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Description:


Gravity meter model No. 70 is one of the last of the old-style (25-lb) LaCoste & Romberg meters built. The 25-lb instrument was manufactured by LaCoste & Romberg between 1941 and 1957. It had a range of 200 mGals and about 80 were sold. A few of these are still in service today. This model was superceded by the Model G meters (Item No 87.05.02).

How does it work? Gravity was measured using a lever-arm system that balanced a mass at the end of a zero-length spring. An optical system (the eyepiece is the tube at the top of the box) was used to observe the position of the mass relative to a reading line. The mass (beam) was moved to the reading line by means of a screw connected to the big dial at the top. Gravity was read directly from the dial by using a dual-dial and vernier system. The three black knobs at the top are used to level the instrument. A light inside the case illuminated the beam. In addition, the sensor was heated to a constant temperature. This required a large lead-acid automotive battery to provide the power during field operations.

One advantage of the 25-lb gravity meter was that it was relatively light compared with other gravity meters available at the time and only required a two man crew to operate it in the field. Smaller crews moved faster and could collect more gravity data. At 0.01 mGal accuracy, the 25-lb LaCoste & Romberg gravity meter was the most accurate gravity meter of its era.

The 25-lb meter was used extensively in the Gulf coast for oil exploration. This type of meter was noted for finding a number of salt domes in Mississippi and Alabama, many of which had large petroleum reservoirs associated with them.

For more information, see the article: Gravity instruments: Past, present, future, The Leading Edge, January 1998, p. 100-112.