Production of Ultrasonic waves Magnetostriction Method

Principle

When a rod of ferromagnetic material like nickel is magnetized. Longitudinally, it undergoes a very small change in length. This is called Magnetostriction effect.

Between two knife edges, a small nickel rod that is permanently magnetised is clamped. On the right-hand side of the rod, a coil L1 is wrapped. A variable capacitor is C. The collector-tuned oscillator's resonant circuit is made up of L1 and C1. The base circuit is connected to coil L2, which is wrapped on the LHS of the rod. A feed-back loop is created using coil L2.

Working

When the battery is switched on, the resonant circuit L1C1?sets up an alternating current o frequency.

This current lowing round the coil L1 produces an alternating magnetic iels of frequency f along the length of the nickel rod. The rod starts vibrating due to magnetostrictive effect. The vibrations of the rod create ultrasonic waves.

The longitudinal expansion and contraction of the rod produces an E.M. in the coil L2. This e.m.f is applied to the base of the transistor. Hence the amplitude of high frequency of high oscillations in coil L1 is increased due to positive feedback.

The developed alternating current frequency can be turned with the natural frequency of the rod by adjusting the capacitor.

Frequency of the oscillator circuit = Frequency of the vibrating rod

Advantages

?Magnetostriction Oscillators are mechanically rugged.

The construction cost is low.

They are capable of producing large acoustical power with fairly good efficiency.

Limitations

?It can produce frequencies up to 3MHz only.

They frequency o oscillation depends upon the temperature.

Breadth o the resonance curve is large. It is due to vibrations of elastic constants of ferromagnetic material with the degree of magnetization. So we cannot get a constant single frequency.

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