GRAVITATIONAL METHODS 



381 



through two springs 9 and 10. The weight arm 7 is fixed and normally 

 extends in a substantially horizontal position from the torsion fiber 5. 



The center of gravity of the suspended system, which includes the arm 

 7, its extension 13, pointer 12, and counterweight 14, is placed as nearly 

 as possible at the same level as the torsion fiber 5 by adjusting the counter- 

 weight 14. The substantially-horizontal equilibrium position of the weight 



Fig. 228a. — Exterior view of Mott-Smith gravity meter. 

 (Courtesy of Mott-Smith Corporation.) 



A — Bath stirring motor 



B — Thermometer housing 



C — Observing microscope housing 



D — Leveling screws 



E — Electric control panel for thermostat 



F — Foot mounting bracket 



G — Truck mounting bracket 



H — Carrying handle 



/ — - Reading light 



/ — Thermometer reading microscope 



K — Thermometer reading light 



arm 7 is adjusted by adding or removing quartz by fusion, or by bending 

 the extension 13. 



Operation of the instrument is as follows : The weight arm is adjusted 

 at a ])ase station until it is in a substantially horizontal position and the 

 position of the pointer arm 12a attached to the pointer 12 is observed with 

 the aid of the microscope 15. In this position, the auxiliary fiber 8 passes 

 through the center of gravity of the suspended system and hence exerts 

 no torque on it. The instrument is then moved to another station. If the 

 acceleration due to gravity is greater at this station, the weight arm 7 will 

 be pulled downwardly against the resistance of the torsion fiber 5 ; at the 

 same time the fiber 8 exerts a torque on the suspended system, tending to 



