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Ernst Alexanderson: The Machine That Made a Continuous Wave

By Infinite Creation · Profile

Historical photograph of apparatus associated with Ernst Alexanderson.

At a Glance

Ernst Frederik Werner Alexanderson (1878-1975) built the high-frequency alternator that made continuous-wave radio practical at power, at Fessenden's prompting. It was a rotating machine generating radio frequencies directly, and it carried transatlantic communication before valve transmitters matured. He held around 340 patents and later worked on early television at GE.

Lived: 1878-1975, Sweden, worked in the United States
Anchor year: 1906, the high-frequency alternator
Strand: canon

Career

The problem as it stood

Fessenden had established that voice needs a continuous carrier. Nobody could generate one at useful power. Spark transmitters produced damped bursts. Arc converters were unstable and noisy.

What he actually did

Alexanderson built an alternator, in principle an ordinary AC generator, engineered to spin fast enough, with enough poles, to output tens of kilohertz directly at high power. Machines of 200 kW were eventually built and installed at stations including Grimeton in Sweden, which still exists and is operated occasionally.

How it worked

Output frequency is rotor speed multiplied by pole count. Reaching radio frequencies means extreme rotational speed and very fine pole spacing, with the rotor under enormous mechanical stress. It is a brute-force mechanical answer to what became an electronic problem, and it worked reliably for decades.

What it made possible

Reliable long-wave transatlantic radio communication and the first serious voice transmissions. It also demonstrated the commercial value of continuous-wave working, which valve transmitters then delivered more cheaply.

What happened to him

He remained at General Electric for a very long career, contributing to power transmission, railway electrification and early television. He was involved in GE's television demonstrations of the late 1920s. He died in 1975 at 97.

Where the credit landed

Alexanderson is reasonably credited in radio engineering history and almost unknown outside it, partly because his machine was superseded within twenty years by valve transmitters. Being the last and best implementation of an approach that is about to be replaced is a reliable route to obscurity: the same shape as Baird's mechanical television.

Known For

Output frequency is rotor speed times pole count, hence the extreme engineering.
The Grimeton alternator survives and is still occasionally run.
Being the best version of a soon-obsolete approach is a route to obscurity.

Knowledge Base

What You'll Learn

Sometimes the mechanical answer arrives first and works for twenty years.

Why valves killed the alternator

An Alexanderson alternator is a building-sized machine with a rotor under severe stress, tied to one frequency band, requiring constant maintenance. A valve oscillator is a component: cheap, small, tunable across wide ranges, with no moving parts. Once de Forest's grid made valve oscillators practical at power, the mechanical approach had no future, but for two decades it was the only thing that worked.

Where This Fits

The alternator made a continuous wave for transmission, the middle link of A History of Broadcast Technology.

FAQ

Q: What is an Alexanderson alternator?
A: A high-speed rotating machine generating radio-frequency alternating current directly, used for long-wave transmission before valve transmitters became practical. Machines of up to 200 kW were built.

Q: Are any still working?
A: The alternator at Grimeton in Sweden survives as a UNESCO World Heritage Site and is operated on special occasions.

Vocabulary

Continuous wave. A steady, unbroken radio wave whose strength or frequency can be varied to carry information. Necessary for voice transmission, which a series of separate pulses cannot carry.

Spark gap. The earlier method of generating radio signals, producing a broad burst of energy across many frequencies. Suitable for telegraphy and unusable for voice, and wasteful of spectrum.

Alternator. A rotating machine generating alternating current. Producing one that turned fast enough to generate radio frequencies directly was a mechanical problem as much as an electrical one.

Radio frequency. The range of frequencies used for radio transmission, far above audible sound. A sound signal has to be imposed on a radio frequency wave to travel any distance.

Sources

The patent was checked against the record. Checked 20 August 2026.

US Patent 1,008,577, High-Frequency Alternator. Ernst F. W. Alexanderson as sole named inventor, assigned to General Electric. Filed 26 April 1909 and granted 14 November 1911. It is patented in Alexanderson's name and owned by his employer. The 1909 filing postdates the alternator Fessenden commissioned, so it documents the machine line rather than dating it.