft iron, and that when the soft iron was affected by
an ordinary magnet instead of the voltaic wire, the same effect still
recurred. The momentary electric current was produced either by moving
the magnet or by moving the wire with reference to the magnet. Finally,
it was found that the earth itself might be substituted for a magnet,
not only in this experiment but also in others. Mere motion of a wire,
under proper conditions, produced the effect.
Here, then, was the true explanation of Arago’s experiment: by the
rapid revolution of the plate the momentary effect became continuous.
Without using the magnet, a revolving plate became an electrical
machine. A revolving globe was found to exhibit electromagnetic action,
the circuit being complete in the globe itself without the addition of
any wire. It was later found by Faraday that mere motion of the wire of
a galvanometer produced an electrodynamic effect upon the needle.[10]
Meanwhile, Ampère, “by a combination of mathematical skill and
experimental ingenuity, first proved that two electric currents act
on one another, and then analyzed this action into the resultant of a
system of push-and-pull forces between the elementary parts of these
currents.”[11]
Örsted having shown that electric currents produced certain effects on
magnets without being in actual contact, and Ampère having demonstrated
that magnets can in their turn be supplemented by electric currents,—a
magnetic needle being deflected not only by a current passing through a
wire, but also by another magnet brought into its neighborhood, and two
electric currents acting on one another at a distance—the question now
arose as to whether or not electrical attraction and repulsion could be
reduced to an action at a distance proportional to the inverse square
of the distance.
As early as 1773, Henry Cavendish (1731-1810)—one of the foremost
chemists and experimentalists of his day—answered this question
affirmatively by experiment.[12] Coulomb (1736-1806)—inventor of the
torsion balance—showed that ponderable matter charged with electricity
followed the same formula for attraction and repulsion as gravitating
bodies did. Poisson (1781-1840) worked out the difficult mathematics of
fluids actuated by repelling forces depending on the inverse square of
the distance. Laplace (1749-1827) had very early become convinced that
the actions of ponderable substances in which electric currents were
flowing could be reduced to an action at a distance proportional to the
inverse square of the elements of the electric current.
Faraday regarded the electric field as full of lines of electric force,
in a state of tension, and naturally repelling each other. To him, as
to a number of his contemporaries, the idea of “action at a distance”
was repugnant; though such a possibility seemed to be indicated by the
action of gravitation—the relation of the forces between two charged
bodies to the distance between them being very similar to that of the
gravitational forces between two bodies to the distance between them.
But Faraday, like the great Descartes long before him, rejected the
theory of action at a distance in favor of “action through a medium.”
Ampère had sought for some sort of mechanism for the transmission of
electromagnetic currents. His own discoveries and those of Örsted
led him to formulate the hypothesis that the field in the vicinity
of a magnetic body is produced by a number of exceedingly small
circular currents which flow undamped in or around the molecules
and that magnetization consists merely of the bringing of these
molecular currents into a parallel direction. But it was difficult for
some physicists, even in Ampère’s day, to accept the hypothesis of
undiminished currents _possessing no resistance_.
If we transform the idea of the “molecular currents” of Ampère into
the language of today, substituting for these molecular currents
electrons revolving in atoms, it can be shown that the great French
scientist was substantially correct in his assumptions. In 1915 Dr.
Albert Einstein and W. J. de Haas astonished the world of physicists by
showing experimentally—by means of a most ingenious apparatus—that the
“molecular currents” or revolving electrons really exist.
In 1919, Professor Kramerlingh-Onnes, at the University of Leyden, was
able to produce what he called _imitations of ampere currents_—i.
e., “undiminished currents producing no resistance.” It was
demonstrated that the resistance of pure gold and pure platinum differ
very little if at all from nil at low temperatures. But wires of these
metals, of absolute purity, are difficult to obtain, so mercury was
selected for the experiments. The resistance of the metal at the lowest
attainable temperature of liquefied helium,-271.5° C., (at a pressure
of 3 mm. of the mercury column), proved to be immeasurably small. The
resistance down to a position shortly below 4.2° K. (Kelvin’s absolute
scale) suddenly dropped from a measurable amount to a value practically
nil. It was found that the induced current remained in a state of
circulation, and that the decrease in the strength of the current
amounted to less than 1 per cent per hour, from which it followed that
the “time of relaxation” must amount to more than four days![13]
At the absolute zero of temperature, it is supposed that the orbits of
electrons in atoms are perfect circles, whatever their paths may be at
measurable temperatures. This motion of the electrons remains when all
heat has disappeared, since it is not this motion of the revolution of
the electrons in their orbits that is associated with the energy of
heat. Heat is _a mode of motion of the atoms themselves_, not of
their contained electrons; though increase of heat doubtless results in
an increase in the average orbital velocity of the electrons.
Since Ampère’s day we have learned at all events, that an electric
current means the flow of electrons, either from atom to atom, or
passing between the atoms, along conductors. In 1920, Lord Kelvin came
to the conclusion that at the absolute zero resistance of metals must
be infinitely great, the degrees of dissociation of the electron being,
he supposed, nil at the zero hour. If any free electrons remained, he
believed they would lose their power of motion, condensing like a vapor
upon the metal atoms and freezing fast to them (to borrow a phrase
from Kamerlingh-Onnes). The experiments of the celebrated Holland
physicist show that the resistance of metals decreases with lowering of
temperature, and would probably become nil at the absolute zero with
employment of a perfectly pure platinum wire. If this is true, then
would a current of electricity, once set up in a conductor, continue
forever?
FOOTNOTES:
[9] _Philosophical Transactions_, Page 127, 1832; First Series,
Article 10.
[10] One of the first electrical experimenters to devise the instrument
known as a “galvanometer” was Professor Schweigger, of Halle. There are
now eight or more varieties of this instrument (or apparatus) in use.
It enables the investigator to measure extremely minute electrodynamic
actions, or the very weakest intensity of an electric current, as well
as to detect its presence or direction, usually by the deflection of a
magnetic needle.
[11] Maxwell, Clerk, “On Action at a Distance,” (_Scientific
Papers_, Vol. II, Page 317).
[12] The scientific papers of Cavendish were published (in 1879)
under the title, “The Electrical Researches of the Hon. Henry
Cavendish,” edited by Clerk Maxwell. Cavendish anticipated many later
investigations of British and Continental writers, including Ohm’s
law—i. e., the proportionality between the electromotive force and
the current in the same conductor; and anticipated also Faraday’s
discovery of the specific inductive capacity of different substances,
even measuring its numerical value in several substances. He had also
arrived at the conceptions of electrical capacity and of “potential.”
[13] See _Die Naturwissenschaften_ (Berlin), January 28, 1921.