Interest in the subject of self-oscillation was also stimulated by the work of Heinrich Hertz, starting in 1887, in which he used a spark-gap transmitter to generate radio waves that he showed correspond to electrical oscillations with frequencies of hundreds of millions of cycles per second. Hertz's work led to the development of wireless telegraphy. The first detailed theoretical work on such electrical self-oscillation was carried out by Henri Poincaré in the early 20th century.