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Magnetostriction | Ultrasonic

Magnetostriction Effect

When a ferromagnetic material in the form of rod is subjected to an alternating magnetic field parallel to its length shown in figure, the rod undergoes alternate constructions and expansion at a frequency equal to the frequency of the applied magnetic field. This phenomenon is known as magnetostriction effect.

Due to resonance the rod is thrown into longitudinal vibrations, thereby producing ultrasonic waves in the surrounding medium. Such ferromagnetic materials which are used for the production of ultrasonic waves are called  ' Magnetostriction materials '.

Construction

The circuit diagram is shown in figure.

Circuit Diagram


The ferromagnetic rod AB is clamped at the middle X. The coils L1 and L2 are wound at the ends of the rod. To the coil L1 a variable capacitor C1 is connected in parallel and this combination forms the resonator or tank circuit. One side of the resonator circuit is connected to the collector of transistor through a milliammeter. The other side of the resonator circuit is connected to the emitter through the battery. The coil L2 is connected between the base and emitter and is used as a feedback loop.

Working

When the battery is switched on, the resonator circuit L1C1 in the transistor sets up an alternating current or frequency,


As a result, the rod gets magnetised by the collector current. Amy change in the collector current brings about a change in the magnetisation, and consequently a change in the length of the rod. This gives rise to a change in the flux in coil L2 in the base circuit, thereby including an emf into coil L2. This varying emf is applied to the base of the transistor and it fed back to the coil L1, thereby maintaining the oscillations.
By varying capacitor C1, the frequency of oscillation of the tank circuit gets varied. If the frequency of tank circuit matches with the natural frequency of the material, then due to resonance the rod vibrates vigorously producing ultrasonic waves at the ends of the rod. The milliammeter reading gives maximum value at the resonance condition. The frequency of ultrasonic waves produced by this method depends upon the length l, density ρ and elastic constant E of the rod.


Thus, by varying l and E of the rod, ultrasonic waves can be generated at any desired frequency. Hence, at resonance condition,

Frequency of the oscillatory circuit= frequency of the vibrating rod


Merits

  1. The design of oscillator is very simple and production cost is low.
  2. At the low ultrasonic frequency, large power output is possible without the risk of damage to the oscillatory circuit.
  3. Frequency ranging from 20kHz to 300kHz can be produced.

Demerits

  1. It cannot generate ultrasonic waves of frequency above 300kHz.
  2. The frequency of oscillations depends on greatly on temperature.
  3. There will be loses of energy due to hysteresis and eddy current.




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