The instrument described here has been designed for the purpose of measuring gravity values on board a moving vessel. It is essentially a short period self-exciting bifilar gravity pendulum mounted on an appropriate gimbal. The natural frequency of the pendulum is approximately 10 c/s. This frequency which depends on the gravity value, after being stepped up electronically to 60 c/s, is compared with a 60 c/s standard frequency given by a crystal oscillator by means of a differential gear device.
In order to avoid the effect due to up and down motions of the vessel, variations in apparent gravity which are shorter than 20 sec, in period are cut off by means of a suitable filter attached to the pendulum.
The whole instrument consists of the following parts.
1) Pendulum
2) Exciter amplifier
3) Frequency measuring equipment
a) Frequency multiplier
b) Crystal frequency standard
c) Power amplifier
d) Differential gear and recording drum
e) Arbitrary time interval generator for time marker
4) Temperature regulator
a) Temperature variation detector and regulator
b) Air bath
5) Gimbal
6) Power source
a) 2.K.V.A. generator driven by light oil engine
b) Electronical A.C. voltage stabilizer.
Mechanical characteristics of the pendulum and the low pass filter; The working principle of the Bifilar gravity pendulum is shown in Fig. 1. A schematic diagram of the pendulum assembly with a low pass filter attached is shown in the Fig. 3. The dimensions of the pendulum are as shown in the Fig. 4. Frequency variation of the pendulum caused by variation in its oscillation amplitude were determined experimentally and is shown in the Fig. 7.
Temperature coefficient of the pendulum frequency is shown in the Fig. 5. Characteristics of the oil damper is shown in the Fig. 10. Buoyancy and surface tension due to the damper liquid are compensated by means of a double damper device which is schema tically shown in the Fig. 11.
Effects of various attachments to the pendulum : The pendulum has various attach ments to its main body. Variations in natural frequency of the pendulum caused by these attachments were calculated, and the instrument was designed in such a way that their whole effect on relative frequency variation will not exceed 5×10^-7.
Self exciting devise: The pendulum is excited by successive impulsive forces. Block diagram of the exciting circuit is given in the Fig. 15. Optical system used is schemati cally shown in the Fig. 14. Circuit diagram of the exciter amplifier is shown in the Fig. 16, together with frequency multiplier circuit.
In the self-exciting pendulum, exciting pulses must be given to the pendulum at the moment when it pass through its zero position . Accutually, this is not exactly the case, and the effect of phase variation of exciting pulses on frequency is estimated. Phase vaniation of exciting pulses is chiefly caused by relative variations in brightness of the lamp and in the grid bias of the 1st stage tube . The exciting amplifier is designed in such a way that 10 % variation in the lamp input voltage will not cause frequency variation greater than 5×10^-7.
Frequency measuring device: Frequency measuring device is designed in such a way that the gravity value can be determined within an accuracy of 1×10^-6 in ten min .. The 10 c/s square wave given by the pendulum is differentiated and rectified, and shaped into a 20 c/s impulsive wave, which is supplied to an electronic frequency multiplier. The circuit diagram of the frequency multiplier is given in the Fig . 21. A 60 c/s cathode coupled multi-bivrator is controled by this 20 c/s impulsive wave which sends out signals once every three oscillasions of the former .
Frequency multipliers by means of resonator of high resolving power have been developed previously, and these are shown in Fig. 25, 26. 60 c/

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