Goldschmidt alternator

Like the similar Alexanderson alternator, it was used briefly around World War I in a few high power longwave radio stations to transmit transoceanic radiotelegraphy traffic, until the 1920s when it was made obsolete by vacuum tube transmitters.

Only alternator transmitters like the Goldschmidt and Alexanderson could produce the high powers (50 to 200 kW) necessary to communicate reliably at transoceanic distances.

The stations themselves resembled a utility powerhouse, with large electric motors turning the humming alternators, which were connected through huge loading coils to enormous wire antenna systems stretching for miles, suspended on steel towers.

Efforts were made to design a transmitter that would generate sinusoidal continuous waves, because they could be received at a longer range, and also could be modulated to transmit audio (sound) in addition to Morse code.

[4] In 1906 Reginald Fessenden and Ernst Alexanderson at General Electric began to solve the problems and build alternators which could produce frequencies in the radio range, above 20 kHz.

[4] It was 1916 before Alexanderson machines achieved the high power needed for transatlantic communication, and they were extremely complex and expensive.

In 1908 Westinghouse engineer Rudolph Goldschmidt devised an intricate method to enable an alternator to generate high frequency without requiring excessive speeds.

[1] His technique was to exploit resonance and the nonlinear saturation characteristic of the iron rotor to use the alternator as a frequency multiplier as well as a generator.

The machine was developed and manufactured by the German company Hochfrequenz-Maschinen Aktiengesellschaft für Drahtlose Telegraphie ("Homag") and was mostly used in Europe.

World War I had brought home to nations the strategic importance of radio communication, as without it they could easily be isolated by enemies cutting their submarine telegraph cables.

The invention of the triode vacuum tube in 1906 by Lee De Forest, and the feedback oscillator circuit in 1912 by Edwin Armstrong and Alexander Meissner, made possible smaller and cheaper vacuum tube transmitters which by the end of World War 1 could produce as much radio power as the alternators.

By 1921 the Marconi Co. had installed 100 kW vacuum tube transmitters for transatlantic message traffic at its stations at Carnarvon, Wales and Glace Bay, Newfoundland.

Due to their huge capital costs, legacy alternator transmitters remained in use through the 1930s and were used in World War 2 to communicate with submarines.

100 kW Goldschmidt alternator at Eilvese, Germany. The 250 HP DC electric motor (right) , turned the 3 ft. diameter, 5 ton rotor (center) , at 4000 RPM. The rotor had 360 poles, and the fundamental frequency of the alternator was 24 kHz. Complicated "reflector" circuits (capacitor banks against walls) forced the machine to produce alternating current at four times this frequency, 96 kHz. The transmitter was used for transatlantic radiotelegraphy traffic, exchanging Morse code messages with a similar Goldschmidt station at Tuckerton, New Jersey, USA. During World War I it was Germany's main communication channel to the outside world, and was used for diplomatic negotiations between Woodrow Wilson and Kaiser Wilhelm II leading to the Armistice .
A more powerful 200 kW Goldschmidt machine replaced the one above at Eilvese station around 1920. The rotor had 400 poles and produced a fundamental frequency of 12.5 kHz, which was multiplied by 4 to give an output frequency of 50 kHz.
12.5 kW Goldschmidt alternator installed in 1910 at a wireless station in Eberswald, Germany. It had an output power of 12.5 kW at a frequency of 30 kHz, or 8 to 10 kW at 60 kHz. It consists of a DC electric motor (left) driving the alternator (right) through a gearbox (center) which steps up the rotation speed.
Rotor of Eilvese machine