Q. Explain Magnetic Read and Write Mechanisms?

Data are recorded on and then retrieved from disk via a conducting coil called the head. In numerous systems there are two heads, a read head and a write head. At the time of a read or write operation head is stationary while platter rotates beneath it.

Figure: Read /write Heads

Write mechanism is based on fact that electricity flowing via a coil generates a magnetic field. Pulses are sent to write head and magnetic patterns are recorded on surface below with different patterns for negative and positive currents. Write head itself is made of easily magnetisable material and is in shape of a rectangular doughnut with a gap along one side as well as a few turns of conducting wire along opposite side (Figure above). An electric current in wire induces a magnetic field across gap that in turn magnetizes a small area of recording medium. Reversing the direction of current reverses direction of magnetization on recording medium.  

Traditional read mechanism is based on fact that a magnetic field moving relative to a coil generates an electrical current in the coil. When surface of disk passes under head it produces a current of same polarity as one already recorded. Structure of the head for reading is in this case fundamentally same as for writing and so same head can be used for both. Such single heads are employed in floppy disk systems and in older rigid disk systems.