COMPARISONS BETWEEN RELATIVE GRAVITY MEASUREMENTS WITH THE CAMBRIDGE PENDULUM APPARATUS AND WORDEN GRAVITY METERS IN NORTH AMERICA, SOUTH AFRICA, AUSTRALIA, NEW ZEALAND, AND GREAT BRITAIN

Summary The invar pendulums of the Cambridge pendulum apparatus are ferromagnetic and their periods are affected by the Earth's magnetic field. Before 1952, the pendulums were shielded from the Earth's field by a mu‐metal screen; since 1952 Helmholtz coils have been used to annul the Earth's field. To see if there is any discrepancy between results with the two systems, pendulum observations made with the screened system in South Africa, Australia, New Zealand, and Great Britian, and with the Helmholtz coil system in North America, have been compared with gravity meter observations made by Dr. G. P. Woollard and his collaborators. The observations, in three groups‐South Africa; Australasia and Great Britain; and North America‐have been adjusted by the method of least squares, a calibration factor for the gravity meter being calculated for each group. There is no significant difference between the factors so calculated, and accordingly no detectable difference between the two pendulum systems. The calculated calibration factors of Dr. Woollard's gravity meters IOE and 41 b are 0.1 per cent greater than those used in calculating the values published in 1952 and 1953 (Woollard, Harding, Muckenfuss, Bonini, and Black, 1952; Woollard, Bonini, Black, Rose, and Ault, 1953). The pendulum observation between Cambridge and Melbourne has a residual of nearly 1.2 mgal; otherwise the pendulum observations in South Africa and Australasia form a satisfactory basis for gravity surveys there. Two pendulum observations between Ottawa and Washington differ by 2.8 mgal and the differences of gravity between Ottawa, Washington, and Teddington (NPL) may be uncertain by about I mgal.