Following King's theoretical work, a series of measurements has been carried out with bronze spheres varying in diameter from ¼ to 1½ inches. In plane stationary waves in water at frequencies between 100 and 200 kc. King′s predicted dependence of radiation pressure on the location of the spheres relative to the position of nodes and loops is verified. Neglecting the viscosity of the medium and the compressibility of the spheres, it was found that the radiation pressure approximates inverse variation with the radius a of the sphere when the latter′s size is such as to make 2πa/λ greater than unity and less than four.
Considering the spherical torsion balance a likely laboratory acoustic standard in liquids, it is possible to calibrate as secondary standards any pressure microphone for subaqueous use in the supersonic range. Such calibration was effected upon tourmaline crystal microphones by the usual resonance tube method. The piston source was telescopically fitted into the end of a horizontal rigid cylinder about 6 in. in diameter and 15 in. long. The other end was machined and covered either by a blank quarter wave plate which acted as a perfect reflector or a quarter wave plate in the reflecting face of which the tourmaline crystal was inserted. The spherical torsion balance was suspended over the center of the cylinder so as always to maintain the sphere on the axis of the cylinder. The whole suspended system was provided with horizontal motion by means of a micrometer screw of 4 in. travel.
The results obtained by this calibration compare favorably with other methods and check roughly with the pressures calculated from the static piezoelectric constant of the tourmaline crystals.