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MagneticRecording | ![]() |
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Returning to a common theme, physics is good for inventing and improving valuable devices that make all of our lives better and, if we do the inventing, make of us very very rich.
One of the most important industries built on magnetic technology is magnetic recording. Actually, magnetic recording is so large and diverse a category that we are really talking about several industries and many different technologies. This document will provide only the barest overview of a rich and rapidly expanding area. First, let's distinguish among some of the basic industries and technologies that fall under the Magnetic Recording umbrella.
![]() Data storage systems are always digital. However, they are in some ways less complicated. On a computer, most of the information (except sound files) remains in digital form. Thus, there is no need to convert from analog to digital and back. The difference between analog and digital can be understood in terms of the difference between telephone (analog) and telegraph (digital). When you speak into your phone, it converts the pitch and volume of your voice into an electrical signal of proportional frquency and amplitude. The other persons phone inverts the process. For telegraph, the operator converts your message into a series of discontinuous sounds‚the dits and dahs (dots and dashes) of a morse code message. These are transmitted, and an operator on the other end decodes the message.
A third important distinction is the form and content of the magnetic material on the medium. For instance most tapes of any kind have "particulate" media. That is, the magnetic material is in the form of small particles which are stuck to a plastic tape by some kind binder. The content of the particles has evolved over the years. Early cassette tapes used ferric oxide particles. Since then, chromium dioxide and pure iron particles have been used. Floppy disks are also coated with a particulate medium, and early hard disks were as well. However, the high storage density hard disks that are available today have a continuous coating of magnetic material. In fact, the surface of a hard disk is a complex set of layers each of which has a specific purpose. In a current hard disk, the "recording layer" which is composed of a cobalt alloy such as Co-Ni-Pt is less than 100 nanometers thick.
Underneath the distinctions, at the level where the magnetic signal is recorded and played back, all of these technologies rely on Ampere's Law for writing and (with a few recent exceptions) on Faraday's Law for reading. The basic device is the "read/write head." In its simplest form, the read/write head consists of a single ferromagnetic yoke with a small gap at one edge and two coils wound around it. When the write coils (right coils) are energized Ampere's law says a magnetic field is produced. The yoke is magnetized by this field
producing a very large "fringe" field at the gap. This field magnetizes a small region of the magnetic medium just below. The medium, which is magnetically hard, retains that
mangetization after the head field is removed.
When it comes time to read the recorded signal, the process works in reverse. Now, as the tape passes by the magnetic field of the bit partially magnetizes the yoke. As the magnetization of the yoke chages, Faraday's Law says that a small electrical signal is induced in the read coils. This signal is amplified and decoded (if data) or sent to an output device such as a loudspeaker.
Additional Sources of Information.
Please mail your responses to Dr. Gavrin.
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