How Is Data Stored on Magnetic Storage Devices?

Analog Tape

Magnetic recorders have been around in one form or another since the end of the 19th century and were used to make audio recordings long before any of their other uses. The first form to come into widespread use was the analog tape. In an analog tape, a strip of plastic coated with a thin magnet coating is wound between two reels. To make a recording, the motor in the tape recorder unwinds the tape past an electromagnet, called the write head, at a steady rate.

The Write Head

A current in the shape of the analog signal pulses through the write head. The current continuously goes from positive to negative as the sound wave being recorded goes up or down. This current creates a moving magnetic field in the write head, which induces a magnetic field in the nearby read head. The magnetic field stays on the tape, creating a recording of the sound.

Digital Recording

Digital tapes do the same thing as analog tapes except that, instead of storing a continuous signal, they store digital information. The current in the write head does not go from negative to positive in a continuous wave, but instead jumps quickly between two different values to represent the binary digits 1 and 0. Digital tapes are used as backup, but they are a bit too slow to use for normal day-to-day work, since the machine has to wind and unwind the tape to find every piece of information.

Hard Drives

Hard drives are much quicker than tapes and are currently the most important magnetic storage devices. They read and write in the same way tapes do, but they have a different physical structure. A hard drive has several different disk-shaped "platters" that are covered with grooves like records. Each has a write head that hovers above it as it spins very quickly. A control motor can move the head to any spot on the disk, where it retrieves the desired information from the disk as it spins.