The gravimeter construction has been shown in Figure 5. It consists of two magnetically coupled pendulums. The magnets are attached on both pendulums in such a manner that they attract each other, thus the pendulums are in the vertical positions when no horizontal gravitation component affects on them. This double pendulum arrangement makes the system more sensitive to the horizontal forces comparing to the simple pendulum.
Figure 5. The gravimeter consisting of two magnetically coupled pendulums.
If there is a horizontal gravitation component, both pendulums rotate slightly and a new equilibrium state is created, see Figure 6.
Figure 6. The positionn of pendulums with a horizontal gravitation component.
However, similar situation arises if the whole gravimeter systems is tilted, and therefore it's not possible to distinguish these two cases just by measuring the tilt angles of the pendulums, as shown in Figure 7.
Figure 7. The position of the pendulums when the the whole system is tilted.
In the gravimeter devices used in this project the lengt of the pendulums were 100 mm and the distance between the magnets was 11 mm. The bearing of the pendulums is critical since the tilt angles are extremely small. We found that razor plates reclining in a narrow and shallow groove worked nicely. With these parameters the oscillation period of the double pendulum was about 3,5 seconds and the tilt gain comparing to the simple pendulum was about 100. The position of the upper pendulum was measured by the position densitive detector, a special photodiode construction, whose own position resolution was 50 nm. The tilt angle resolution of the gravimeter was 20 nrad based on the total noise level of the detector and the electronics. Some pictures of the gravimeters can be found on GALLERY page.