126 



GRAVITATIONAL METHODS 



[Chap. 7 



In the Hartley gravimeter the mass is supported approximately from 

 the center of a beam hinged on one end. The movement of the beam is 

 transferred to two rocking mirrors that rotate m opposite directions when 

 the beam is displaced. The main spring supporting the beam above the 

 mass is made of an alloy of tungsten and tantalum, carries 99.9 per cent of 

 the total load, is wound with high initial tension, and- in extended position 

 is about 10 cm long. A small additional spring is provided for compensat- 

 ing the beam deflections by rotation of a micrometer screw. 



In the Gulf (Hoyt) gravimeter, Fig. 7-286 (U. S. Patent 2,131,737, Oct. 4, 

 1938) a spider weighing about 100 grams is suspended from a helical spring 

 of rectangular section. A spring section whose width is much greater than 

 its thickness produces a rotation of the suspended mass when the weight 

 changes, this action being comparable with that of a bifilar suspension 

 (see below). The dimensions may be so selected that a change of 0.1 



Fig. 7-28a. Lindblad-Malmquist gravimeter (schematic). 



milligal produces a deflection of the order of 1 arc-second. Deflections 

 are measured by a multiple reflection setup involving two semireflecting 

 lenses. One of these is attached to the spider. Scale and light source are 

 in the conjugate foci of the lens combination. Reading the tenth multiple 

 reflection, a deflection of 1 arc-second corresponds to about 20 scale 

 divisions. 



In the Askania-Graf gravimeter a mass is suspended freely from a 

 helical spring. Its deflection is measured with an electrical displacement 

 meter (presumably operating without amplifier) with a magnification of 

 about 4 • 10*. Temperature compensation and a double battery-operated 

 thermostat is provided. The accuracy is of the order of 0.1 milligal. It 



" Physics, 2(3), 123-130 (March, 1932). 



" A. Graf., Zeit. Geophys., 14(6/6), 154-172 (1938). 



