Description
A gravimeter is a high precision instrument designed to measure minute variations in the Earth's gravitational attraction, and they are most commonly used for geophysical investigation of the nature of the earth.

The Holweck-Lejay inverted-pendulum dynamic gravimeter uses a pendulum as the means for determining gravitational attraction. The inverted pendulum, a fused quartz rod about 6 x 0.4 cm diameter, is driven by a combination of gravity and the elastic restoring force of its elinvar spring. The quartz is coated with platinum and is surrounded by a Faraday cage to eliminate electrostatic action. The pendulum is observed by a microscope focussed on a fine quartz thread on its upper end. A thermometer is set in the metal frame, the entire apparatus being sealed in a low-pressure vacuum inside a glass cover. The apparatus sits on a brass plate with levelling screws; one knob liberates the pendulum, the other rotates the central plate. The apparatus is transported in a three-compartment fitted wooden box, with spirit level and xerox of 19-page pamphlet ‘Principe, description, emploi sur le terrain du gravimètre Holweck-Lejay.’

The instrument has an oscillation period of five or six seconds and retains a measurable ampliture for several hours. Very small variations of gravity have a large effect on the observed time of oscillation, so that accurate observations can be made by timing the series with a tenth-second-divided pocket chronometer. This enables campaigns to be completed faster than with previous apparatus requiring wireless signals, and to be undertaken more easily in difficult country, besides needing fewer trained personnel.

The instrument is named after Father Pierre Lejay SJ (1898-1958), of Zi Ka Wei Observatory, Shanghai, and Professor Fernand Holweck (1889-1941), a French physicist murdered by the Gestapo.

14/01/2008
Created by: Dr. Anita McConnell on 14/01/2008