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Hendrik van der Bijl was born in Pretoria in 1887 and studied at Victoria College (now Stellenbosch University) before travelling to Germany to complete his doctorate in electrical engineering (1910).
While investigating the speed at which electrons travel when ultraviolet light hits a metallic surface, he placed a tiny metal grid in the path of the particles. In so doing, he had unwittingly reinvented the thermionic valve that had first been developed by the American Lee de Forest in 1904.
Van der Bijl’s research came to the attention of the American physicist Robert Millikan, who invited him to join his staff at the University of Chicago. The American Telephone and Telegraph Company had just acquired the rights to De Forest’s Audion, an early thermionic valve.
Van der Bijl was tasked with improving it so that it could produce “continuous waves at even intensity” for long distance radio broadcasts.
Van der Bijl said of De Forest’s invention: “Here was merely a modified form of the equipment I had designed in Dresden.” Using his considerable mathematical ability, he quickly developed the Van der Bijl Equation, which described exactly how it worked.
He subsequently fine-tuned the thermionic valve for wireless radio, including a transmission over 4 800km from New York to the Eiffel Tower and then over 8 000km from New York to Honolulu. His improved thermionic valve became the standard design for the next 20 years.
Van der Bijl also developed an early form of cathode ray oscillograph, the essential element in a television set and, in 1915, applied the thermionic vacuum tube to a photoelectric selenium cell, which allowed photographs to be sent by radio. He also developed a simplified thermionic valve, the French valve, which was widely used in World War 1.
- What A Great Idea! is published by Jacana Media and can be purchased at major bookstores at a recommended selling price of R295
