Tidal Oscillations of Gravity

Abstract

IN an earlier communication,¹ we reported the results of experiments to determine as exactly as possible the variations of gravity. We have now succeeded in increasing the accuracy and in eliminating the disturbances to such a degree that now the oscillations of gravity due to the attraction of the sun and the moon are directly perceptible in the photographic registrations. The arrangement of the apparatus is the same as stated in our previous letter.² The principal success was achieved by using the alloy WT 10 made by Krupp as material for the spiral spring. This material has an infinitesimal temperature coefficient of elasticity. By careful and repeated heating over a period of several months, it was also possible to reduce to a small factor the elastic after-effect. By this means the disturbances due to temperature are strongly suppressed, and also the uniform shift is very much diminished. The sensitiveness amounted to a deflexion of 1 mm. for dg/g = 10⁻⁸ at a distance of registration of 5 metres. With the aid of an electric method similar to the attracted disk electrometer of Lord Kelvin, the sensitiveness of the apparatus was often determined; therefore the indications of the registrations can be regarded as accurate to within 10⁻⁹g