2170 RPM, for an output frequency near 21.7 kHz. To obtain higher frequencies, higher rotor speeds were required, up to 20,000 RPM. Along with the arc converter invented in 1903, the Alexanderson alternator was one of the first radio transmitters that generated continuous waves. In contrast, the earlier spark-gap transmitters generated a string of damped waves. 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. In addition, continuous waves could be modulated with an audio signal to carry sound. The Alexanderson alternator was one of the first transmitters to be used for AM transmission. The Alexanderson alternator produced "purer" continuous waves than the arc converter, whose nonsinusoidal output generated significant harmonics, so the alternator was preferred for long-distance telegraphy. Disadvantages Because of the extremely high rotational speed compared to a conventional alternator, the Alexanderson alternator required continuous maintenance by skilled personnel. Efficient lubrication and oil or water cooling was essential for reliability which was difficult to achieve with the lubricants available at the time. In fact, early editions of the Royal Navy's "Admiralty Handbook of Wireless Telegraphy" cover this in considerable detail, mostly as an explanation as to why the navy did not use that particular technology. However, the US Navy did. 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. The latter problem greatly complicated "listening through" (that is, stopping the transmission to listen for any answer). There was also the risk that it would allow enemy vessels to detect the presence of the ship. Because of the limits of the number of poles and rotational speed of a machine, the Alexanderson alternator is capable of generating transmission frequencies up to around 600kHz in the lower Medium wave band, with shortwave and higher frequencies being physically impossible. See also Alexanderson Day Tonewheel Resolver (electrical) Notes References Further reading Antique Wireless Association - column edited by Frank Lotito David E. Fisher and Marshall J. Fisher, Tube, the Invention of Television Counterpoint, Washington D.C. USA, (1996) Hammond, John Winthrop. Men and Volts, the Story of