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HAROLD DELOS BABCOCK
January 24, 1882-April 8, 1968
BY IRA S. BOWEN ~
HAROLD DELOS BABCOCK came from a family whose members
have made many contributions to science. Harold him-
self was elected to the National Academy of Sciences in 1933;
his brother Ernest B., a biologist, was elected in 1946; and his
son Horace W., an astronomer, was elected in 1954.
Harold Babcock was born {anuary 24, 1882, in Edgerton,
Wisconsin, a town of 2,000 inhabitants, twenty-five miles south
of Madison. He was the youngest of seven children of Emilus
W. and Mary Eliza (Brown) Babcock. His father's ancestry is
traced to fames Badcock (later spelled Babcock), who was born
in England in 1614 and settled in Rhode Island in 1642. His
mother's grandparents, German and English, traveled on a raft
down the Ohio River from Pittsburgh to Cincinnati to build
a home there, about 1800.
Harold's father owned and operated a general store in
Edgerton and a farm nearby. The environment, while isolated,
was wholesome. Family life was busy and congenial. The
father and mother and an older sister had all been teachers.
With their help Harold learned to read before reaching school
age, and he acquired a lifelong love of music. From an old
~ The Academy is indebted to Horace W. Babcock, son of Harold D. Bab-
cock, for his assistance in the final preparation of this memoir for publication
after the death of the author.
1
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2 BIOGRAPHICAL MEMOIRS
book, Natural Philosophy, and a copy of S. P. Thompson's
Elementary Lessons in Electricity and Magnesium, he developed
an early interest in these subjects, performing many experiments
in static electricity and constructing a simple telegraph. A pin-
hole camera and a few photographic plates, a reward for obtain-
ing a subscription to the Youth's Companion, introduced him
to photographic techniques.
Harold's health was never robust, possibly because of an
attack of rheumatic fever in early youth. He attended public
school and had completed one year of high school when, in
1896, the family, except for the two oldest sons, moved to Los
Angeles.
In Los An~eles Harold entered the only high school then
operating and continued for four and a half years until Febru-
ary 1901. In addition to the usual mathematics, physics, and
chemistry, the extra year and a half spent at the school enabled
him to take four years of Latin and a year each of German and
Greek, as well as short courses in geometry and astronomy. He
engaged in dramatics and was president of the school literary
society. At the high school he came under gifted teachers who
aroused his interest in physics and chemistry. During these
years he carried out at home such experimental work in these
subjects and in photography as meager equipment would per-
mit.
Marconi's successes in radio communications impressed
young Babcock greatly. In 1900 he used the facilities of the
physics laboratory of the Los Angeles High School to demon-
strate the transmission of radio signals over a distance of one
hundred feet. The discharge of a high-voltage condenser was
used to produce the signal, which was received by a "coherer"
patterned after Marconi's apparatus. Years later Babcock built
his own apparatus for receiving early radio broadcasts and in
1940 received his license as an amateur radio operator. The
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HAROLD DELOS BABCOCK
3
familiarity with electronics obtained in these early studies
proved useful in much later work.
In August 1901 Babcock enrolled in the College of Elec-
trical Engineering at the University of California in Berkeley.
He found his chief interest in physics and had an opportunity
for unscheduled laboratory study in electrical measurements
and spectroscopy under the guidance of Professors W. I. Ray-
mond and E. P. Lewis. The death of Babcock's father and his
own illness delayed the completion of the requirements for the
B.S. degree until 1906. The degree was conferred in absentia
the following year.
Summer vacations during these college years brought diverse
experiences. In 1903 Babcock was a member of a party making
reconnaissance surveys for new construction by the Pacific Elec-
tric Railroad. In 1904 he accompanied Dr. H. M. Hall of the
Department of Biology of the university on a five hundred-mile
botanical collecting expedition in the southern High Sierra.
For two months, in the tradition of John Muir, the party lived
in magnificent scenery in regions remote from settlements and
often without trails. Several peaks, including Olancha Peak
and Mount Whitney, were climbed. The exhilaration of this
. .
experience was astlng.
In July 1906 Babcock received an appointment as laboratory
assistant at the National Bureau of Standards. At the bureau he
and Edward B. Rosa made an extensive study of the instability
of laboratory standards of electrical resistance. They found the
cause to be fluctuations in atmospheric humidity, a result at
first disputed but later confirmed by the corresponding bureaus
in England and Germany.
Babcock was united in marriage to Mary G. Henderson in
1907. To this union one son, Horace, was born in 1912.
After a few months of teaching physics at the University of
California, in 1908, Babcock was invited by George E. Hale to
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4
BIOGRAPHICAL MEMOIRS
join the staff of the Mount Wilson Observatory of the Carnegie
Institution of Washington; this he did on February 1, 1909.
He continued this connection with the observatory for the
remainder of his active life.
At Mount Wilson, Babcock's first assignment was the pho-
to'~,raphy of selected star fields at the Newtonian focus of the
newly completed sixty-inch telescope as part of Professor J. C.
Kapteyn's program for the study of the structure and kinematics
of the Galaxy. The plates obtained provided some of the first
evidence for interstellar absorption of light. Later Babcock col-
laborated with Walter S. Adams in a spectroscopic program
using the 60-inch telescope. Very high-dispersion spectrograms
of seven of the brightest stars and some five hundred spectro-
grams of fainter stars at lower dispersions were obtained.
In lS96 Zeeman had discovered that spectral lines emitted
in a strong magnetic field were split into three or more com-
ponents, the width of the pattern of lines being proportional
to the strength of the field. Twelve years later Hale observed
with the sixty-foot solar tower telescope on Mount Wilson the
~. . . · r I' ~ · · r
same splitting of the lines coming from sun spots. Obviously,
the Zeeman effect provided a powerful tool for the study of
magnetism in astronomical bodies. The number of components
into which a line is split, however, and the ratio of the width of
the pattern to the magnetic field, vary from line to line and from
chemical element to chemical element. Extensive laboratory
work was obviously required before the method could be ap-
plied to astronomical studies. Using newly developed equip-
ment that provided herds of up to 35,000 gauss, Babcock made
detailed observations of the Zeeman patterns in vanadium and
chromium, two elements whose lines are prominent in the solar
spectrum.
With the development of atomic structure theory and, in
particular, of the vector model early in the 1920s, it became
possible to predict the structures of the complicated Zeeman
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HAROLD DELOS BABCOCK
patterns and their widths as functions of the strength of the
magnetic field and certain atomic constants. At that time
Babcock returned to the problem and from very precise meas-
ures of the width of the Zeeman patterns and of the magnetic
fields was able to obtain one of the most accurate values then
available for e/m or the ratio of the charge of the electron to its
mass.
During the first decades of the present century, spectroscopy
experienced a tremendous development. This was especially
so in astronomy, since it was realized that spectroscopy held the
key to many problems, including chemical compositions, tem-
peratures, pressures, radial velocities, and magnetic fields of
astronomical bodies. Progress in most of these problems de-
pended on very precise measurement of wavelengths, and this
in turn required accurate and easily reproducible standards for
comparison. The publication of Rowland's Preliminary Table
of Solar Spectrum Wavelengths between 1895 and 1897 provided
the first such set of standards. They were used as the basis
for both laboratory and astronomical observation of spectra for
the next quarter century. However, by the second decade of the
present century it became evident that these Rowland wave-
lengths were not only too large by 0.0036 percent, but had
erratic fluctuations of up to 0.03 to 0.04 A throughout the range
from 3000 to 7330 A.
One of the first problems here was to find at new source,
preferably a laboratory source, that produced a large number of
lines well distributed through the ordinarily observed spectral
range and the wavelengths of whose lines remained constant
under all ordinary conditions of operation. The first criterion,
a satisfactory distribution of lines throughout the spectrum,
was satisfied by an electric arc between iron electrodes. Begin-
ning in 1914, Charles E. St. John of the observatory staff and
Babcock carried out an extensive study of the second criterion,
the constancy of the wavelength of the light emitted from
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6
BIOGRAPHICAL MEMOIRS
various points in such an arc under different conditions of
operation using the most precise techniques available, namely,
a Fabry-Perot interferometer combined with a grating. In
general, they found substantial wavelength shifts of many of the
lines, depending on operating conditions and the position in
the arc from which the light originated. However, by carefully
defining the operating conditions and the location of the source
of the light, they were able to achieve highly reproducible re-
sults. The specifications for the arc that they set up were later
adopted officially by the International Astronomical Union for
the source of the iron wavelength standards.
Using this standard source, St. John and Babcock proceeded
in 1921 to the measurements of the wavelengths of the lines
emitted. Later, in 1927, Babcock repeated many of the measure-
ments and measured additional lines. For adoption as an official
wavelength standard, the rules of the International Astronomical
Union required that at least three independent observers agree
on the wavelength of a line within certain very close limits.
The values by St. John and Babcock were used as one of these
three measures.
Because of the important role that these studies played in
the establishment of the basic wavelength standards, Babcock
was asked in 1925 and again in 1928 to serve as president of
the Commission des Etalons du Longueur d'Onde et des Tables
de Spectres Solaires of the International Astronomical Union.
Using the same precision techniques, St. John and Babcock
then investigated certain pressure-sensitive lines in the solar
spectrum to measure the pressure in the photosphere. They also
studied the constancy of the wavelengths of both terrestrial and
solar lines in the solar spectrum as a means of detecting motions
in the earth's and the sun's atmospheres.
Having established the necessary wavelength standards and
investigated the constancy of the wavelengths of the lines in the
solar spectrum, St. John and Babcock, with the assistance of
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HAROLD DELOS BABCOCK
7
Charlotte E. Moore, Louise M. Ware, and Edward F. Adams,
carried out a revision of Rowland's table of solar wavelengths.
All wavelengths were related to the new wavelength standards.
A large fraction of the lines that had not been classified as to
chemical element in Rowland's list were classified on the basis
of later laboratory work. Temperature classifications and ex-
citation potentials were added when available. The tables were
extended from Rowland's limit in the red at 7730 A to 10,218 A
on the basis of new observations. A total of some 22,000 solar
and terrestrial lines were listed. The volume containing these
results was published in 1928 and at once became the basis for
many investigations of both the sun and other stars.
In 1947 Babcock and Charlotte Moore published a second
volume repeating many of the earlier infrared measurements
and extending them to 13,500 A in the far infrared. The fol-
lowing year Babcock, with Miss Moore and Mary F. Coffeen,
extended the observations of the solar spectrum in the ultra-
o
violet to 2935 A and increased the accuracy and detail of the
measurements from 3063 A to the former limit.
These observations of the solar spectrum included a large
number of sharp lines originating in the earth's atmosphere,
especially in the red and infrared. Many of these lines were
caused by absorption by the oxygen molecule. Observations
made when the sun was near the horizon and the light passed
through some hundreds of kilometers of air brought out many
very faint lines that could not have been observed otherwise.
In 1927, G. H. Dieke and Babcock published the wavelengths
of these lines and their classification as bands of the oxygen
molecule, each of whose atoms was considered to be of mass 16.
A little more than a year later Giauque and Johnson of the
University of California noted that several of the faint bands
could be explained best as arising from an i60~80 molecule.
Babcock again went over the observational data and listed a
number of hitherto unclassified lines. Some of these proved to
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8 BIOGRAPHICAL MEMOIRS
be missing i60~80 lines, while some of the faintest were caused
by the ]60~70 molecule. From measures of the relative in-
tensities of the lines of i60~6O, 070, and ]60iSO, Babcock was
able to estimate the relative abundances of the i60, i70, and
RIO isotopes. Likewise Birge and Babcock were able to fix the
relative masses of the i60 and iso isotopes from the constants
of the band structures. Some twenty years later, Babcock and
Louise Herzberg, using new measurements, made new pre-
cision determinations of the constants of the ]60~6O, 070,
and i60~80 molecules.
This discovery that oxygen had isotopes of mass seventeen
and eighteen and that ordinary oxygen was a mixture of these
with the predominant isotope of mass sixteen had a fundamental
impact on the atomic weight systems as determined from chem-
ical analyses and from mass-spectrograph observations. Because
of the procedures used, the chemical system was relative to the
average weight of all isotopes of oxygen, while the mass-spectro-
graph system was relative to the mass of the i60 atom alone.
Since the basic assumption of both systems was that the atomic
weight of oxygen was exactly sixteen, it became necessary to
shift one or the other system to a new base to bring them into
agreement.
In 1923 Babcock used the interferometer techniques that
he had developed to make the first precise measurement of the
wavelength of the brightest but as yet unidentified line in the
spectrum of the aurora—the well-known "green line." He
achieved at least a one hundred-fold increase in accuracy, show-
ing that the line's wavelength was 5577.350 A and that its width
was less than 0.035 A. This led to its identification as due to a
forbidden transition in the oxygen atom.
Starting in 1912, a very large engine for ruling diffraction
gratings was designed and constructed at the Mount Wilson
Observatory under the direction of J. A. Anderson. Because of
the friction and flexure inherent in such a large engine, it never
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HAROLD DELOS BABCOCK
9
succeeded in ruling the very large gratings for which it was
designed. When the 200-inch telescope project was initiated,
in 1928, Anderson was made its executive officer, and Bab-
cock was asked to take charge of the ruling of gratings. A
careful review of the program indicated that for the ruling of
small and moderate size gratings (up to 10 x 7 inches), a
smaller engine would have a much larger probability of suc-
cess. Under Babcock's direction, such an engine was designed
and constructed between 1928 and 1932 by Francis G. Pease,
Edgar C. Nichols, Clement iacomini, and Elmer Prall. In the
course of this development much attention was given to the
selection and the shaping of the ruling diamond in order to
control the exact shape of the grooves ruled. With the proper
groove shape, it is possible to throw most of the incident light
into one order of the spectrum. A still further increase in grat-
ing efficiency was achieved by Babcock by shifting from specu-
lum metal (an alloy of tin and copper) to a coat of aluminum
evaporated onto glass as a ruling surface.
These procedures
were so successful that the gratings he produced had a higher
efficiency than a prism train of the same dispersive power.
Moreover, when ruled on aluminum evaporated onto Pyrex
blanks, the gratings had a sensitivity to temperature only about
one twenty-fifth of that of prisms. Because of these advantages,
all prisms in the spectrographs at Mount Wilson were replaced
with Babcock's gratings, and noteworthy improvements were
achieved in resolving power, speed, and stability.
On Babcock's retirement from regular duties at the ob-
servatory on February 1, 1948, he was asked to continue the
supervision of the ruling engine for another year. By the end
of that year the ruling engine was in regular production of
grating up to 6 x 7.5 inches, which approached closely the
capacity of the engine. The gratings produced in this and the
following year met the needs of the large spectrographs of the
200-inch Hale telescope.
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10
BIOGRAPHICAL MEMOIRS
After Hale's discovery of magnetic fields in sunspots in
1908, Hale and a number of collaborators had attempted for
many years to detect and measure the general magnetic field of
the sun, but were never able to achieve conclusive results. Bab-
cock began work on the problem in 1938 using a Lummer plate,
which provided somewhat higher resolution than had been
used in the earlier studies. The photographs obtained, how-
ever, failed to yield a definite answer.
Shortly after World War II, Babcock and his son Horace
attacked the problem again, using new optical and electronic
techniques that had been developed since the earlier studies.
The Babcocks achieved not only unambiguous measures of the
field, but were able to push the sensitivity to the point that it
was possible to scan the sun's surface rapidly and plot the de-
tailed distribution of the intensity of the field over the surface.
A program was then set up for producing daily maps of this
distribution of the field. In the course of these observations it
was found that this general magnetic field of the sun reverses
with the eleven-year period of the sunspot cycle.
Babcock participated in the Mount Wilson Observatory
expeditions to observe solar eclipses in 1918, 1923, 1930, and
1932.
In World War I Babcock served in the Research Information
Service of the National Research Council. Later he engaged
in supersonic research that was part of an antisubmarine effort.
In World War II he served as consultant on a number of
projects and produced special ruled surfaces for the Manhattan
District, the U.S. atomic bomb project.
In summary, Harold Babcock's scientific life was devoted to
pushing the precision of measurements and of techniques to
the furthest possible limits. In spectroscopy this resulted in a
set of standards that are basic to most spectroscopic measures in
both astronomy and physics. His accurate measurements of the
oxygen bands provided the basis for the discovery by Giauque
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HAROLD DELOS BABCOCK
11
and Johnson of the isotopes of oxygen of mass seventeen and
eighteen, which required a major revision of the atomic weight
system. His development of precision techniques made possible
the final solution of the problem of the general magnetic field
of the sun and the ruling of the first large gratings of high
efficiency for astronomy.
Harold Babcock's results were never published until they
had been carefully considered and were fully established on a
sound basis. Underlying this patience and thoroughness was an
unusual awareness of nature. As Gerald E. Kron remarked in
1953, when presenting to him the Bruce Medal of the Astro-
nomical Society of the Pacific, Babcock was a person with a
high degree of interest in his environment and in people. He
sought to understand and appreciate in depth the elements of
nature that he encountered, on whatever scale, and he had the
ability to transmit this appreciation, especially to younger
associates.
Always considerate of his colleagues, he unobtrusively ac-
complished many kindnesses for them and their families, espe-
cially in later years. He died suddenly on April 8, 1968.
Babcock was a member of the American Association for the
Advancement of Science and received its Pacific Division Prize
in 1929. He was a member of the American Physical Society,
the American Astronomical Society, and the Astronomical So-
ciety of the Pacific and was an Associate of the Royal Astro-
nomical Society. The University of California conferred the
honorary LL.D. degree on him in 1957.
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BIOGRAPHICAL MEMOIRS
BIBLIOGRAPHY
KEY TO ABBREVIA TIONS
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Phys. Rev. Physical Review
Phys. Soc. London Opt. Soc. Physical Society of London Optical Society
Popular Astron. Popular Astronomy
Proc. Nat. Acad. Sci. Proceedings of the National Academy of Sciences
Publ. Am. Astron. Soc. Publications of the American Astronomical Society
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1907
With E. B. Rosa. The variation of resistances with atmospheric
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1911
The Zeeman effect for chromium.
The Zeeman effect for vanadium.
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Astrophys. l., 34: 209-24.
Note on the grouping of triplet separations produced by a magnetic
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1914
With C. E. St. John. A displacement of arc lines not due to pres-
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1915
Review of laboratory studies of the Zeeman effect at Mount Wilson
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OCR for page 14
HAROLD DELOS BABCOCK
13
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1917
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1920
With C. E. St. John.
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BIOGRAPHICAL MEMOIRS
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(A)
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The effect of pressure on the spectrum of the iron arc. Astrophys.
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HAROLD DELOS BABCOCK
15
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Revision of the value of e/m derived from measurements of the Zee-
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All~oOOA. Publ. Astron. Soc. Pacific, 41:274-76. (A)
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1930
Beyond the red in the spectrum. Publ. Astron. Soc. Pacific, 42:83-
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OCR for page 17
16
BIOGRAPHICAL MEMOIRS
1931
With W. P. Hoge. New data on the absorption bands of atmo-
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oxygen 18 and 16. Phys. Rev., 37:233. (A)
1932
With W. P. Hoge.
spheric oxygen.
New measurements of the 1,1 band of atmo-
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1933
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(A)
The construction and characteristics of some diffraction gratings.
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1934
With H. W. Babock. Some new features of the solar spectrum.
Publ. Astron. Soc. Pacific, 46:132-33.
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Wave numbers of the infrared spectral lines beyond A10,000. Phys.
Rev., 46:382-83.
With H. N. Russell and C. E. Moore. Series lines of magnesium
in the solar spectrum. Phys. Rev., 46:826-27.
With C. E. Moore and C. C. Kiess. The presence of phosphorus in
the sun. Astrophys. i., 80:59-60.
The description of the infrared solar spectrum by photography.
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1935
Charles E. St. John. Publ. Astron. Soc. Pacific, 47:115-20.
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OCR for page 21
Representative terms from entire chapter:
magnetic field