Principles of electricity

THE ZEEMAN EFFECT

Heinrich Hertz demonstrated in 1887 that he could produce in the “ether”—or at least in space—what are now known as “wireless waves,” by allowing a charge of electricity to oscillate to and fro. Larmor and Lorentz were, at the same time, endeavoring to formulate a theory which would account for the production of the far shorter light-waves.

Lorentz supposed that each atom contained one or more infinitesimal particles, or electric charges (electrons), whose excessively rapid vibrations caused the emission of light-rays. Maxwell showed that there must be a close connection between light and electricity, a theory converted into demonstrable fact by the work of Hertz.

That there is a similar relation between light and magnetism was the firm conviction of Faraday. In 1845, he placed a block of very dense glass between the poles of the most powerful electromagnet produceable at the time. Before turning on the switch, he allowed a beam of light to pass through the glass, producing “polarization”—a modification of light-rays resulting from their reflection (in this case from a crystalline substance), imparting to the beam a definite direction—the plane of vibration or plane of polarization. When the switch was closed, permitting the flow of the electric current, which produced the magnetic field, the beam of light was “rotated.” That is, the beam of light was “plane-polarized” by the crystal, and “rotated” by the magnetic field; i. e., now changed into two “circularly polarized” rays, one a left-handed motion and the other a right-handed motion (in the direction of the hands of a watch).

This could be accounted for only on the theory that light is affected by magnetism, since the beam was not rotated by the glass alone—in itself a very important discovery. But the experiment did not yield Faraday an answer to the question uppermost in his mind: namely, can a magnetic field change the rate of vibration of a light-emitting particle? That is to say, in effect, can a magnetic field cause a ray of light to shift its normal place in the spectrum?

It was not until 1862, seventeen years after the experiment just described, that Faraday attempted to solve this important theoretical problem. He now placed a sodium flame in front of the slit of the spectroscope, which normally yields two characteristic yellow lines (the D lines of the spectrum), and observed them with the best spectroscope at his command, under the most powerful electromagnetic field which he could produce. No change from the normal could be detected. Other observers tried the same experiment, but with negative results. We know that his theory was well founded, and that only the lack of a better spectroscope and a more powerful magnet prevented his discovery of what is now known as the Zeeman effect—a discovery which has already thrown a flood of light on a number of difficult physical problems.[23]

Working with much more powerful apparatus, but following the same method of procedure employed by the immortal Faraday, Dr. Pieter Zeeman, of Leyden, succeeded, in 1896, in experimentally demonstrating the close relationship between light and magnetism. Dr. H. A. Lorentz, then Professor of Physics in the University of Leyden, now mathematical physicist at the Norman Bridge Laboratory of Physics, Pasadena, California, had predicted the nature of the change in the spectral lines to be expected, and this knowledge was used by Dr. Zeeman as a check on his results.

Using a Rowland grating, instead of a less efficient prism spectroscope, Dr. Zeeman found that when a relatively weak electric current was applied, the two sodium lines were merely widened. In a still more powerful magnetic field, each of the lines was decomposed into two or three components, when the lines of force were parallel to the line of sight.[24] Moreover, the rays of the components of each line “were not those of natural light,” but were “polarized in a characteristic way,” i. e., were circularly polarized in opposite directions—“the direction of the vibration depending in a simple manner on the direction of the magnetic lines of force.”[25]

The same effect has more recently been produced in the case of the spectral rays of nearly—if not quite—all the other elements. The process, as described by Dr. George Ellery Hale, is very simple: “We place our iron ore or spark between the poles of a powerful magnet, and photograph its spectrum. The lines behave in the most diverse way, some splitting into triplets, others into quadruplets, quintuplets, sextuplets, etc. One chromium line is resolved by the magnet into twenty-one components.... The distance between the components of a line is directly proportional to the strength of the magnetic field.”[26]

The meaning of this splitting and polarization of light-rays in the magnetic field is that, as Lorentz had predicted, there are present in the luminous vapor vibrating particles negatively charged, or “electrons.” Measurement of the distances apart of the components of the triple line reveals the relation between the charge and the mass of the particles.[27]

It is interesting to add that the disturbances in the magnetic field, as observed by Zeeman, were precisely of the amount calculated by Lorentz purely on theoretical grounds, and the mass of the electron was found by this method to be ¹/₁₈₄₀ that of the hydrogen atom. By a different method, Sir J. J. Thomson obtained a value of ¹/₁₈₀₀ the mass of the hydrogen atom; while Dr. Robert A. Millikan, by means of his famous “electrical balance,” derived a value of ¹/₁₈₄₅ that of the hydrogen atom.[28]

In his monograph of 1913, Zeeman remarked that in discoveries of optics “we may always cherish the hope that they will lead ultimately to applications to astronomy.” So far as study of solar phenomena and the Zeeman effect are concerned, this hope has been fully realized, and attempts are being made to extend the applications of this method of investigation to other stellar bodies. Of the general value of Zeeman’s discovery, Dr. Hale writes: “The complex phenomena of the Zeeman effect (as revealed in a comparative study, with powerful spectrographs, and an intense magnetic field, of the lines of a long list of elements) furnish material available for wide generalization, important in their bearing on theories of radiation and atomic structure” (Op. cit., Page 36).

Discovery by Hale and his co-workers at Mount Wilson of the Zeeman effect in sun-spots led to the very important conclusion that these disturbances represent whirling vortices of electrons, producing a magnetic field. “The strength of the magnetic field produced, which is measured by the degree of separation of the triple lines, increases with the diameter of the spot.... It has long been known that sun-spots usually occur in pairs, and our study of the Zeeman effect indicates that the two principal spots in such a group are almost invariably of opposite polarity” (Hale, Op. cit., Pages 28-31).

The sun, like the earth is now known to be a magnet, whose general magnetic field is about 80 times as intense as that of the earth. At the distance of the earth the solar magnetic field is not appreciable, “since the effect of one pole counteracts the equal and opposite effect of the other pole.”

Were it not for our knowledge concerning the Zeeman effect, it would not yet be known for a certainty that the sun is a vast magnetic globe, since this fact could not be assumed to be a source of the sun’s gravitational power. “Indeed,” says Dr. Hale,[29] “its attraction cannot be felt by the most delicate instruments at the distance of the earth, and would still be unknown were it not for the influence of magnetism on light. Auroras, magnetic storms, and such electric currents as those that recently deranged several Atlantic cables are due, not to the magnetism of the sun or its spots, but probably to streams of electrons, shot out from highly disturbed areas of the solar surface surrounding great sun-spots, traversing 93 million miles of the ether of space, and penetrating deep into the earth’s atmosphere.”

By means of the famous 150-foot tower telescope at Mount Wilson, which produces at a fixed point in a laboratory an image of the sun about sixteen inches in diameter, the magnetic phenomena of sun-spots are being studied to great advantage, the enlarged sun-spots making possible separate observation of their various parts. “This analysis is accomplished with a spectroscope 80 feet in length, mounted in a subterranean chamber beneath the tower.” By this means the very important discovery was made by Director Hale that the entire sun, rotating on its axis, is a great magnet. “Hence,” says Dr. Hale, “we may reasonably infer that every star, and probably every planet, is also a magnet, as the earth has been known to be since the days of Gilbert’s ‘De Magnete.’ Barnett has succeeded in producing magnetism by rapidly whirling masses of metal in the laboratory” (Hale, “The New Heavens,” Pages 69-70).

More recently (October, 1922), Hale, Ellerman and Nicholson, all of the Mount Wilson Observatory, have detected invisible sun-spots by searching for evidences of the Zeeman effect in promising regions, such as areas of flocculi following a large spot. “A special polarizing apparatus permits very small magnetic fields to be found by the alternate widening to red and violet of the iron triplet Lambda 6173,” say Hale and Adams (“Summary of the Year’s Work at Mount Wilson,” Publications of the Astronomical Society of the Pacific, October, 1922, Pages 269-70 [Vol. XXXIV, No. 201]). “The results confirm the view that a spot represents a vortex, which becomes visible only when the cooling due to the expansion (of gases) is sufficiently great to produce a perceptible decrease in the brightness of the photosphere.”

From what has been said, it is evident that Dr. Zeeman’s desire to see the results of his discovery applied to the study of astronomical problems has been fully realized.

THE STARK EFFECT

Lorentz’s prediction regarding the effect of a strong magnetic field on spectral rays, and the movements of electrons in the field having been confirmed so brilliantly by Zeeman, it remained to ascertain what effect, if any, would be exerted by electrical force on light-rays.

The answer to this problem was given by Prof. Johannes Stark, at Aix-la-Chapelle, in 1913, by his skillful demonstration of the electrical decomposition of the spectral rays of hydrogen, helium and lithium.[30]

Stark’s task was a more difficult one than Zeeman’s, owing to the fact that he had to deal with luminescent gases, which, being conductors, exhaust the electrical field almost before any observations can be made, even hurriedly. This condition gives rise to difficulties in connection with the application of the electric field. But these were very ingeniously met by employment of highly evacuated tubes and the light emitted by the “canal rays”—positively charged particles similar to the alpha rays.[31] Where the rays issue from the perforated electrode (or “canal”), the conduction of electricity is weak, and Stark was able to apply intense electric fields in a small space. It was then found that the diffuse rays of the spectrum produced were strongly influenced, while the “sharp” rays were less so.

The attentive reader will note that this result was in marked contrast with the magnetic decomposition produced in the Zeeman experiment, in which the rays did not differ one from another in respect to the degree of their decomposition. In all the details there is a difference between the electric and magnetic decompositions, and analogy existing only in this, namely, that in both cases polarized rays were obtained. In both cases the results produced were due to disturbance of the motions of electrons, giving rise to broadening, displacement or other modifications of spectral laws. Both “effects” confirm the theoretical view of Maxwell, namely, that light is an electromagnetic phenomenon.

Faraday’s famous question is thus more than answered in the affirmative: not only is the rate of vibration of “atoms” (electrons) changed by a magnetic field, but also under the action of an electrostatic field, producing decomposition of certain spectral lines, which are usually polarized, as in the Zeeman effect.

As a result of his intensive investigations of the Zeeman effect, Dr. Henri A. Deslandres, Director of the Astrophysical Observatory at Meudon (a southern suburb of Paris), proposed a new general formula which represents the series relationship of the component lines and heads of bands both for emission and absorption spectra. According to his experimentally-derived law, “the origin of these radiations may be found in the transverse and longitudinal vibrations of the atoms.”

The lamented Dr. P. S. Epstein, a gifted pupil of Sommerfeld, who—like Mosely—fell a martyr to the World War, succeeded in applying the quantum dynamics to the Stark effect, whereby the motions of the electron in producing the H-beta (in the blue-green) and H-gamma (in the violet) lines observed, “are accounted for with great accuracy” (Loring, “Atomic Theories,” Page 67).

It may be said in conclusion, that the most promising attempts fully to explain the phenomena of the Zeeman and Stark effects seem to be made from the point of view of Planck’s Quantum Theory of Light. On the other hand, it must be admitted that there has not been, so far as I can ascertain, any theory proposed which explains all of the phenomena involved.