The observation of gravity by means of invariable pendulums

Abstract

By observing the variation of the gravitational field over the earth’s surface it is possible to obtain information about the distribution of attracting matter within the earth. Of the methods of investigating these variations, the observation of the change in period of invariable pendulums carried from place to place has so far yielded most information. Until recently such observations have been of necessity extremely lengthy, and only about 3 per cent, of the earth’s surface has been at all adequately surveyed. It is therefore important to develop rapid methods of observing, and recently several papers have appeared describing methods of using wireless signals in timing pendulums. The purpose of the present paper is to describe the methods developed in Cambridge. They differ from former methods in that:— (1) No special signals are required. (2) The apparatus is considerably more portable than any so far described. (3) The errors in timing are so small that, with a 1-hour swing, they are negligible compared with other sources of uncertainty.