English: Photo of quartz gravimeter pendulums used in the gravimeter instrument designed by Gulf Research and Development Co. in 1929. Accurate to less than 1 milligal, the Gulf instrument was the most accurate commercial gravimeter into the 1950s. The pendulums are 10.7 inches long and had a period of about 0.89 sec. They were mounted in a sealed vacuum tank and swung synchronized, 180° out of phase, to cancel out swaying of the support. Fused quartz was used because of its small thermal expansion, but stray electrostatic charges on the pendulums had to be removed by exposure to a radioactive salt before use.

Alterations to image: removed aliasing artifacts (striped lines) from scanning of original halftone image using GREYCstoration filter in Gimp.