GRAVITATIONAL METHODS 



373 





Fig. 218. — Diagrammatic sketch 

 of Hartley type gravimeter. 

 (Bryan, Geophysics.) 



reflected image of the lamp L at some fixed position. The main spring Si 



supports the mass M. The cross member AB is hinged at A with a flexible 



metal strip, or Galitzin hinge, which permits the end B to move up and down, 



with movement of M. The movement of B 



causes the small mirror D which bridges the 



gap to tilt from side to side. Adjustment of 



the instrument is accomplished by returning 



the reflected image to a previously chosen 



reference line in the eyepiece E, by turning the 



divided head H. 



The Hartley meter employs no astatizing 

 force, depending instead on mechanical and op- 

 tical magnification of the displacement of the 

 beam for its sensitivity. A movement of the 

 mass of 10"^ mm. results in a movement of 0.6 

 mm. at the eyepiece. The meter employs a 

 thermostat to keep the temperature constant to 

 within 0.01 °C. The instrument is highly damped 

 and relatively insensitive to leveling. The mech- 

 anism is in an airtight container, with the clamps 

 and screw H working through mercury seals. 



Truman Gravimeter. — This instrument is illustrated in Figure 219. 

 An approximately triangular framework A is hinged at N by flexible 



metal strips, t The spring Si supports the mass 

 M which is attached to the outer end of the 

 framework. A second spring S2, which is at- 

 tached to the lower end of the framework at B, 

 pulls directly upward almost through the hinge 

 line. This spring acts to increase the sensitivity 

 of the instrument. By adjusting the tension of 

 S2 for a given position oi B, the moving sys- 

 tem approaches unstable equilibrium with in- 

 creased sensitivity. A third very small spring 

 (not shown in the sketch) is attached to the 

 frame and works in parallel with Si. 



An optical system T is focused from above 

 on a reference line C attached to the frame. 

 Readings are made by observing the lateral mo- 

 tions of this reference line corresponding to 

 vertical motions of M. A lamp L shining 

 through a ground glass D illuminates the reference line or scale. Means 

 are provided to damp the motion of the moving system electromag- 

 netically and to clamp it.* Also, provision is made for the frame to 



Fig. 219. — Diagrammatic sketch 

 of Truman type gravimeter. 



t A. B. Bryan, loc. cit. 



* The instrument usually is either undamped or else damped electromagnettcally. 



