Question

Explain the principle, construction and working of a Magnetostriction Oscillator.

Answer 1

This is based on the principle of magnetostriction effect.

⦁ A ferromagnetic rod MN is clamped in the middle. Two coils L₁ and L₂ are wound round its two halves.

⦁ The coil L₁  is connected in the plate – cathode circuit and the coil L₂ is connected in the grid cathode circuit of a triode valve oscillator.
⦁ The capacitor C has a variable capacitance .
⦁ As the circuit is switched on the plate current I_p starts flowing through coil L₁ producing  magnetic field B along the axis of the rod as shown:-

⦁ Now the rod is placed in a magnetic field along its length as a result of which its overall length will change by a small amount.

⦁ As the length changes the flux linked with L₁ and L₂ also changes giving rise to an induced emf , e= -dφ/dt across L₂ .

⦁ The induced emf is fed to the grid-cathode circuit and the oscillator valve gives an amplified alternating voltages as output to the plate-cathode circuit causing an alternating plate current.

⦁ Due to the alternating current flowing through coil L₁ the magnetic field becomes an alternating magnetic field giving rise to the vibration of the rod.
⦁ The frequency of mechanical vibration of the rod is given by

`f_(rod) = p/(2L) √Y/ρ`

Where p = mode of vibration

L = length of the rod.
rod

Y = Young’s modulus of the material of the rod and, density of the material of the rod.

For first harmonic p =1 and the frequency of vibration becomes

`f(rod) = p/(2L) √Y/ρ`

The frequency of the triode valve oscillator is given by
`f_(osc) = 1/(2π√L_1 C)`

By varying the capacitance C of the variable capacitor the oscillator frequency , f_osc can be  adjusted to the mechanical frequency of the rod f_rod . In the case

                            f_{rod  =}  f_osc      
And resonance takes place.

⦁ At resonance the rod vibrates vigorously emitting ultrasonic waves from both its ends.
⦁ By adjusting the length of the rod and the capacitance of the variable capacitor ultrasonic waves of frequency up to 300KHz can be produced .