GRAVITATIONAL METHODS 



385 



Sufficient sensitivity and output can generally be obtained with one or two stages of 

 vacuum-tube amplification. 



An electric gauge, bridge-type gravimeter is indicated schematically in Figure 

 230. The displacement of the mass 1 causes a displacement of a suspension lever 2 

 to the end of which is attached a light arm composed of a magnetic material. Move- 

 ment of the magnetic arm between the pole pieces of the electromagnet varies the 

 current through the reactors 12 and 12', thereby producing a deflection of the volt- 

 meter 13. 





Fig. 231. — Exterior view of the Graf-Askania mechanical- 

 electrical gravity meter. 1, arresting clamp; 2, scale range 

 adjustment; 3, calibration weight; 4, spring adjustment; 5, 

 leveling screw knob; 6, leveling screw shaft; 7, microam- 

 meter; 8, cable to storage battery. (Courtesy of the Askania 

 Corporation.) 



In another electric gauge-type gravimeter, determinations of the relative gravity 

 are based on electrical measurements which show the change in length of a vertical 

 spring to the lower end of which is attached a mass M. t Measurements of the relative 

 gravity are made on a microammeter (without amplifier) or on a reading drum. The 

 sensitivity of the instrument can be modified within a fairly wide range ; for field 

 investigations, 3 to 6 scale divisions of the microammeter are made to correspond 

 to 1 millidyne. A double thermostat is provided which, however, is used only when 

 the apparatus is moved between two observation stations far apart from each other. 

 No temperature control is necessary for local investigations. 



In one American modification of this method, the electric measuring device utilizes 

 a change in impedance as a measure of the movement of the mass. In this system, a 

 medium frequency alternating current is passed through a differential coil system, and 

 any movement of M causes a change in impedance, with resultant unbalance of an 

 alternating current, bridge measuring circuit. 



t A. Graf, "Ein neuer statischer Schweremesser zur Messung und Registrierung lokaler und 

 Zeitlicher Schwereanderungen," Zeit. fiir Geophysik, No. 14, Vol. 5/6, 1938, pp. 153-172. 



