Canadian-American radio pioneer who broadcast the first program of music and voice ever transmitted over long distances.

After study at Trinity College School, in Port Hope, Ont., and Bishop's College in Lennoxville, Que., Fessenden went to Bermuda as principal of the Whitney Institute, where he developed an interest in science that led him to resign and go to New York. Working as a tester at the Thomas Edison Machine Works, he met Thomas Edison and in 1887 became chief chemist of the Edison Laboratory at Orange, N.J. In 1890 he became chief electrician at the Westinghouse works at Pittsfield, Mass., and in 1892 turned to an academic career, as professor of electrical engineering first at Purdue University, West Lafayette, Ind., then at the Western University of Pennsylvania (now the University of Pittsburgh), where he worked on the problem of wireless communication.

In 1900 Fessenden left the university to conduct experiments in wireless telegraphy (telegraph) for the U.S. Weather Bureau, which wanted to adapt radiotelegraphy to weather forecasting. He then became interested in voice transmission and developed the idea of superimposing electric waves, vibrating at the frequencies of sound waves, upon a constant radio frequency, so as to modulate the amplitude of the radio wave into the shape of the sound wave. (This is the principle of amplitude modulation, or AM.) Fessenden also invented an electrolytic radio detector sensitive enough for use in radiotelephony.

In 1902 Fessenden joined two financiers in organizing the National Electric Signalling Company to manufacture his inventions. He directed Ernst Alexanderson of the General Electric Company in building a 50,000-hertz alternator that made possible the realization of radiotelephony, and Fessenden at once built a transmitting station at Brant Rock, Mass. On Dec. 24, 1906, wireless operators as far away as Norfolk, Va., were startled to hear speech and music from Brant Rock through their own receivers. That same year, Fessenden established two-way transatlantic wireless telegraphic communication between Brant Rock and Scotland.

Fessenden further contributed in 1902 to the development of radio by demonstrating the heterodyne principle of converting high-frequency wireless signals to a lower frequency that is more easily controlled and amplified. This was the forerunner of the superheterodyne (superheterodyne reception) principle, which made easy tuning of radio signals possible and was a critical factor for the growth of commercial broadcasting.

Fessenden has also been credited with inventing the radio compass, the sonic depth finder, submarine signalling devices, and the turboelectric drive for battleships.