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Aim of the experiment
To determine the speed of ultrasonic waves in a non-electrolytic liquid by ultrasonic
High frequency generator
An Ultrasonic Interferometer is a simple and direct device to determine the ultrasonic velocity
in liquids with a high degree of accuracy.
The principle used in the measurement of velocity (V) is based on the accurate determination of
the wavelength (l) in the medium. Ultrasonic waves of known frequency (f) are produced by a quartz
plate fixed at the bottom of the cell. These waves are reflected by a movable metallic plate kept
parallel to the quartz plate. If the separation between these two plates is exactly a whole multiple of
the sound wavelength, standing waves are formed in the medium. This acoustic resonance gives rise
to an electrical reaction on the generator driving the quartz plate and the anode current of the generator
If the distance is now increased or decreased and the variation is exactly one half wavelength
(l/2) or multiple of it, anode current again becomes maximum. If the separation between quartz plate
and metallic plate is changed by d between two successive maximum anode current
1. Adjustments of ultrasonic interferometer :
(a) Insert the cell in the square base socket and clamp to it with the help of a screw provided
on one of its side.
(b) Unscrew the knurled cap of cell and lift it away from double walled construction of the cell. In
the middle portion of it pour experimental liquid and screw the knurled cap.
© Two chutes in double wall construction are provided for water circulation to maintain desired
(d) Connect the High frequency Generator with cell by co-axial cable provided with the
2. For initial adjustment two knobs are provided on high frequency generator, one is marked ‘Adj’
and the other with ‘Gain’. With knob marked ‘Adj’ the position of the needle on the Ammeter is
adjusted and the knob marked ‘Gain’ is used to increase the sensitivity of the instrument for
greater deflection if desired. The meter is used to notice the number of maximum deflections
while micrometer is moved up and down in liquid.
3. The micrometer is slowly moved till the anode current on the meter on the High Frequency
Generator shows a maximum. Note the anode current and the micrometer reading.