Principles of electricity

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?