:- This is based on the principle of magnetostriction effect. A ferromagnetic rod MN is clamped in the middle. Two coils $L_1$ and$ L_2$ are wound round its two halves.

The coil $L_1$ is connected in the plate – cathode circuit and the coil$ L_2$ 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 $L_1$ producing a 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$ L_1$ and $L_2$ also changes giving rise to an induced emf ,$ e= -dφ/dt$ across$ L_2$.

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 $ L_1 $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.

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

ρ= 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 .