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Hendrik van der Bijl

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Hendrik Johannes van der Bijl was born in Pretoria, South Africa, on November 23, 1887 to a prosperous farmer. He was the fifth child in a family which eventually grew to eight. Their South African origins dated back to 1668 when Gerrit van der Bijl arrived at the Cape from Holland to serve the Dutch East India Company. During van der Bijl’s childhood, Pretoria was the capital of President Kruger’s South African Republic, an area also known as the Transvaal.

The Anglo–Boer War started when van der Bijl was 12 years old. His family remained in Pretoria until the city was occupied by the British in mid 1900, when his moved to the Cape, where van der Bijl was able to complete his schooling while removed from the war. After graduating from Franschhoek High School he spent three years at Victoria College (now the University of Stellenbosch). Here he received the B.A. degree in physics, the third person to do so from the university. Van der Bijl also won a college prize for physics and the van der Horst Prize for the most deserving student of mathematics and physical science in the college.


Van der Bijl spent a semester at Halle University studying philosophy and inorganic chemistry. His he moved to Leipzig to work under Professors Wiener, des Coudres, and Jaffe, who had impressed him by their work. By March 1912 he had successfully completed a Doctorate on the behavior of ions produced by a strong radium source in selected liquid dielectrics. Wiener recommended van der Bijl for a Physics Assistant post at the Royal School of Technology, Dresden, generally reserved for German students and required the incumbent to remain in the position for at least two years. However, he managed to persuade the authorities to take him on for only one year (or longer if he so desired).

The Head of the Department of Physics at Dresden was Prof. Hallwachs, who had done much of the early research on the photoelectric effect. In 1900 the German physicist Planck had proposed the Quantum Theory to explain the mechanism of absorption and emission of electromagnetic waves by resonators of atomic or subatomic dimensions. In 1905 Einstein applied the Quantum Theory to the photoelectric effect and proposed a linear relationship between the wavelength of the light and the maximum velocity of electrons emitted from the irradiated metal. Quantum mechanics had not yet been widely accepted by the scientific community, and this relationship was seen as a means to test its validity. Several attempts to do this had already been made but all they had achieved was to convince the doubters that the Quantum Theory should be abandoned. Hallwachs brought this problem to van der Bijl’s attention and suggested that he look into it.


At that time the apparatus usually chosen to determine the maximum velocity of emission consisted of an irradiated photocathode and an anode with a metallic grid interposed between them (see Fig. 3). Both the anode and the grid had a central hole to allow the light to reach the photocathode . Electrons emitted by were drawn to the anode through the grid . The negative potential on the grid was then raised to the point where the anode current was cut off, and this was taken to be the maximum velocity (measured in volts) of electrons emitted by the photocathode. It was generally accepted that the plane containing the grid would provide a uniform equipotential surface which could be varied by changing the voltage.

To satisfy Einstein’s equation, the maximum electron velocity should be in the region of a few volts, but most workers had found grid voltages more than ten times greater. Van der Bijl suspected that the field due to the relatively high anode potential penetrated the grid and produced a “stray field” between the grid and the cathode. He designed a special version of the photoelectric tube (see Fig. 3) which allowed the distance between the grid and the anode to be preset to convenient values. The distance between the cathode and the grid could be changed while the tube was under vacuum to facilitate measurement of the contact potential between the grid and the cathode. With this apparatus he was able to find the combinations of grid and anode voltage which would just reduce the anode current to zero for various distances between the anode and the grid.

The relationship is clearly linear, and by extrapolating it to the vertical axis the inferred grid voltage corresponding to zero anode potential was the same for both spacings (about 4 V). Under these inferred conditions the influence of the anode-grid field on the grid-cathode field being removed, the true retarding potential depended only on the cathode-grid potential. In this way he eventually reached voltages which came close to the value satisfying Einstein’s photoelectric equation. He concluded that the field between the anode and the grid penetrated the grid and that its effect on the cathode-grid field was proportional to the anode voltage.


One of the leading specialists in electron theory was Millikan, a Professor of Physics at Chicago University. Millikan had also been trying to satisfy the Einstein photoelectric equation using an apparatus similar to that used by van der Bijl. His unsatisfactory results had turned him into “an avowed opponent of light quanta and was trying to prove Einstein wrong,”. Soon after van der Bijl had completed his experimental work, Millikan came to Germany to read a paper relating to his discovery of very high-emission velocities. Subsequently he paid a visit to Dresden and van der Bijl (possibly because he spoke English) was given the job of showing him around. During the course of the visit, they compared notes and found that his extraordinarily high velocities were due to the same cause, viz.: the stray field

Millikan thereafter changed from being an opponent of Einstein’s photoelectric theory to become the man who eventually proved Einstein’s equation to be correct. In 1916 Millikan published the results of his subsequent experimental work which fully justified Einstein’s photoelectric theory. This paper gives a more rigorous confirmation of Einstein’s theory than the work done by van der Bijl, and as the apparatus used did not require a grid he did not make use of van der Bijl’s stray field relationship. Millikan does not acknowledge van der Bijl’s paper and van der Bijl nowhere refers to Millikan’s subsequent work, but it seems clear that, at the very least, van der Bijl’s work must have moved Millikan to return to the problem and finally find the solution.

Millikan was at that time Technical Adviser to the American Telephone and Telegraph Company and was aware that their subsidiary, the Western Electric Company, was negotiating for the rights to use de Forest’s Audion as a telephone amplifier. He could see that van der Bijl’s investigation was not only relevant to the Quantum Theory but also to the thermionic vacuum tube.

On March 20, 1913 he wrote to Millikan asking for help in finding a suitable research position at one of the American universities. Millikan showed van der Bijl’s letter to Colpitts, a senior research engineer at Western Electric (best known for his oscillator circuit) with the suggestion that they should offer van der Bijl a position in their laboratories. Colpitts wrote to Dresden on May 28 suggesting that van der Bijl should visit them in New York to see whether he might like to join their industrial research team. By the time the letter reached Germany, van der Bijl had already left for Chicago. In July, Jewett, Assistant Chief Engineer of Western Electric, met van der Bijl in Chicago and offered him employment in their Research Department at a starting salary of $36 a week. Van der Bijl presumably spent some time vacationing before joining the Western Electric Laboratories in September 1913.


By 1912, Arnold of Western Electric had developed a successful telephone amplifier using a mercury-arc tube. De Forest offered Western Electric the Audion (for the second time) at the end of October 1912 and Arnold saw that, despite a lack of understanding of how it functioned, this device had possible advantages as a telephone amplifier (see Fig. 6). An agreement must have been struck with de Forest almost immediately because Arnold was asked to organize a study of the device to see what was needed to remove its shortcomings (de Forest was eventually paid $50 000 for the use of the Audion in telephony but not until July 1913). Arnold and his team started by removing the series capacitor in the grid circuit which was effective in a radio detector but which blocked the operation of an audio amplifier at higher input voltages. De Forest believed that the Audion needed vestiges of gas to operate efficiently, which was true when it was used as a detector. As an amplifier Arnold could see that this caused erratic behavior and limited the maximum anode voltage and hence the output power. By November they had already improved the vacuum sufficiently to operate the tube at 80 V. In April 1913 they received a Gaede Molecular Pump which was used to further improve the vacuum so that they could safely raise the anode to 200 V (de Forest’s tube was limited to about 20 V). Glass arbors were introduced to support the electrodes firmly. The filaments of de Forest’s Audions were heated to incandescence which limited their life to between 35 and 100 hours [6]. Arnold introduced Wehnelt oxide-coated filaments which could

produce adequate emission at much lower temperatures with less power and a longer life. Finally, larger anodes and grids were placed on both sides of the filament to increase the power handling of the tube. By the fall of 1913 these improvements were incorporated into prototype tubes which were successfully used in a tests between New York and Washington.

Van der Bijl joined the company at about this time and recognized that the thermionic triode was very similar to the photoelectric tube he had used in Dresden. He developed the stray field relationship which he had discovered in Germany and the factor (appearing in (3) of his photoelectric paper [1]), became the amplification factor . Together with the anode resistance and transconductance , these remained the basic amplifier parameters until the transistor took over after World War II. With his team he investigated the effects of electrode spacing and grid proportions on tube performance making it possible to design tubes for particular purposes (see Figs. 7, 8, and 10 and [7, pp. 227–236]). His Classic Paper, reprinted in this PROCEEDINGS, sets out his analysis of vacuum tube behavior in detail.

His first tube was the type M, or 101A (see Fig. 9) designed for use as a telephone line amplifier and the first Western Electric tube to be provided with a base mating with a mounting socket to facilitate replacement. Late in 1909, during talks with the management of the Panama-Pacific Exposition due to open in San Francisco, CA, in 1914, Vail and Carty, two senior executives of the company, had virtually promised that they would have a telephone working between New York and San Francisco in time for the opening of the exhibition. On their return to New York it is said that Vail told his engineers, “We’ve promised it; now you find a way to do it,” [8]. Three lines were constructed so that comparative tests could be made between the three types of amplifiers then available. The first, designed by Shreeve, was a mechanical amplifier using an electromagnetic receiver coupled directly to a carbon microphone which provided adequate gain but produced distortion which limited the number which could be cascaded. The second was Arnold’s mercury-vapordischarge tube, which was suitable for telephone line use but required skilled maintainance. The third line was fitted with amplifiers using van der Bijl’s type 101A tube. The vacuum tube emerged as a clear winner and van der Bijl always treasured the certificate confirming his membership in the Society of Planners and Builders of the First Transcontinental Telephone Line which was issued to each of the main participants [9]. By then, management was convinced that the vacuum tube held great promise, and van der Bijl was encouraged to develop and expand his knowledge of the triode. An immediate benefit came from a study of filament performance which revealed that the life could be improved considerably by increasing the electron-emitting area. The 101A tube had an inverted Vee filament drawing 1.45 A at 4 V. He replaced this with a double inverted Vee form which required 1.3 A at 5 V and increased the life from 400 h to 4500 h (see Fig. 9 and [6]).

In September 1914, just after the start of World War I, van der Bijl designed a robust tube with coaxial cylindrical electrodes suitable for radio work which could be produced economically for military purposes. Fig. 11 includes the drawing of this tube which was prepared for U.S. Patent Application 1 738 269 (Dec. 1918) and which also appears in [7, p. 244]. Presumably, Western Electric ignored this design because it did not measure up to the needs of the telephone industry and failed to see the potential of the emerging military market. In three separate statements spread over almost 30 years, van der Bijl claims that this design forms the basis of the historical French Telegraphe Militaire (TM) tube (see Fig. 11). Several millions of these were made in France and the U.K. (where it was known as the valve). The author has tried, unsucessfully thus far, to find independent confirmation of this claim. As this is an important step in the history of the vacuum tube it justifies further investigation and it is hoped that this exposure will stimulate other researchers to contribute to the debate. The author will be glad to send copies of his notes and references to anyone wishing to participate [10]. In 1915, after successfully linking the country coast-tocoast by telephone, Western Electric turned its attention to transoceanic communication by experimenting with radio telephony. Speech had already been transmitted by radio but no method suitable for commercial use had thus far emerged. A team of engineers was assembled at Western Electric to explore the use of the vacuum tube for this purpose. Van der Bijl’s main contribution was the grid modulation system which was applied at a low level and amplified by up to 500 power tubes connected in parallel (the special power tubes for the transmitter were not designed by van der Bijl). The equipment was installed at the Navy Signaling station at Arlington, VA, so that it could take advantage of their large and efficient long-wave antennas. It succeeded in reaching Honolulu (7800 km) and Paris (6000 km) in 1915. For a more detailed account of this initiative see [11].