The rotary grating ultrasonic velocity meter is a convenient apparatus by which the sound velocity of the liquid can be measured rapidly. It is based on a variation of Debye-Sears ultrasonic diffraction method. The principle , the experimental process of measurement and some data taken by means of it have been published befote. In the preceding cases, the rotary grating whose proper constant is 1.369 x 10^<-2> cm, slightly smaller than the wavelength of the measured liquid has been used. The author reports here the case in which the proper constant of the rotary grating is 2.000 x 10^<-3> cm, far smaller than the wavelength of the measured liquid. By using such a rotary grating, detection of the coincidence of diffraction lines has become easier. Consequently, the accuracy of the apparatus has been promoted.
In the preceding papers, the measured values were somewhat higher than the acknowledged values and the reason could not be clarified. Here are reported the results of the analysis of the reason on the basis of the following four hypotheses.
a) The deviation from the exact parallelism of the light beam might affect the velocity value.
b) The friction between the liquid and the wall of the sample container might affect the velocity value, when the width of the ultrasonic beam is larger than that of the sample container. In order to test this hypothesis, a " Beam Forming Cylinder " is used to control the width of the beam.
c) The curving up of the free surface of the sample liquid by intense ultrasonic waves might change the proper wavelength near the surface. In order to test the effect, the apparatus is equipped with a reflector.
d) The deviation from tne parallelism of the wave front and the slit might affect the velocity.