Magnetostriction oscillator

Principle :

When an alternating magnetic field is applied to a rod of ferromagnetic material such as nickel, iron , cobalt , then the rod is thrown into longitudinal vibrations producing ultrasonic waves at resonance.

Construction :

The magnetostriction generator consists of a ferromagnetic rod clamped at the centre. The two ends of the rod are wound by coils L1 and L2. The coil L1 is connected to the collector output of the transistor and the coil L2 is connected to the base of the transistor as shown. The frequency of the oscillatory circuit can be adjusted by the capacitor C1 connected across the coil L1. A battery is connected in the circuit that acts as a source.

Working :

The battery is switched on and oscillating current is produced by the collector of the transistor . This current is passed through the coil L1 which magnetises the ferromagnetic rod. The magnetization changes due to the oscillating current from collector and the rod begins to vibrate . This is due to magnetostriction effect produced on the ferromagnetic rod.


The magnetization of rod causes an induced e.mf in the neighbouring coil L2 due to mutual induction and the induced current is given to the base of transistor as positive feedback. Hence , the oscillations are sustained without being damped. Now the rod is capable of vibrating with its natural frequency .

Resonance condition :

When the frequency of oscillatory circuit is adjusted with the help of capacitor to match the natural frequency of the rod , then resonant condition is said to have been achieved and ultrasonics waves are produced from the two ends of the rod.

Merits :

1. The oscillatory circuit is easy to construct.
2. The experiment is inexpensive
3. It can produce frequency upto 3 MHZ.

Demerits :
1. It cannot produce frequency above 3 MHz.
2. It cannot produce a stable frequency at the output
3. As frequency is inversely proportional to length of rod, frequency cannot be increased as length of rod cannot be reduced.