Leopold Stokowski, Harvey Fletcher and Bell Laboratories
Experimental High Fidelity and Stereophonic Recordings 1931-1932
Leopold Stokowski, Harvey Fletcher and Bell Laboratories
Experimental High Fidelity and Stereophonic Recordings 1931-1932
Leopold Stokowski with Dr. Harvey Fletcher
In 1929, the Philadelphia Orchestra began live broadcasts of concerts from the Academy of Music, Philadelphia via the NBC radio network. Arthur Judson, the famous orchestra manager had been urging Stokowski to broadcast for some time, but Stokowski was dissatisfied by the sound of these early broadcasts.
Harvey Fletcher and the Bell Laboratories Research in Recording Technology
Harvey Fletcher was a brilliant physicist who had studied and worked at the University of Chicago, where he aided Dr. Robert A. Millikan (1868-1953) in his research. Millikan was famous for his measurement of the charge of the electron (aided by Harvey Fletcher), and for his work on the photoelectric effect (confirming Einstein's theory of the photon theory of light). Harvey Fletcher's contribution to this work on the charge of the electron may have been understated 16, 17, but was a key part of Millikan's work. Millikan was awarded the Nobel Prize in Physics in 1923 for his measurement of the electron charge, one of the fundamental physical constants in particle physics. Millikan then went on to serve as president of the California Institute of Technology 1921-1945.
Harvey Fletcher received his Ph.D. in Physics from the University of Chicago in 1911 summa cum laude. Harvey Fletcher later became Director of Research at Bell Laboratories, where he oversaw three decades of research and improvement in sound, hearing, transmission, and sound reproduction.
Stokowski, Harvey Fletcher and Robert Millikan in 1936 13
At Bell Laboratories, Fletcher oversaw research by (among others) Joseph P. Maxfield, Henry C. Harrison, Edward Christopher 'E.C.' Wente, K. P. Secord, Rogers H. Galt, Harold Black (who invented the negative feedback amplifier in 1927) 12, Arthur C. Keller and others working on amplification, electronic sound transmission, recording and reproduction.
Harrison and Maxfield made a number of developments which, together allowed the creation of an electrical sound recording system. One of these was the development of a matched-impedance electronic system, with a carbon microphone, linked to a tube or valve amplifier, driving a moving magnet (called also a "moving armature") cutting head to scribe the sound in the wax master.
This matched-impedance electrical recording system had a recording bandwidth from 50 Hertz to 6,000 Hertz, beyond which its sensitivity declined. The new electrical recording system dramatically improved on the acoustic recording system, invented by Thomas Edison and gradually improved until it could record approximately from 250 Hertz to about 2,400 Hertz (sometimes). This new, wider bandwidth of the electrical recording system added another octave of sound reproduction, compared with the acoustic process, and greatly reduced the harmonic distortion of the acoustic process, and produced a generally more realistic sound image. You can read about these developments in The Development of Electrical Recording , elsewhere on this site.
Harvey Fletcher, K. P. Secord, and Rogers H. Galt at the Bell Laboratories
In a 1981 BBC Radio 3 interview, Arthur C. Keller, who worked for Henry Harrison, told of his and the Bell Lab's early sound and recording efforts and those of his colleagues in the 1920s and 1930s at the Bell Laboratories. They were working initially on long line transmission with better sound fidelity, and later on high fidelity and binaural or stereophonic recording. In the interview which you can listen to by clicking below, Keller spoke of Bell Laboratories work with Stokowski and the Philadelphia Orchestra in test recordings to achieve high quality recordings and broadcast transmissions from the Academy of Music in Philadelphia in 1930 and 1931.
1930 Advertisement for the Sunday Stokowski - Philadelphia Orchestra broadcast
In April, 1931, Bell Labs began recording Stokowski and the Philadelphia Orchestra in the Academy of Music, using new equipment installed in the basement, with Stokowski's permission. This equipment, did not use the conventional lateral cutting heads used for contemporary 78 rpm disks. Keller’s disk cutting equipment used a vertical cut ("hill and dale") recording method, using a magnetic moving coil pickup fitted with a sapphire stylus cutting the wax master.
The vertical cutting device improved stylus tracking and thereby reduced harmonic distortion. It also helped to expand the dynamic range of the recording.
Moving coil microphones, capable of capturing frequencies above 10,000 Hz, developed by Bell Laboratories were used in both the monophonic and "binaural" (or stereophonic) recordings. Reportedly, more than one thousand 78 RPM sides were cut in the Academy of Music during the 1931 and 1932 experimental Philadelphia Orchestra recordings.
Bell Labs had also earlier determined that surface noise on 78 wax recording masters (called the “matrix”) was caused by the graphite which was deposited on the wax surface during the manufacturing process. In the web page Eldridge Johnson, Victor, and the Development of Acoustic Recording you can read that first lead powder, and later copper powder was brushed on the wax master to make it conductive for electroplating. Later, graphite was adopted. The graphite allowed the surface of the master to become conductive, so it could be electroplated, preliminary to the later steps in producing record "stampers". This electroplating technique was the key to permitting creation of multiple versions of "masters", used in the mass production of records.
Arthur Keller and A.G. Russell devised the approach of processing the wax masters by means of gold sputtering in a vacuum chamber, which laid down a one-molecule thickness of gold onto surface of the wax. This conductive layer allowed them to electroplate a copper layer onto the gold surface, thus bypassing the need for the conductive graphite surface. This eliminated the surface noise resulting from the graphite on the surface of the recording.
Pressings of the recordings from these quiet masters were then made using cellulose acetate disks, rather than the typical noisy shellac material of the usual 78 rpm disks of that era.
In December, 1931, the first electrical recordings with this improved process were made and the experiments continued throughout the 1931-1932 concert season. The audio spectrum was extended first to 9,000 Hertz and then to 10,000 Hertz, giving for the first time good fidelity in the overtones and treble range of instruments.
Bell Laboratories asked Arthur Keller to come out of retirement in 1979 to catalogue, and assist in transcribing some of the gold sputtered disks still in storage. Keller identified Stokowski - Philadelphia recordings from among 600 metal masters at the Bell Laboratories in Murray Hill, New Jersey4. Of these, more than 100 were preserved by transcription, done by the legendary remastering engineer Ward Marston. I should add that Arthur Keller in his interviews does not give (in my opinion) the proper recognition of the contributions of Ward Marston in restoring and therefore making available these historic, and wonderful recordings.
Ward Marston, Arthur Keller and their associates were able to save many of the old metal masters. These masters were then cataloged and transcribed, including some of the stereo masters. Based on Ward Marston and Arthur Keller's work, Bell Telephone issued two commemorative albums with some of these transcriptions in 1979 and 1980, Bell Telephone BTL-7901 and BTL-8001. As far as I have been able to determine, all of the CDs and other media which circulate with some of this material come from these Bell LP disks compiled by Arthur Keller and Ward Marston, whatever these 'knock-off' CDs may claim (or leave unsaid). The Bell Laboratories disks are the only sources of this pioneering recordings (at least so far) and we are indebted to Bell Laboratories of 1979-1981, and to Keller and Marston for the surperb examples of these important early sound experiments.
The 1981 Bell Laboratories LP of 1931 and 1932 Philadelphia Performances
The Berlioz Roman Carnival Overture was recorded December 5, 1931 in the Academy of Music, without the knowledge of the orchestra members. The musicians likely did not notice anything unusual, since radio microphones were routinely hanging from the Academy of Music ceiling. Arthur Keller had installed his recording equipment in the basement below the orchestra stage, where the Victor Talking Machine Company electrical recording equipment had also been installed in 1926. Stokowski later said, when he heard the Roman Carnival recording at the Bell Laboratories in New York City that it was the best quality recording he had ever heard.
Arthur Keller said that their recorded response in the Roman Carnival extended to 13,000 Hertz, the highest frequency response achieve up until that time by Bell Laboratories5. The restored recording in the links, below, were done by the mastering and restoration engineer Marcos Abreu, from the original Bell Labs LP disks and they are both excellent and subtle restorations. You can contact him at Marcos Abreu - Audio mastering and restoration services, email address: email@example.com Thanks Marcos ! Listen to Marcos' results by clicking on the link, below.
As well as developing higher-fidelity sound reproduction with expanded frequency range and reduced harmonic distortion, Bell Laboratories also developed and used stereophonic recording technology for the first time. In March, 1932, Bell Laboratories recorded the Philadelphia Orchestra in "binaural" or stereophonic sound, by connecting two different microphones each to its own cutting stylus, with each moving magnet cutting stylus. The two cutting styli were each in its own arm, parallel to the other, but one recording from the outer edge of the wax disk (as was normal), and the other beginning half-way into the disk. As a result, each stylus would cut half of the 78 RPM disk with a record groove containing a right or a left audio channel. Playback was the reverse process, using two playback styli.
Using this stereophonic equipment, the Bell Laboratories engineers recorded Stokowski and the Philadelphia Orchestra in a Russian program on March 12, 1932 in the Academy of Music. They recorded the Poem of Fire opus 60 by Alexander Scriabin (1874 - 1915) and the Mussorgsky-Ravel Pictures at an Exhibition in this format. These recordings are the earliest surviving examples of stereophonic recording.
You may hear two stereo tracks of the Poem of Fire transcribed from the old metal masters done by Ward Marston by clicking on the links below.
Alexander Scriabin in about 1900
On March 12, 1932, the Bell Laboratories experimental equipment recorded Stokowski and the Philadelphia Orchestra performing Maurice Ravel's orchestration of Tableaux d'une Exposition in both their high fidelity and stereophonic recording technology.
The original composition was written by Mussorgsky in 1874 and were his musical impressions of 10 (or perhaps 11) pictures, or tableaux by Mussorgsky's friend Viktor Hartmann (1834–1873), shown at a retrospective exhibition of Hartmann's works. Hartmann had died unexpectedly of an aneurysm the year before Mussorgsky wrote 'Pictures at an Exhibition'. Hartmann's death is said to have made a deep impression on Mussorgsky, and Mussorgsky (who also died young 1839-1881) later recounted that he composed these piano pieces in only six weeks.
Serge Koussevitzky commissioned Maurice Ravel to orchestrate the Tableaux d'une Exposition. After Koussevitzky's exclusive period of performance of the transcription ended in the late 1920s, Stokowski performed the Ravel transcription a number of times.
However, as you may read elsewhere in this www.stokowski.org site, Stokowski was not totally satisfied with Ravel's orchestration. Stokowski did not commercially record the full Ravel orchestration, but rather made his own orchestration. You can read about Stokowski's efforts and also listen to the Stokowski transcription by clicking on the link to the page 1939 - Mussorgsky - Pictures at an Exhibition - Stokowski Orchestration.
Stokowski's score for 'Tableaux d'une Exposition' 1939
The March 12, 1932 performances Tableaux d'une Exposition resulted in four excerpts from the work which Ward Marston was able to expertly assemble together to provide these satisfying, extended excerpts.
These excerpts have been restored from the Bell Laboratories LP recording by the mastering and restoration engineer Marcos Abreu in an excellent and subtle restoration. (You can contact him at Marcos Abreu - Audio mastering and restoration services, email address: firstname.lastname@example.org
Listen to these excerpts by clicking on the links, below. The excerpts include:
1. The conclusion of the initial Promenade, leading into the full movement of Gnomus , which constitute most of the music of the first two sections of Tableaux. This track is a stereophonic recording.
2. Most of Il vecchio castello (the Old Castle) , missing a few seconds at the beginning and the end. This track is a stereophonic recording.
3. The conclusion of Bydlo (the Polish wooden cart), the Promenade transition and the complete Ballet des poussins dans leur coque (Ballet of the Unhatched Chicks), and the beginning of Samuel Goldenberg and Schmuyle. The laughter of the audience in this live recording at the end of the Ballet des poussins indicates that the audience was not yet familiar with this work. This track is a monophonic recording.
4. All of La cabane sur des pattes de poule (Baba Yaga's Hut on Fowls' Legs) and La grande porte de Kiev (The Great Gate of Kiev) , except perhaps the first second of Baba Yaga. Also, Stokowski makes cuts at the conclusion of 'La grande porte de Kiev'. This track is a monophonic recording.
Hartmann's sketch for Unhatched Chicks
Those who know Tableaux d'une Exposition well will notice that both Les Tuileries, the third 'picture' and Limoges, le marché, the seventh 'picture' are not included in these excerpts. This is because Stokowski did not perform them. He seems to have formed the theory that these two pictures were not really intended by Mussorgsky, but rather inserted by Ravel from other music. Stokowski also found Ravel's orchestration in these two cases too French, and not sufficiently dark and Slavic. So, he did not include them when he performed the Ravel orchestration. He also did not include these tableaux at all in his own later orchestration of Tableaux d'une Exposition.
On Friday April 29 and Saturday April 30, 1932, Stokowski and the Philadelphia Orchestra gave two performances of music from Wagner's Ring des Nibelungen. Bell Laboratories recorded excerpts of these concerts in what are likely the best Wagner performances Stokowski made with the Philadelphia Orchestra during the 1930s. These excerpts were from from Die Walküre, Siegfried, and Die Götterdämmerung.
Ward Marston in his notes to the Bell Laboratories LP writes:
"...Portions of the Saturday performance were recorded in two-band stereo, and in every case these stereo excerpts have been incorporated here - however fleeting in duration they may be. Throughout the first side of this record [note: the Die Walküre excerpts and the Siegfried Forest Murmurs] the listener will be aware of a change in sound from monaural into stereo and back again. This approach was adopted to maintain musical continuity while preserving what stereophonic excerpts survive...".
As Ward Marston's notes remark, this famous excerpt from Die Walküre: Wotan's Farewell and Magic Fire Music, recorded during concerts of 29 and 30 April 1932 are partially in stereo. This dramatic scene from the finale of the opera, Act 3, represents the confrontation between Wotan and his daughter Brünnhilde, when he condemns her to mortality and to a magic sleep. However, he relents and surrounds her with a magic fire, so that only the bravest hero (here the Siegfried motif is heard in the music) will penetrate the ring of fire to save Brünnhilde.
This performance is poignant and magical, as of course it should be. The sound, amazing for this era, and of a 'live' performance is yet more striking.
Ward Marston in his notes for the Bell LP states:
"...In this recording, a 17 second gap in the Magic Fire Music has been filled from a 1939 recording, with the kind permission of RCA Records, and there is a 90 second gap near the beginning of Wotan's Farewell, which was impossible to fill from any other Stokowski/Philadelphia source..."
Concluding the first half of the April 29 and 30, 1932 all Wagner concert was the Siegfried Forest Murmurs scene from Act 2 of Siegfried when Siegfried is in the woods in front of Fafner's cave. After slaying the dragon, Siegfried's hand touches the dragon's blood, allowing him to understand the song of the birds, who tell him of Brünnhilde. This wonderfully evocative music is played with beauty and sensitivity by Stokowski and the Philadelphians.
Another beautiful extended excerpt from Wagner's Ring des Nibelungen from this concert was the finale of Die Götterdämmerung. This includes some of the most dramatic music of the Ring. It depicts the funeral and the funeral pyre of Siegfried, where Brünnhilde sings of her love for Siegfried. She then rides her steed Grane onto the funeral pyre. The Rhine overflows, Hagen attempts to seize the ring, but the Rhine maidens regain it. In the distance, the sky is filled with fire, and Valhalla is revealed consumed in flames. Valhalla then collapses with Wotan and the gods seated within. Brünnhilde, through her love for Siegfried has cleansed the world of its corruption. Underlining this transformation, the key of the final music changes from E flat to D flat, concluding Wagner's four operas telling the story of the Ring.
Incidentally, in this excerpt, there is a very faint audio image of what seems to be an soprano operatic voice singing faintly in the background at about 6 minutes into these excerpts. This is present on every pressing of the Bell Laboratories 33 RPM Long Playing disk I have heard, so I conclude it intruded in some way into the master tape of this excellent transcription of the original 78 RPM master disks. It is noticeable only at elevated volumes, however, and should not detract from the enjoyment these performances provide, more than 75 years after the original live concerts.
Caution: the file below is very large (>28 MB)
Also from this concert is the famous Ride of the Valkyries from Die Walküre. This excerpt is partially in stereo. This has been restored from the Bell Laboratories LP recording by the mastering and restoration engineer Marcos Abreu , in an excellent and subtle restoration. (You can contact him at Marcos Abreu - Audio mastering and restoration services, email address: email@example.com
Keller and Bell Laboratories 45° Stereophonic Grooves
Arthur Keller later had the inspiration of a technique to record two channels of stereophonic signal in one record groove. His idea was to record each channels at 45 degrees from vertical and 90 degrees from each other. Keller was awarded US patent 2,114,471 for this method. As described below, Alan Blumlein working for EMI in Britain also developed this idea, but from the information I have seen, it would seem that Arthur Keller and Bell Labs probably were the first. This seems yet another example of the many in science and technology of two independent minds reaching the same idea or discovery at about the same time.
The 45 degree stereophonic recording of two channels within one record groove was not exploited commercially in the 1930s or 1940s, and Arthur Keller's patent eventually lapsed, and seemed to have been forgotten. This solution to stereophonic reproduction seems to have been re-invented in the 1950s by Westrex of Hollywood, California. This Westrex was a later spin-off of Bell Labs technology. Westrex of Hollywood, California used the the 45 degree stereophonic recording system for their Westrex stereo LP long-playing stereo disks.
Another early pioneer of stereophonic recording was done at EMI in Hayes, Middlesex, UK by the brilliant young scientist Alan Blumlein.
Alan Blumlein in the 1930s
Blumlein had joined the Columbia Graphophone Company in March, 1929 reporting to another great man and engineer, Isaac Schoenberg (in 1962, Sir Isaac Schoenberg) who had become General Manager of Columbia in 1928. Schoenberg had previously been General Manager of the Marconi Wireless and Telegraph Company.
Sir Isaac Schoenberg
Schoenberg, who had emigrated from Russia to England in 1914, hired Blumlein to join him at Columbia Gramophone. He assigned Blumlein the job of inventing a new electrical recording process not dependent on the Bell Labs/Western Electric technology and patents. The UK Columbia company had purchased the failing US Columbia company in 1924, and had then licensed the Westrex process from Western Electric (who initial licensed only US firms) in early 1925, somewhat before the Victor Talking Machine Company. (Read about this on the page Licensing the Westrex Electrical System ). This Columbia license may have influenced Victor also to license the Western Electric process.
In 1929 and 1930, Blumlein developed a superior disk cutting technology, using moving coil cutting heads, rather than the moving magnet technology of the Western Electric process. Blumlein also developed a moving coil microphone at about this time. Interestingly Edward Christopher 'E.C.' Wente of Bell Laboratories had developed a moving coil microphone in 1928, which received US patent 1,766,473 in 19319. Wente's Western Electric Model 618A of 1931 was nearly flat in response from 30 to 15,000 Hertz, and its low impedance (30 Ohm) allowed long cables without significant signal loss10.
The E. C. Wente Western Electric Moving Coil Microphone Model 618A of 1931
These inventions by Blumlein eliminated the royalties paid to Western Electric on each disk using the 'Westrex' process. These inventions are particularly impressive, given that Blumlein was working for the most part alone, with some assistants, whereas E. C. Wente, Joseph P. Maxfield, Henry C. Harrison were working as part of a large Bell Laboratories team. Blumlein, as well as saving EMI the Westrex royalty payment, developed a moving coil cutting head which was superior to the Westrex system, since it reduced distortion and increased frequency response, and tended to be more linear in frequency response during the critical step of cutting the wax master.
In 1931, in part because of the effects of the great depression, the Gramophone Company (HMV) merged with Blumlein's employer, Columbia Gramophone Company (Columbia) to form Electric and Musical Industries: EMI. In November, 1931, EMI also built the famous new recording studios at 3 Abbey Road, in St. John's Wood, London, at that time, the largest recording studio in the world.
In 1933, using the stereophonic developments which Blumlein patented (patent issued in June 14, 1933), EMI cut a stereophonic disk with two channels in one groove, 90 degrees apart. Blumlein's first recording apparatus is described by A. J. Lodge of EMI Labs, in R. W. Burn's excellent The Life and Times of A D Blumlein 15:
"...The stereo wax cutter survives as well. It was made from two Western Electric moving-armature units coupled to a single stylus by a lightweight lever system, so that one unit moved the stylus vertically, and the other horizontally. The first calibration of the recorder is believed to have been on 12th July 1933. Bandwidth is reported to have been about 4kHz.......It was with this set-up that the well-known 'walking' and 'talking' records, the first complex-cut stereo records ever, were made some time before 16th December 1933......The signals feeding the two cutters were sum, for the lateral cutter, and difference for the vertical... "
This technique was similar to Arthur Keller's patent, but slightly different. Keller's patent, written in 1931 and 1932, but not submitted until 1936, taught having both channels cut 45 degrees from vertical, and 90 degrees from each other.
So, Blumlein's pioneering stereo work resulted in the first pressing of a stereophonic disk with two channels in one groove.
It is interesting that Keller had conceived of this 45 degree rotation so that each channel would have potentially similar reproduction, since he found that cutting purely vertical and horizontal groves having differences in reproduction. In contrast, the first EMI stereo disk had one channel cut horizontally, and the other vertically. This was similar to combining the old 'hill and dale' cutting method of companies such as Pathé with the horizontal cutting of companies such as Victor and Columbia. Additionally, Blumlein did later work on 45 degree oriented groves, which have the advantage of avoiding channel differences arising from factors such as rumble and vibration.
The two channels in one groove, and the 45 degree orientation seems to have been forgotten until the period 1954 - 1958, when Westrex of Hollywood, California reinvented the technique. Westrex, a spin-off of Western Electric was sold to Litton Industries. It prospered for a time in both stereo recording and Hollywood sound systems. Westrex developed the stereo Westrex groove design, reinventing the 45 degree orientation 14.
On April 27, 1933, Bell Labs, Harvey Fletcher and Arthur Keller also arranged long distance transmission of high quality stereophonic sound across telephone long lines capable of sound transmission up to 10,000 Hz. It is interesting to note that what must have been the first telephone transmission of music also involved the Academy of Music. As cited in Gerson's Music in Philadelphia 11
"...Mr. Bocovitz, the renowned pianist, played...Home Sweet Home...and other airs in New York. The audience heard this program via telephone at the Academy of Music in Philadelphia."
A concert of the Philadelphia Orchestra at the Academy of Music, sponsored by AT&T was captured by three microphones spaced across the front of the orchestra and transmitted via three long lines to Constitution Hall in Washington, D.C. where three amplified loudspeakers reproduced the orchestra sound. The orchestra was conducted by Alexander Smallens, assistant conductor of the Philadelphia Orchestra, with Stokowski controlling sound balance.
Stokowski at the controls in the 1933 Washington DC stereo broadcast with Harvey Fletcher observing (looking somewhat unhappy)
The Washington DC broadcast concert was advertised as demonstrating “…the recent advances in high-quality telephonic transmission and reproduction of music…" On April 9 and April 10, 1940, Harvey Fletcher and Stokowski arranged another demonstration of stereo sound in Carnegie Hall with music recorded onto a three channel system using sound recorded optically on film with a frequency range of 30 Hz to 15,000 Hz.
1 Alexander, Robert Charles. The Inventor of Stereo: The Life and Works of Alan Dower Blumlein Focal Press 2000 ISBN 0240516281
4 page 805 Journal of the Audio Engineering Society. Volume 31 no 10. October, 1983.
5 Tebo, Julien. Arthur Keller: an Interview. March 13, 1973 IEEE History Center Interview #6
6 Fox, Barry A Hundred Years of Stereo, New Scientist December 1981
7 Fagen, M.D., ed. A History of Engineering and Science in the Bell System: The Early Years (1875-1925). New York: Bell Telephone Laboratories, 1975
8 Arthur Keller: BBC Interviews, broadcast on Radio 3, 1981
9 Bell Laboratories Patents
10 Schoenherr, Steven Recording Technology History July 6, 2005 http://history.sandiego.edu/GEN/recording/notes.html
11 page 160. Gerson, Robert A. Music in Philadelphia Theodore Presser Co. Philadelphia. 1940.
12 Thanks to Christine Rankovic, Ph. D. for this information on Rogers Harrison Galt.
13 Photograph from Harvey Fletcher "My work with Millikan on the oil-drop experiment" Physics Today. American Institute of Physics, College Park, Maryland, USA June 1982. Again, thanks to Christine Rankovic, Ph. D. for this material
14 pages 1686-1693. Davis, C.C. and Frayne, J.G. The Westrex Stereo Disk System. Westrex Corporation, Hollywood, CA Proceedings of the IRE October, 1958 Volume: 46, Issue: 10.
15 Burns, R. W. The Life and Times of A D Blumlein Institution of Engineering and Technology. Hertfordshire, UK. 2000. ISBN 0-8529677-3-X
16 Knudsen, Vern and King, W.J. King. Oral History Transcript - Dr. Harvey Fletcher American Institute of Physics. May 15, 1964. Niels Bohr Library ∓ Archives, American Institute of Physics, College Park, Maryland
17 David Goodstein In defense of Robert Andrews Millikan. The American Scientist: 54–60. January-February 2001.