Magnetic tape

Magnetic tape (as distinguished from paper tape, another early popular secondary storage medium) has been a popular bulk secondary storage medium for computers from the earliest days.

Once disks became cheap, it was relegated to use for backups, but it remained popular in that use until recently. It has always had a very low cost per bit, since a reel of tape has much more surface area (and thus space for data) than any other physical form.

It consists of a magnetic medium applied to flexible medium, usually plastic tape (although the UNIVAC I used thin metal tape), and wound on a reel. Mounted on a tape drive, the tape is moved across a head onto a takeup reel, and data can be written to, and read from, the tape as it moves over the head.

For many years the standard width was 1/2"; lengths were typically 1,200 feet and 2,400 feet, on a maxium reel size of 10-1/2". Originally data was stored in 7 parallel tracks (6 data, and 1 parity), but this was soon replaced with 9 tracks (8 data, and 1 parity). 7-track recording densities started at 200 bit per inch, increased to 556, and ended at 800. 9-track started at 800, and increaed to 1600 and 6250.

More recently, open reels of tape started to be supplanted by various types of cartridges; over times, cartridges took over, and open reel tape is now more or less extinct.