Danish telephone engineer and inventor, best known for his Telegraphone, which he patented in 1898. It was the first practical apparatus for magnetic sound recording and reproduction. It recorded, on a wire, the varying magnetic fields produced by a sound. The magnetized wire could then be used to play back the sound.

The Telegraphone received considerable attention when it was exhibited at the Exposition Universelle in Paris in 1900. The few words that the Austrian emperor Francis Joseph spoke into it at that exhibition are believed to be the earliest surviving magnetic recording.

Even with this encouragement, he could not find financial backers in Europe. In 1903, with American associates, he founded the American Telegraphone Company for the manufacture and sale of an improved version of his device. The telegraphone recorded continuously for 30 minutes on a length of steel piano-wire moving at a speed of 84 inches (213 cm) per second. The device did not have wide application, however.

Also in 1903 Poulsen obtained an English patent on his adaptation of a "singing arc" for radio purposes. Invented by the Englishman William Duddell, the singing arc could generate continuous audio waves (hence its name). Poulsen transformed this device so that it could generate radio waves; the arc was formed between a copper cathode (positive terminal) and a carbon anode (negative terminal) in an atmosphere of a hydrocarbon gas and a transverse magnetic field. Subsequent efforts with this device by Poulsen and others made long-wave radio broadcasting possible by 1920.

Magnetic recording dates to 1898, when Danish engineer Valdemar Poulsen developed a method for recording sound on a steel wire. He stretched this wire across his laboratory and put the recording apparatus on a trolley that traveled along that wire. He would run along with the moving trolley, talking into its microphone to record sound on the wire. To play back this sound, he would roll a second trolley containing the playback equipment along the wire and it would reproduce the sound.

Having proven the principle of magnetic recording, Poulsen and others began to develop wire recorders. In these devices, a wire rolling from one drum to another was used to record and play back sound. In 1927, American inventor J. A. O'Neill replaced the wire with a magnetically coated ribbon and since then magnetic tape recorders have dominated the recording industry.