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EXPLORATION GEOPHYSICS 



pendulum (Figure 140) is supported by agate knife edges, swinging on agate 

 planes. Essential equipment used with this pendulum apparatus includes 

 a chronograph, a radio receiver, and photoelectric and power amplifiers. 



The chronograph records simultaneously both the pendulum beats and 

 mean-time seconds (U.S. Naval Observatory time signals). Light pulses, 

 controlled by the pendulum beats, are picked up by the photoelectric cell and 

 amplified to operate a chronograph pen, using a relay. The time signals 

 are received by radio, amplified, and then fed to a second chronograph pen. 

 Both pens are adjusted to follow the same track, with one pen moving to 

 the left and the other to the right. A micrometer-microscope is used to 

 scale the chronograph records to 1/1000 of one second. 



Fig. 140. — Brown gravity apparatus. A, pendulum (swing is parallel to plane of paper) ; A', 

 pendulum support arm; B, lever for starting oscillation; C, release mechanism; D, pendulum clamp; 

 E, clamp screw; F, clamp knob. (Courtesy of U. S. Coast and Geodetic Survey.) 



The number of beats during the observations (which last for six hours) 

 is determined, as is the time interval. The error in the latter is less than 

 0.005 second, using as control the time signals of the U.S. Naval 

 Observatory. 



The time periods of the pendulums are then corrected for pressure, 

 temperature, and amplitude of oscillation, as well as for the slight motion 

 of the supporting framework caused by the swinging of the pendulum. A 

 specially designed interferometer is used to measure this movement caused 

 by elastic yielding of the support, in units of wave lengths of monochromatic 

 light. The instrument operates from batteries, charged in the field by a 

 gasoline-driven generator.f 



t U. S. Coast and Geodetic Survey, Special Publication No. 233, 1943. 



