Van der Bijl was born on 23 November 1887 in
Pretoria to Pieter Gerhard van der Bijl and Hester Elizabeth Groenewald. He was the fifth of eight children. Pieter van der Bijl had been an
ox-wagon driver between
Cape Town and
Kimberley and moved to Pretoria in 1887 where he became a prosperous grain and produce merchant. Pieter van der Bijl was acquainted with high-profile politicians such as
Louis Botha and
Jan Smuts. After the ill-fated
Jameson Raid (1895) there was increased tension in the
Transvaal Republic, of which Pretoria was the capital, and the young Van der Bijl assisted his father in stacking rifles and ammunition collected by the
Boer forces in preparation for further British attacks. He was thirteen and attending the
Staatsch Model School in Pretoria when the
second Anglo-Boer War broke out. He witnessed the occupation of Pretoria by the British armed forces and he was forced to leave the school as it was converted into a concentration camp. In 1902 Pieter van der Bijl moved his family to
Gordon's Bay in Cape Town where Hendrik attended a farm school in
Sir Lowry's Pass and later Boy's High School in
Franschhoek. He matriculated in 1904 and went to Victoria College (now
Stellenbosch University) where he completed his BA with distinctions in Physics, Mathematics and Chemistry. He received the college prize for physics and the Van der Horst Prize for "most deserving student of mathematics and physical science at the College." In 1908 Van der Bijl went to
Germany where he studied physics. He spent one semester at
Halle and then moved to
Leipzig University where he was supervised by Wiener, des Coudres and Jaffé. He studied
ions produced by a strong
Radium source moving through a
dielectric liquid, the results of which verified the equation: :{dn \over dt}= q - \alpha n^2 and its description of the behaviour of such ions. He obtained his PhD in March 1912 and was appointed assistant in physics at the
Königlich Sächsische Technische Hochschule in
Dresden, where he worked under
Wilhelm Hallwachs. While there he met
Robert Andrews Millikan, who arranged for him to work the following year at
Western Electric in
New York at what would become part of the
Bell Telephone Company (in 1925). In April 1913 he published a paper on the research he had conducted in Dresden on the photoelectric effect entitled
"Zur Bestimmung der Erstenergien lichtelektrisch ausgelöster Elektronen" [The Determination of the Initial Energies of Photoelectrically Liberated Electrons]. During the seven years he spent in New York, he studied the performance of the first three-electrode
thermionic valve, known as the "
Audion", developed by
Lee de Forest. His research, assisted by H.D. Arnold, led to the installation of the first Audion as a
repeater on the New York to
San Francisco telephone line. In 1915 he co-developed the master oscillator circuit that was used with the Audion for wireless communication between New York and
Wilmington, Delaware; and between
Paris,
France and
Honolulu,
Hawaii. Van der Bijl published the design and theory of the devices he worked on in a book,
The Thermionic Vacuum Tube-Physics and Electronics in 1920. It became the standard textbook on the subject for more than 20 years. He is now perhaps best remembered for the van der Bijl equation which describes the relationship of the three 'constants' of a
vacuum tube, the
transconductance gm, the
gain μ and the plate resistance rp or ra. The van der Bijl equation defines their relationship as follows: :g_m = {\mu \over r_p} In 1919, Van der Bijl wrote a paper on
Scientific research and industrial development and related this to the development of secondary industries in South Africa. As a result of this paper, he was invited to return to South Africa by Jan Smuts and was offered the position of Scientific and Industrial Advisor in the Department of Mines and Industries. He took up Smuts' offer in 1920 and returned to South Africa. == Career in South Africa ==