We've seen this question several times now so we'll ask our computer scientist Pierre Kerr, ac210@freenet.carleton.ca, one more time.

Computer disks are very similar to video and audio tapes, the kinds you may use at home. They are made with a brownish material that is actually a very fine powder of iron. This material is able to hold a magnetic charge, that is, if you magnetize a tiny part of it, it will stay magnetized for many years. The computer disk drive is able to use this tiny bit of magnetism to remember all kinds of things.

Like all things in a computer's memory, it only knows of on or off. This is called binary information since there are only two numbers to remember. This is also called digital since the information is precisely on or off, never sort of on or off. The ON is represented by the number 1 and the OFF is a zero.

There are a few different kinds of computer discs (or disks). I've given you an example of one kind of disk, a floppy disk.

This one is an old 8 inch floppy disk. It's called floppy because it can bend a little bit. You shouldn't bend it too much but this one is not used anymore so you can do what you like with it. I have cut the end of the case off so you can slip out the shiny brown disk that's inside. That disk turns around very quickly when it's in the computer and an arm moves a disk head back and forth to read different parts of the disk. This head actually rubs against the disk in a floppy disk, but in a hard disk it flies just above the surface.

There are other sizes of floppy disks as well. The most common type these days is a 3 1/2" disk which is in a hard plastic shell. This is still called a floppy disk though, because inside is the same kind of floppy brown material. A 3 1/2" floppy disk can hold over a million characters. That's a lot of words! And they only cost about fifty cents.

There are also hard disks. These are normally inside a computer so you can't see them. They still use the same brown material to store the magnetic information but it is on a hard surface such as aluminum. When it is turned on it will spin very quickly. When this is in a computer, you wouldn't be able to see the disk part at all.

Show the floppy disks I've given you to the other students in your class and perhaps you would like to set them up on a table so everyone can see them. They're yours to keep. They don't work anymore since they're quite old.

Take a look in your library in the science corner. There is now a complete computer on display including a hard disk with the top off. If you look closely you will see the disk heads. That hard disk has four of them, two for each disk platter.

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