GRAVITATIONAL METHODS 



375 



a greater or less pull on the spring, as the case may be, and this causes an 

 angular deflection of the lower end of the spring. 



The angular deflection is observed with the aid of the lens 96 attached 

 to the lower portion of the post 82 and additional optical apparatus in- 

 cluding a source of light, scale and two totally reflecting prisms. 



With an optical lever arm 22 inches long and using the 4th multiple 

 reflection (although 6 are visible), a displacement of the index of 0.001 

 inch (corresponding to 0.1 milligal of force) can easily be read, with the 

 microscope. The instrument can be quickly 

 adjusted for the magnitude of gravity in an 

 area, without affecting the calibration, by 

 means of a torsion head from which the system 

 is suspended. 



The scale constant is 100 divisions per milli- 

 gal on the 4th reflection, and readings to 0.05 

 mg. or better are possible under ordinary field 

 conditions. Tests have shown a probable error 

 for any one station of 0.02 mg. 



In the Hoyt gravimeter the direction of 

 instrument drift (or creep of the spring) can 

 be controlled by adjusting the mass to the 

 proper value. This adjustment is made so the 

 meter is operating near the reversal point. A 

 typical meter showed a drift over a 30-day 

 period of only about 0.3 mg. in 24 hours. 



Electromagnetic damping is applied by a 

 metallic vane (attached to the mass) moving 

 through the air gap of a permanent magnet. 

 Leveling is a critical feature of spring sus- 

 pension arrangements because, when released, 



the system is essentially a plumb bob. In this case the system was made 

 less sensitive to leveling by tapering the spring at the point of suspension 

 to compensate for the bending of the spring caused by its being off level. 

 Such bending is converted, in part, into rotation. Two 10-second spirit 

 levels set at right angles are provided with special leveling screws which 

 operate in an oil bath and are therefore dust free. As expressed in terms 

 of level arc causing a 0.1 mg. change in reading, the meters varied from 

 10 to 60 seconds, so that the 10-second levels were sufficiently delicate. 



The meter is clamped by means of 3 ball-tipped clamping arms actuated 

 by a control shaft and gear train and engaging 3 lugs on the moving mass. 

 The instrument is suspended in a heater shell surrovnided by insulating 

 material covered with thin aluminum. Temperature is controlled by a 

 thermostat and the heater elements are supplied by a 12-volt battery. 

 Temperature can be held constant to 0.02 C.° in field operations. 



Fig. 220 — Hoyt Gravimeter — 

 schematic vertical section. 

 (Hoyt, U. S. Patent 2,131,737.) 



