The magnetic recording was demonstrated in principle as early as 1898 by Valdemar Poulsen in his Telegraphone. Magnetic wire recording, and its successor, magnetic tape recording, involve the use of a magnetizable medium which moves past a recording head. An electrical signal, which is analogous to the sound that is to be recorded, is fed to the recording head, inducing a pattern of magnetization similar to the signal. A playback head (which may be the same as the recording head) can then pick up the changes in the magnetic field from the tape and convert them into an electrical signal.
Poulsen obtained a Telegraphone Patent in 1898, and with his assistant, Peder O. Pedersen, later developed other magnetic recorders that recorded on steel wire, tape, or disks. None of these devices had electronic amplification, but the recorded signal was easily strong enough to be heard through a headset or even transmitted on telephone wires. On the 1900 World Exposition in Paris, Poulsen had the chance to record the voice of Emperor Franz Josef of Austria which happens to be the oldest surviving magnetic audio recording today.
Poulsen developed an arc converter in 1908, referred to as the "Poulsen Arc Transmitter," which was widely used in radio before the advent of vacuum tube technology. A stamp was issued in honor of Poulsen in 1969.