Efforts were made to invent transmitters that would produce continuous waves – a sinusoidal alternating current at a single frequency.

In an 1891 lecture, Frederick Thomas Trouton pointed out that, if an electrical alternator were run at a great enough cycle speed (that is, if it turned fast enough and was built with a large enough number of magnetic poles on its armature) it would generate continuous waves at radio frequency.

[5] In 1904, Reginald Fessenden contracted with General Electric for an alternator that generated a frequency of 100,000 hertz[citation needed] for continuous wave radio.

The Alexanderson alternator was extensively used for long-wave radio communications by shore stations, but was too large and heavy to be installed on most ships.

British Marconi offered General Electric $5,000 in business in exchange for exclusive rights to use the alternator, but just as the deal was about to go through, the American president Woodrow Wilson requested that GE decline the offer, which would have given the British (who were the leader in submarine communications cables) dominance over worldwide radio communications.

[9] Thorn L. Mayes identified the production of ten pairs of 200 kW Alexanderson alternators, totaling twenty transmitters, in the period up to 1924:[10][11] Beginning in 1941, seven of the twenty original 200 kW alternators were put into service by the U.S. Navy and Air Force:[12] During World War II the U.S. Navy recognized the need for reliable distant longwave (VLF) transmissions to the Pacific fleet.

The Air Force found that longwave transmissions were more reliable than shortwave when sending weather information to Arctic researchers as well as bases in Greenland, Labrador, and Iceland.

The space between the teeth is filled with nonmagnetic material, to give the rotor a smooth surface to decrease aerodynamic drag.

The machine operates by variable reluctance (similar to an electric guitar pickup), changing the magnetic flux linking two coils.

One set of coils is energized with direct current and produces a magnetic field in the air gap in the stator, which passes axially (sideways) through the rotor.

Alexanderson alternators were used to produce radio waves in the very low frequency (VLF) range, for transcontinental wireless communication.

The manufacture of precision machines rotating at such high speeds presented many new problems, and Alexanderson transmitters were bulky and very expensive.

Transmitters installed in Europe, operating on 50 Hz power, had a wavelength range of 12,500 to 28,800 meters due to the lower speed of the driving motor.

These were electrically "noisy"; the energy of the transmitter was spread over a wide frequency range, so they interfered with other transmissions and operated inefficiently.

With a continuous-wave transmitter, all of the energy was concentrated within a narrow frequency band, so for a given output power they could communicate over longer distances.

Other major problems were that changing the operating frequency was a lengthy and complicated process, and unlike a spark transmitter, the carrier signal could not be switched on and off at will.