Gravity measurements in North America with the Cambridge pendulum apparatus

Abstract

The Cambridge pendulum apparatus has been used to establish a line of nine fundamental gravity stations in North America, between Mexico City and Winnipeg. For this work considerable care was given to the elimination of magnetic disturbances, the most important precaution being the compensation of the vertical component of the earth’s magnetic field by means of Helmholtz coils. The crystal frequency standard used to time the pendulums was rated for each observation against the precise carrier-wave frequencies broadcast by radio station WWV. In general, the values of gravity obtained are in agreement with those of previous observers, but detailed comparisons are difficult because of the need for a more consistent North American network. The total range in gravity covered by the line of stations is over 2000 mgals, the extreme values being Mexico City: g = 977·9415 cm/s 2 , Winnipeg: g = 980·9952 cm/s 2 , relative to the adopted base value Ottawa: g = 980·6220 cm/s 2 . The standard deviation of the difference of gravity has a mean value of 0·0003 cm/s 2 .