138 



WORZEI. AND HARRISON* 



[chap. 9 



length of a simple pendulum with the same period as the compound pendulum 

 formed by the meter unit. In this case the correction for the horizontal accelera- 

 tions amounts to the time average of IgO-, where 9 is the angle between the 

 axis of the gravity meter and the true vertical. The direction of the true 

 vertical must be established using long-period pendulums or a gyroscopically 

 controlled device. The forces perpendicular to the measuring axis of the meter 

 are very small, so the cross-coupling effect is not important. The theory holds 

 to an accuracy of I mgal for horizontal accelerations up to about GO, 000 mgal. 

 For larger accelerations, fourth-order terms must be considered. 



Fig. 1. Schematic diagi'am of the Graf Sea Gravimeter. 1. Photocell lamp and optics. 

 2. Diaphragm. 3. Photo-electric cell. 4. Damping magnet. 5. Measuring spring 

 and scale. 6. Torsion spring. 7. Fibei's for constraining weight beam motion. 

 8. Weight beam. 9. Amplifier. 10. Recorder. 



C. The Graf Sea Gravimeter 



The main features of the Graf Sea Gravimeter are shown in Fig. 1. It consists 

 of a horizontal beam of aluminum supported on nearly horizontal helical ribbon 

 springs. These springs act as the main springs and the pivots for the beam 

 motion. Eight ligatiu"es are arranged to restrain all beam motions except 

 rotations about the main axis. The main sjmngs are set under sufficient tension 

 that the center of gravity of the beam would be raised above the axis of rotation 

 of the beam at all values of gravity likely to be encoimtered on the siu'face of 

 the seas. A weak reading spring is attached to a lever arm on the beam and to 

 a nut whose position is controlled by a dial reading screw. By adjusting the 

 reading screw, sufficient tension can be applied by the reading spring to bring 

 the center of gravity of the beam to the same horizontal plane as the axis of 

 rotation of the beam. A dial, attached to the reading screw, permits a reading 

 of the tension of the spring which can be related to the value of gravity by 



