Although tape recorders have been replaced in everyday use by Cd players and devices such as iPods they still provide a really useful way to understand the ideas of electromagnetic induction. Tape recorders store the information music, speech or data,
on plastic tape that is coated with iron oxide powder. You can tell that the tape is magnetic by
attracting it with a magnet. Warning – don't try to do this with a tape that already has some
music or data stored on it – you will ruin it!
Each grain of iron oxide acts like a tiny
magnet and on a tape that has no data stored on it these gains are arranged irregularly on
the tape – the tape is unmagnetised.
When you record data, lets say some music,
on the tape the following things happen:
(a) the microphone picks up the sound wave
and converts it to a small changing voltage
(b) the amplifier amplifies this voltage
(c)
the output from the amplifier is fed to the recording head where a changing magnetic field is
produced
(d) this changing magnetic field arranges the grains of iron oxide on the tape
into a pattern that "mirrors" the changing sound received by the
microphone.
The faster the
tape moves the better the recording because the information (lets say the music) is spread
out over a longer piece of tape. Slow tape speed compresses the information into a small
length and a poorer recording results. The recording head should be as close to the tape as
possible so that the changing magnetic field can affect the iron oxide grains more easily.
The reverse happens at
the playback stage. A changing magnetic field on the tape is converted to a voltage by the
playback head, this is amplified by the amplifier and then fed to a loudspeaker.
The
tape recorder also has an erase head. This is fed by a high frequency signal (60 kHz) which
is well about the range of human hearing. This signal jumbles up the tiny iron oxide grains
before a new recording takes place. This is done to make sure that none of the previous
recording is left on the tape.
