Electromagnetic waves

Question

With neat diagram, explain the magnetostriction method (Oscillator) for production of

ultrasonics.

Answer 1

Production of Ultrasonic waves

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.

Construction:

The circuit diagram of magnetostriction ultrasonic generator is as shown in the figure1.3.2. A short permanently magnetized nickel rod is clamped in the middle between two knife edges. A coil L₁ is wound on the right hand portion of the rod. C is a variable capacitor. L₁ and C₁ form the resonant circuit of the collector-tuned oscillator. Coil L₂ wound on the LHS of the rod is connected in the base circuit. The coil L₂ is used as a feed back loop.

Working:

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

\(F = \frac{1}{2{\pi}{\sqrt{L_1C_1}}}\)

This current lowing round the coil L₁ produces an alternating magnetic fields 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.

Condition for Resonance:

Frequency of the oscillator circuit = Frequency of the vibrating rod

\(F = \frac{1}{2{\pi}{\sqrt{L_1C_1}}}\) = \(\frac{1}{2l}\sqrt{\frac{E}{\rho}}\)

Where ‘l’ is the length o the rod

‘E’ is the Young’s modulus o the rod

‘ρ’ is the density of the material of the rod.

The resonance condition is indicated by the rise in the collector current shown in the milliammeter.

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.