SECT. 1] 



GRAVITY AT SEA 



139 



suitably calibrating the instrument. The position of the beam is determined 

 by the differential output of two photocells mounted behind a slit attached to 

 the end of the beam. These photocells are so mounted and adjusted that they 

 give a null reading when the center of gravity of the beam is in the same 

 horizontal plane as the axis of rotation of the beam, and give equal readings 

 for the same upward and downward deflections. A large permanent magnet is 

 placed so that the beam moves in the gap between the pole faces, thereby 

 generating eddy currents in the aluminum beam which strongly damp the 

 beam motion. A pressure compensator and temperature compensator are 



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Fig. 2. Comparison of two Graf Sea Gravinieters operating simultaneously on the same 

 stable platform. 



mounted on the beam. The mechanism is also housed in a pressure-tight case 

 and is surrounded by an oven maintained at a constant temperature (to within 

 about O.orC). 



The output of the photocells is amplified by a d.c. amplifier. The output of 

 the d.c. amplifier is further damjjed by a suitable R-C circuit and a record of 

 this output is made on a potentiometer-type recorder. The record allows a 

 static variation of + 50 mgals from the true null position, which is arranged to 

 fall at the center of the recorder paper. 



The method of operation, then, is to adjust the dial reading until the recorder 

 pen is near the center of the record. The highly damped wave motions appear 

 as small oscillations on the record (see Fig. 2). The changes of gravity appear 



