Valdemar Poulsen

Poulsen invented the telegraphone in 1898, which was the first practical magnetic sound recording device. It used a magnetized steel wire as the recording medium, a principle that directly anticipated later magnetic tape recording technology.

Poulsen developed an arc transmitter for continuous-wave radio transmission that was a major improvement over earlier spark-gap transmitters. The Poulsen arc was used for long-distance radio communication in the early 20th century before being superseded by vacuum tube transmitters.

The telegraphone's principle of magnetic recording on steel wire directly preceded magnetic tape recording, which German engineers developed further in the 1930s and 1940s using paper and plastic tape coated with magnetic material. Magnetic recording ultimately evolved into the audio cassettes, videotapes, and hard disk drives that became ubiquitous in the 20th century.

Poulsen was a Danish engineer who lived from 1869 to 1942. He worked in Copenhagen, where he conducted his pioneering experiments in magnetic recording and radio transmission at the turn of the 20th century.