382 EXPLORATION GEOPHYSICS 



TABLE 12 



CLOSURE DATA, ILLUSTRATING ACCURACY OF 



MOTT-SMITH GRAVIMETER t 



displace it still further. The total displacement of the pointer arm 12a is 

 then measured by means of the microscope 15. 



An exterior view of the Mott-Sniith gravity meter is shown in Figure 

 228a, 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 neces- 

 sary to observe various precautions. The casing must be air-tight and 

 contain a drying agent to keep the air inside at a constant density. Also, 

 it is necessary that the temperature within the case be kept constant to 

 within about 0.001° C. The latter requirement is fulfilled by immersing 

 the casing and level (not shown) in a water bath whose temperature is 

 controlled by a mercury-toluene thermostat element. 



The accuracy of the instrument is indicated by Table 12, which is a sum- 

 mary of closure data for eight loops. The first column gives the total 

 numerical change in gravity around the loop, i.e., the sum of the dififer- 

 ences added without regard to sign ; the second column gives the number 

 of observed differences which were added together to form the loop ; the 

 third column gives the error of closure. The probable error of closure 

 was 0.11 millidyne, the average error 0.13 and the largest error 0.26 

 millidyne. 



Portable Gravimeter. — An extremely portable meter has been devel- 

 oped$ which utiHzes a modified quartz mechanism similar to that illustrated 

 in Figure 228. The entire mechanism is housed in a sealed steel chamber 

 about 3 inches in diameter and 6 inches long. Micrometer screws, acting 

 through sealed copper bellows, actuate the null-point setting. The steel 

 chamber is housed in a 35^-inch Dewar glass flask for thermal insulation. 

 Due to the high thermal efficiency of the Dewar flask, coupled with the 

 temperature compensation incorporated into the mechanism, the temper- 

 ature effects are small and gradual, and may be treated as a drift in the 

 readings. 



t Mott-Smith, Geophysics, Jan. 1937, p. 29. 

 t V. J. Meyer, Atlas Exploration Company. 



