PROOF THAT ELECTRONS ARE ATOMS 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.
FOOTNOTES:
[23] For a good summary of the main results concerning the Zeeman
effect, see von Auerbach, Felix, “_Moderne Magnetik_,” Leipsic,
1921. An excellent account of the quantum treatment of the Zeeman
effect may be found in Chapter XV (Series Spectra) of Dr. N. R.
Campbell’s “Modern Electrical Theory, Supplementary Chapters,”
Cambridge University Press, 1921.
[24] It seems that this phenomenon had previously been observed by
M. Fievez. (Cf. Michelson, Dr. Albert A., “Light Waves and Their
Uses,” Page 107.) “He thought that each separate line was doubled or
quadrupled.” Lockyer, in 1866, observed that some of the lines in a
sun spot spectrum were widened. Prof. Charles Young and W. M. Mitchell
observed that some of the lines were even double, but it was not
suspected that these phenomena were caused by a strong magnetic field
in sun-spots, brought about by free electrons being driven around
in a vortex movement. In fact, Mitchell referred to the doublets as
“reversals.”
[25] Zeeman, “_Les Lignes Spectrales et les Theories Modernes_,”
_Scientia_, January 1, 1921, Page 18 (Vol. XIX, No. CV—I).
[26] Hale, “Ten Years Work of a Mountain Observatory,” Pages 29-30,
Washington, D. C. (Carnegie Institution of Washington), 1915. See also,
Babcock, Harold D., “The Zeeman Effect for Chromium,” _Contributions
from Mount Wilson Observatory_, Vol. II, Paper No. 52; also “The
Correspondence between Zeeman Effect and Pressure Displacement for the
Spectra of Iron, Chromium and Titanium,” Arthur S. King, Loc. cit.,
Paper No. 46; and “The Zeeman Effect on the Sun,” Adriaan van Maanen,
_Publications of the Astronomical society of the Pacific_, Page
24, Vol. XXXIV, No. 197 (February, 1922).
[27] Zeeman, _Loc. cit._, Page 18. See also the classical
monograph by the same author, “Researches in Magneto-Optics,” London,
1913.
[28] Millikan, _Physical Review_, 2, 143 (1913); “The Electron,”
1917 (revised edition, 1924). See also, _Proceedings of the National
Academy of Sciences_, 3, 314 (1917).
[29] “The New Heavens,” Page 70, New York, 1922.
[30] Cf. Stark, “_Die Atomionen chemischere Elemente und ihre
Kanastrahlenspektra_,” Berlin, 1913. See also, “_Elektrische
Spektralanalyse chemischen Atome_,” Leipsic, 1914.
[31] Called “canal rays” by the German physicist, Eugen Goldstein, who,
in 1886, first obtained them by the use of a perforated cathode; that
is, he used a metallic tube for a cathode, through which tube, called
by Goldstein a “canal,” the rays issued.