Rectification

After two years, a full university post still eluded him and he settled for teaching at a high school in Leipzig; father had been right. A university was still his aim, but meanwhile teaching gave some time for scientific research.

Braun had already experimented with electrical conduction through elec-trolytes and salts and he now studied earlier work on conduction through mineral crystals. He solved the vexing problem of making contacts to the crystals by an example of what was to become his elegant experimental style. He simply bent two silver wires. One became a ring which supported the crystal; the other became a spring whose point pressed onto the crystal. It was a near-perfect set-up for the discovery of the point-contact crystal rectifier. Just when he discovered rectification is not known, but he announced it on the 23rd November, 1874; "the resistance varies according to current direction, intensity, and duration," he wrote. At first the discovery did not make a big impact in scientific circles, though Arthur Schuster repeated the results at Cambridge with clean and oxidized copper wires.

Two years later, Braun expressed his work as departures from Ohm's Law. He recognized that the effect took place at the surface of the crystal, that a point contact was needed, and that it happened very rapidly (in less than l/500th second). It was 30 years before an important application was found, as a crystal detector for radio receivers.

The word rectification now has implications which go beyond the original discovery. What Braun discovered was that there are experimental set ups in which a direct flow of electricity is conducted better in one direction than the other. When he made his fourth and last publication on the rectifier effect in 1883 he was able to refute every accusation of experimental error and to extend the observations to alternating current. His last observation showed that the effect held true even for very short pulses of current.

The latter experiments took place at Strasbourg University, which he joined as an associate professor in the spring of 1880, having held a similar position for three years at Marburg after leaving his teaching job in Leipzig. For a number of years he played musical academic chairs as he strode along the path of success from university to university. The next move was to a full professorship at the Technical University of Karlsruhe (1883), then to Tubingen (1885), and back to Strasbourg (1895).

It was at the time of the move to Tubingen that he got married. He and his wife, Amalie, set up house in the castle that was the physics institute. Unlike the domestic quarters the laboratory was in the unheated castle tower and subject to the vagaries of the weather; in winter the temperature hovered for weeks at -1°C. Braun's replacement at Karlsruhe was fortunate in inheriting a spacious lab in which, three years later, he performed the most famous experiments ever carried out there; his name - Heinrich Hertz. Braun meanwhile set about getting a new lab.