390 EXPLORATION GEOPHYSICS 



grams). Certain other meters of proper design may be calibrated by the 

 "tilt" method. Tilting the instrument decreases the effective value of 

 gravity by a factor equal to the cosine of the angle of tilt. Hence, an in- 

 dication of the sensitivity may be obtained by observing the scale deflec- 

 tions for various angles of tilt. 



In all cases, it is desirable to provide a means by which calibration can 

 be carried out in the field because changes in sensitivity may occur during 

 the course of time. (Compare p. 371.) For instance, when dealing with a 

 delicately balanced elastic system, small changes in geometry caused by 

 relative motion of supports, or by readjustment to compensate for creep 

 or large changes of gravity, may cavise bothersome changes in calibration. 



The calibration of gravimeters by taking measurements at different 

 heights in tall buildings makes use of the change in gravity with elevation 

 as expressed in the constant used in the "free air" reduction, (page 256). 

 The constant, worked out by Helmert in 1910, is 0.3086 milligals per meter 

 or 0.09406 milligals per foot, and carries a minus sign to indicate a lower 

 force of gravity at a higher elevation. The vertical gradient of gravity 

 has a slight variation which was investigated by Hammer. f It was con- 

 cluded that the figure cited is accurate for all calibration purposes. 



The calibration of a gravity meter is basically similar to the calibration 

 of a magnetometer prior to field work. It is necessary to know the value 

 of the change in gravity represented by one scale division (for direct read- 

 ing instruments) or dial division (for null point instruments). This value 

 is usually expressed as a constant k. As an example, for the Brown Gravi- 

 meter No. 21, (July, 1942), the k value was 0.0352 milligals per scale 

 point. The term scale point is often used instead of scale division, in gravi- 

 meter work. 



In certain meters the effect of barometric pressure must be considered. 

 The moving mass in the instrument is buoyed up by the air in the case. 

 If the case is not hermetically sealed, the buoyancy of the air will be a func- 

 tion of the barometer reading. By taking a series of readings at the same 

 station, and varying the pressure within the case, the barometric correction 

 may be determined. During field work with such meters it is necessary to 

 take a barometer reading at each station. A barometer of the necessary 

 accuracy is carried in the field truck for that purpose. Instruments with 

 sealed cases do not require a barometric or air-pressure correction. 



Temperature Effects. — Temperature changes have an appreciable 

 effect on instrument readings. It is usual to protect the instrument from 

 temperature changes by carefully-constructed housings or casings which 

 maintain the working parts of the meter at a constant temperature. Tem- 

 perature is held constant to about 0.2 of a centigrade degree by thermostat- 

 controlled heating elements. 



Power for these elements is usually supplied by a storage battery and 

 the meter is kept under a constant temperature day and night during a 



t S. Hammer, "Investigation of the Vertical Gradient of Gravity," Trans. Amer. Geophys. 

 Union; Nat. Research Council, Wash., D.C., Part I, pp. 72-82, Aug. 1930. 



