The amplitude of the oscillatory pressures, at in the course to be transmitted to the manometer through a long pipe, will be diminished and at the same time be accompanied with a phase lag owing to the inertia of the fluid (air) existing in the pipe and the frictional resistance of the pipe wall.
In my experiments a basic theory was developed at first in which the fluid in the pipe line as an incompressible tube was considered. The results of the experiments coincided, although very roughly, witsh the basic theory at the resonance frequencies and amplitudes. Rigorously speaking, however, it goes without saying that the velocity distribution of the fluid in the pipe and the variation of the frictional resistance with increasing frequencies had to be considered.
The theory of the oscillatory flow of fluid (air) in the circular pipe was developed and the velocity distribution, the increase of the frictional resistance and the total resistance coefficient including the inertia were calculated accordingly.
The corrected basic theory showed that the resonance frequencies coincided very well with the experiments, but the resonance amplitudes did not do so well.

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Two fundamental problems are involved in the designing of permanent magnet; first, the complexity of Ohm's law in this case, because the reluctance of magnetic circuit is not constant, but depends upon the flux passing through it and second, the leakage from the circuit owing to the poor insulating characteristics of the surrounding air to magnetic flux. Concerning the former, it is necessary to determine the equation representing demagnetization curve if analytical solution of the design problem is required. Therefore, the equation; B=aH/b+H was assumed to represent the demagnetization curve of a given magnetic material, after which experiments have been carried out to determine if the assumption is reasonable. For the latter poblem, the relations between the leakage coefficient and the shape or dimensions of a circuit have been studied experimentally. If these two problems could be solved, the next thing would be the determination of the operation point of the magnet. Some methods and examples to do it, therefore, have been considered in case of using MT magnet.

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The writers have tried to manufacture a practical level gauge both for remote measurement and remote control of liquid level which is contained in a closed vessel or which does not allow the access of measuring instrument. For this purpose, the γ rays from C_o⁶⁰ were irradiated either parallel or inclined 30° to the surface of the liquid in a hollow cylinder made of iron or glass through its side wall. After measuring the amount of the rays passed through the cylinder with a counting-ratemeter combined with a G M counter, the relations were checked between the liquid level and the amount of the rays passed. In this article appear four cases of the many experiments conducted, of which the most accurate one shows that the level may be determined within ±0.3mm. Some cautions in manufacturing the ratemeter, protection against the γ rays irradiated, etc. are also given.

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