DENSITY AND COERCIVITY

Table 10-5.—The 5.25-Inch Disk Densities and Coercivities Figure 10-8.—Construction of a floppy disk drive read/write head. HEAD CONSTRUCTION. —Heads are made of a soft ferrous material with electromagnetic coils for reading and writing. The read head picks up changes in magnetic flux as the disk moves past the head surface. An electric current fed through the write head creates a magnetic field around it. If the force of the magnetic field is strong enough, the area on the disk is also magnetized. By controlling the direction of current flow through the head, we can also control the direction of the magnetic field. The write (record) head is centered between two erase heads. Figure 10-8 illustrates the construction of a floppy disk drive read/write head. ERASE HEAD OPERATION. —As data is written on the disk, the erase heads clip each edge of the track, ensuring that data from one track does not “spill over” to the next track. This form of recording is known as tunnel erasure. DENSITY AND COERCIVITY Density is the measure of how much data can be stored on a disk. The higher the density of the disk, the more data can be reliably stored on the disk. Disk density is measured in two ways: longitudinal density and linear density. Longitudinal Density —Longitudinal density is defined by how many tracks per inch can be reliably written on a disk. Longitudinal density is generally expressed in tracks per inch (tpi). Linear Density —Linear density is how many bits per inch (bpi) can be stored on a disk track. Coercivity is the magnetic field strength required to properly record data. It is measured in oersteds. Coercivity is affectedly the magnetic material used and the thickness of the material. The 5.25-Inch Disk Densities and Coercivities The 5.25-inch disks are rated by their density capabilities and whether data can be stored on one or both sides. A 360K disk is rated as DSDD, or double-sided, double-density disk. The rating “double- density” on these disks goes back to the very early days of floppy disk development. Single-density disks are no longer manufactured and the DSDD disk is often called a low-density disk. Table 10-5 show the common 5.25-inch disks in use today with their densities and coercivities. 10-10