1898 V. Poulsen (Denmark) invented wire recorder; Information storage technology by control of magnetic state.
1900 The magnetic recorder was exhibited at the Paris EXPO and was praised as “the most interesting invention of recent years”.
Invention of vacuum tube amplifier by L. De Forest (USA) in 1921, together with development of the ring-type magnetic head and the fine magnetic powder applied tape bring about practical magnetic recorder.
Recording process K. Sato ed., Applied Materials Science (Ohm publishing) Fig. 5.18 Recording current time moving direction of recording media Recorded wavelength
Signal current is applied to a coil in the magnetic head which is placed close to the recording medium to generate the magnetic flux, the intensity and direction of which is proportional to the signal.
The medium is magnetized by the magnetic flux from the head, leading to formation of magnetic domain corresponding to the intensity and polarity of the signal.
Recorded wavelength （ the length of recorded domain corresponding to one period of the signal) is calculated by = v/f where v is the relative velocity between head and medium, and f the signal frequency)
Read out of recorded signal （ 1 ） Inductive head
Electromagnetic induction Electric voltage proportional to the derivative of the magnetic flux is generated
Output has the differential form of the recorded signal
The readout voltage is proportional to the product of the recorded wavelength and relative velocity between the head and the medium.
K. Sato ed., Applied Materials Science (Ohm publishing) Fig. 5.19, 5.20 Running direction Spacing loss Principle of read-out induction
Read out of recorded signal （ 2 ） MR (magneto-resistance) head
Change of the electric resistance of the head by the magnetic flux from the medium is utilized.
AMR (anisotropic magneto-resistance) was utilized in the early stage and was replaced to GMR (giant magneto-resistance).