G. P. Woollard, 1963. "An Evaluation of Tares and Creep in Pendulum Observations on the Basis of Gravimeter Comparisons", International Gravity Measurements, George P. Woollard, John C. Rose
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As indicated in Part VIII, it has been noted by nearly all investigators that gravity pendulums undergo changes in period that may take the form of either abrupt shifts (tares), or gradual changes with time or use (creep). Although these effects might be attributed to knife-edge wear and crumbling, such action should result in a gradual effective lengthening of the pendulums so that the effect on period would always be of the same sign. Observations, however, show that the sign of the change is not constant and that there are frequent reversals. It is also observed that these changes in period occur in an unpredictable fashion as regards frequency of occurrence, the nature of the change, and its magnitude. As it has not been possible to correlate the phenomenon definitely with changes in environmental temperature, pressure, magnetic field or other external influences such as ground motion, and as the problem appears to be a general one affecting all pendulum observations regardless of pendulum material, time standard, or operating procedures used, there are significant discrepancies in pendulum values. These dis-crepancies may take the form of random errors, systematic deviations in values, or offsets in datum. Despite improvements in time standard, more rigorous control of operating pressure and temperature, and more precise recording that have taken modern pendulum operations up to the frontiers of present day technological developments, these effects are still present.
In addition to errors caused by period creep and tares in pendulum observations, comparative gravimeter measurements have also brought out the fact that there are "hidden" errors. These last appear to be related to environmental conditions and do not affect all sets of pendulum
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“Originally established to test reliability of the world network of firstorder international gravity bases, the program was expanded to include establishment of a series of reliable pendulum gravity control measurements in North America to assure a potential accuracy of 1 mGal or better on any global series of measurements. Studies also expanded and standardized existing gravity surveys of the United States for reliable gravity anomaly maps, and evaluated reliability and gravity standards used for global work.”