Modern developments in precision clocks

ULTRA-PRECISE practical timekeepers of today can be placed within two classes, depending upon whether the restoring force is gravity, as in pendulum clocks, or elasticity, as in quartz crystal oscillators. The second class is a development of the last decade and was made possible by the vacuum tube and associated electrical circuits. Intercomparisons between timekeepers of the two classes provide a most valuable means for t he study of changes in gravity and related phenomena.