. Carnegie Institution of Washington publication. 102 DISPLACEMENT INTERFEROMETRY APPLIED TO 0 = radian 2X378 and the displacement, x, of either end of the needle, therefore, if y is the corresponding scale reading in the telescope, All the work was done in the semi-subterranean basement of the laboratory and in the dark, except for the light which came through a ground-glass screen from the lamp at the scale. The room was so damp that electric charges are out of the question. The thermostat indicated temperature changes rarely exceeding a degree per day. 84. Long-period observations.—A number
. Carnegie Institution of Washington publication. 102 DISPLACEMENT INTERFEROMETRY APPLIED TO 0 = radian 2X378 and the displacement, x, of either end of the needle, therefore, if y is the corresponding scale reading in the telescope, All the work was done in the semi-subterranean basement of the laboratory and in the dark, except for the light which came through a ground-glass screen from the lamp at the scale. The room was so damp that electric charges are out of the question. The thermostat indicated temperature changes rarely exceeding a degree per day. 84. Long-period observations.—A number of early readings, taken at inter- vals of about half an hour with the weight M alternately on opposite sides of the needle, are shown in figure 118. There is some drift, and if the observations be distributed in triplets, the successive double amplitudes Ay (marked on the curve) make a decreasing series. The mean is Ay = From these, the actual double amplitude of the shot may be computed for each case, the mean value being A* = cm. This is about half the value obtained with the former quartz fiber, but it suffices. The reason for the irregularities and variations of figure 118 is at the outset hard to ascertain, even if the needle is never quite at rest. The shot, in other words, undergo something that is analogous to the Brownian motion of motes or very small bodies, except that the cause here may be surmised to be referable to temperature differ- ences and convection currents (dynamic pressure reductions) in the sur- rounding air; but one is at first very far from understanding all these vagaries As the apparatus was arranged, the observer sat in a neutral position (plane of the needle) within about an average meter from the 1 1 ft needle-case and controlled the attracting weight M man- 8 ually. It is possible that control from a remote distance would be preferable, but I did not at first think so. Later I worked from a distance only; but the results were no b
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