Have you ever heard of the Alexanderson alternator?

Electrical Tester - 17 March 2020

Author - Keith Wilson

Though not nearly as well-known as Edison and Marconi, Ernst Alexanderson is nevertheless one of the most important inventors of the last century. He is credited with 344 patented inventions, ranging from variable speed AC motors to a whole host of devices used for radio (or should that be wireless?) communication. 

In 1906, Alexanderson collaborated with Canadian inventor Reginald Aubrey Fessenden in an attempt to devise a method for broadcasting the human voice by radio. At the time, only telegraphy signals – essentially the dots and dashes of Morse code – could be broadcast. Fessenden knew that for voice broadcasting he would need a continuous wave transmitter that would generate a sine wave on a single frequency, which could then be modulated by the voice signal. 


But this was long before the transistor or even the vacuum tube (valve) had been invented, so how could that continuous wave be produced? Well power alternators produce a continuous wave output, usually at 50 or 60 Hz, so why not use an alternator that would produce a radio frequency output? Surely all that’s needed is to spin the alternator faster! It’s not quite that easy, unfortunately, as the frequencies needed are of the order of tens, if not hundreds of kilohertz and any ordinary alternator driven fast enough to even approach these frequencies would simply tear itself apart. 

For help with devising a solution, Fessenden got in touch with General Electric, Alexanderson’s employer at the time, and Alexanderson immediately got to work on the project. He devised a novel alternator that used a disk of magnetic material as its rotor. The disk has slots cut in it to create teeth that function as poles. It is rotated at high speed in a special stator assembly that has pairs of poles, one pole in each pair being energized with DC, the other producing the alternator output. With this arrangement, a typical Alexanderson alternator is capable of generating an output at 100 kHz when rotating at 20 000 rpm. 



An Alexanderson alternator was installed in the Fessenden station at Brant Rock in Massachusetts and, on 24th December 1906, this 2 kW, 100 kHz machine made possible the transmission of the first radio broadcast in history. This first experimental broadcast consisted of Christmas music and Fessenden playing the violin. It was heard by wireless operators on ships and in shore stations down the East Coast as far as the West Indies. The operators are reported to have reacted with disbelief and astonishment – and who could blame them when all that they had ever heard before in their headphones was the rasping sound of Morse code? 

Alexanderson alternators were in use for many years, long into the valve and even the transistor era. The last of the alternators, in Marion, MA, USA, was shut down in 1959. But wait! Was it really the last of its kind? Actually, it wasn’t because there’s still a working Alexanderson alternator in Grimeton, Sweden. Every year, on Alexanderson Day, which is the first Sunday in July, and also on Christmas Eve, this transmitter is fired up and used to transmit short Morse code messages on a frequency of 17.2 kHz. Enthusiasts all over the world listen out for these historic transmissions. 

And Alexanderson certainly wasn’t a one-hit wonder. His other contributions to technology include the magnetic amplifier, the multiple tuned antenna, the antistatic receiving antenna and the directional transmitting antenna. He also devised radio altimeters and his studies in the polarization of radio waves made possible effective radio direction finders. He was instrumental in the development of television and, in 1927, he demonstrated ‘television’ in his own home, with a public exhibition in 1930. His system displayed the picture on a seven-foot screen and was a marvel for the viewers at the time! 



Alexanderson retired from his full-time position with General Electric in 1948 but continued to act as a consultant. From 1952 he worked at Radio Corporation of America (RCA) and was awarded his 321st patent in 1955 for the color television receiver he developed for RCA. 

Alexanderson never really retired completely, and he lived to the ripe old age of 97. He received numerous honours in the USA, as well as the prestigious Swedish Order of the North Star from his native Sweden. His tenacity, wits and creativity paved the way for many of our modern conveniences, and he will forever remain one of the most important of the forefathers of radio and television.