A balance for measuring ultrasonic radiation pressure has been built which has the following features:It employs the parallelogram principle of Roberval'snigme Statique, or the old-fashioned letter-balance, to provide a measuring surface above the balance beam. This also renders measurements independent of the position and uniformity of the sound beam. It uses a suspension system of four horizontal torsion wires to eliminate friction, and its sensitivity can be varied in steps over a wide range by successive replacement of each of the four torsion wires. It employs a 45reflector which acts as a perfect absorber independent of the reflection coefficient of its surface. This balance gives an absolute measure of the total acoustic power in a sound beam.
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