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Description:
The following is taken from Exploration Geophysics by J.J. Jakosky, Second Edition, 1950.

Mott-Smith Gravimeter U.S. Patent 2,117,471 issued May 17, 1938 (Assignor to Shell Development Company)

The Mott-Smith Gravimeter is an astatic meter employing an auxiliary quartz fiber as a labilizing spring to effect a further movement of the weight after it is initially moved by gravity.

The component parts of the meter are a T-shaped frame having arms rigidly attached to a casing by means of a clamp. A torsion fiber which is about 1.5 inches long and about 0.002 inches in diameter, carries a weight arm, which, in turn, is connected to the labilizing fiber. The latter is connected to the T-frame through spring. The weight arm is fixed and normally extends in a substantially horizontal position from the torsion fiber.

The center of gravity of the suspended system, which includes the arm, its extension, pointer, and counterweight, is placed as nearly as possible at the same level as the torsion fiber by adjusting the counterweight. The substantially-horizontal equilibrium position of the weight arm is adjusted by adding or removing quartz by fusion, or by bending the extension.

Operation of the instrument is as follows: The weight arm is adjusted at a base station until it is in a substantially horizontal position and the position of the pointer arm is observed with the aid of a microscope. In this position, the auxiliary fiber 8 passes through the center of gravity of the suspended system and hence exerts no torque on it. The instrument is then moved to another station. If the accereleration due to gravity is greater at this station, the weight arm will be pulled downwardly against the resistance of the torsion fiber; at the same time the fiber exerts a torque on the suspended system, tending to displace it still further. The total displacement of the pointer arm is then measured by means of the microscope.

The instrument is capable of measuring the force of gravity with an accuracy of 0.1 millidyne or better. To achieve this accuracy, it is necessary to observe various precautions. The casing must be air-tight and contain a drying agent to keep the the air inside at a constant density. Also, it is necessary that the temperature within the case be kept constant within 0.001 degree centigrade. The latter requirement is fulfilled by immersing the casing and level in a water bath whose temperature is controlled by a mercury-toluene thermostat element.