A Transportable Apparatus for Absolute Measurement of Gravity.

A transportable apparatus for an absolute measurement of gravity has been constructed for the purpose of detecting secular variations of gravity. First of all, the significance of the absolute measurement of gravity is outlined in this paper. The principle of measurement adopted here is the simple free fall, on the basis of the well-known relation between falling distance, falling time and acceleration of gravity. Using a cat's eye as a falling object, a Michelson interferometer measures the falling distance comparatively with the wavelength of a He-Ne laser light. Interference fringes for 30 cm falling distance are continuously recorded on a recording film with 1 ms sampling intervals which are accurately controlled by a frequency standard. Phase angles of interference fringes at every 1 ms interval are taken into consideration but direct counting of the number of interference fringes is unnecessary in the present apparatus. The method of data processing, in which the falling distance is counted from time to time on phase angle data, is one of the distinct features of the present apparatus, and proves to be very effective in both detecting details of falling motion and simplifying the apparatus. Furthermore, we describe disturbing factors such as the inaccuracy in wavelength of laser light, the vertical gradient of gravity, residual gas and imcompleteness of the optical character of a falling object. Secondly, the result of experimental observations at the Matsushiro Seismological Observatory is reported. The gravity value obtained from twenty-one test drops is as follows : gOBs = 979 770.131 ± 0.010 mGal. This is very close to the corresponding value derived from that of the Matsushiro First-order Gravity Station of the Geographical Survey Institute ; gDRV = 979 770.19 ± 0.1 mGal. The main sources of standard error of a single drop of 0.038 mGal have been estimated as the inaccurate setting of the wavelength of laser light and microtremors. Inaccuracy in wavelength causes a 0.020 mGal error, but it will be overcome by introducing an iodine saturated absorption stabilized He-Ne laser with an accuracy in wavelength of 1×10-9. Microtremor effect on gravity amounts to about 0.026 mGal, but can be minimized by monitoring ground vibrations with a seismometer. It is concluded that the high efficiency and the practicability of the present apparatus have been confirmed through the experiment and the data analyses