There are several instances where electrical resistance of a carbon bar or a coiled wire is applied for measuring piston stroke of the rock-drill. It is considered in these, however, that the manufacture of an apparatus with these things is not simple, and that troubles by frictional force of the sliding piece, wear of it, variation of contact resistance and increase of resistance by sliding frictional heat will present themselves.
Now, the present writers applied electrical resistance of a water column and there were no troubles as above mentioned. As for the water, usual drinking water is found to be good, still more sensibility of measurement being increased with distilled water.
Amplifying is done as follows: bias voltage of an amplifier valve varies according to-notion of a moving pole bar in water, connected to the piston, therefore plate current varies.
This current is recorded by an oscilograph. By this, sensibility about 0.01mA/cm of current in water is amplified to about lmA/cm, and straightness of a calibration curve is very good.
Temperature rise of water is not found and diameter of the moving pole bar may be fi ner (2mm) than in another cases, for fluid resistance to this pole very small.

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It has been pointed out that the influence factor concerning estimation of the surface subsidence is, in strict meaning, to be determined not only by the area mined out but also by the property of the ground, and. that the methods to determine the factor heretofore in use, such as Bails' method, have no theoretical background. A method to determine the factor taking into account the surface subsidence curve obtained from field survey has been suggested, besides a simple and convenient method for the case the data are not available has also been proposed.

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A photo-electric spectrophotometer was used to provide a simple, convenient, and accurate method of measuring the refl ectivity of powdered minerals.
The determination of spectral reflectivity of mineral powders provides a useful aid in the identification and chromaticity specification of minerals.
The powdered minerals were used as samples. in the measurements to eliminate the effects of specular reflection, Beilby layer, etc. on the polished surface.
The determination were carried out for the powders of pure galena, sphalerite, pyrite, chalcopyrite, bornite, enargite, stibnite, magnetite, specularite, ilmenite, and quartz in the range of wave length of 380 to 800 mμ.
The chromaticity of the samples based on the C. I. E. system were given, the three standard sources, A, B, and C, being used.

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In this report as the continuation of report I and II, the author reported the magnetic forces of locked particle - and the theoretical sensitivity of magnetic separation.
In the ordinary magnetic separation, the ores are generally locked particles including ferromagnetic minerals. So these particles show ferromagnetic characteristics, and their properties varie with the content of them. Technically the, most important problem of the magnetic separation of these locked particle lies, from the author's view, in the establishment of the clear relation of magnetic force and the content of ferromagnetic minerals of locked particle.
To ascertain the above problem, the author experimentally determined the relation by using a magnetic balance and over 8 hundreds of locked particles produced in the Kamaishi mine. More over, this experimental result was theoretically studied, and the sensitivity of magnetic separation was obtained. The result was illustrated by Fig. 6. Thus by the above result the practical problems of magnetic separation were discussed.
In the above considerations, the author theoretically solved the relation between apparent susceptibility of locked particle and of constituents. The relation was illustrated by the expression (9). By using the above relation the author numerically calculated the apparent susceptibility concerned with the locked particle of magnetite and quartz. The result obtained was illustrated by Fig. 10.
These results may have a wide use in practical applications of magnetic separation.

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We found the electrification occurring during the melting or the freezing of copper blast furnace slag.
Two iron wires as electrodes and a thermocouple were dipped into the molten slag in an iron crucible. The e. m. f. and the temperature were recorded simultaneously on a photographic paper using two galvanometers.
The e. m. f. occurred when the slag began to melt or freeze, reached to its maximum value, 0.16-0.20V, then it disappeared when the melting or the freezing was complete. The solid had generally a positive charge with respect to the melt.

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