Seismometer

The seismometer is a modification of the Lehman seismometer as described in various articles in Scientific American magazine.

The mechanism can best be described as a horizontal pendulum, much the same as an open, swinging fence-gate that leans a bit downward. The arm of the pendulum can be pushed sideways slightly and it will oscillate about its point of rest until frictional forces bring it again to rest. Properly adjusted, the un-damped pendulum will have a period of oscillation of approximately 12-seconds, making at least eight visible oscillations before the motion dies out completely.

Place the seismometer on a hard, flat, level surface, such as the concrete floor of a basement, well-away from furnace drafts, foot traffic and other potential disturbances.

Put a drop of damping oil on the pivot point of the pendulum. Make sure the damping-oil container is empty, and install the pendulum. The damping flag should be fully within the damping-oil container, but not in contact with either the bottom of the container or its walls.

Adjust the leveling screws as necessary to make the base of the seismometer as level as possible, as a starting point. Adjust the two screws at the swinging end of the seismometer such that the pendulum comes to rest halfway between the stops.

Move the pendulum back and forth to make certain that the damping flag does not contact the oil container throughout its swing. The clearance is adequate if the pendulum can freely move at least 1.5-cm in each direction from its rest-point.

Gently push the pendulum and note its period of oscillation. Adjust the screw at the rear of the seismometer as necessary to achieve a pendulum period of greater than 4-seconds. Continue adjusting the leveling screws, keeping the rest-point halfway between the stops and try to achieve a periodicity of at least 8-seconds. As longer and longer periods are approached, the adjustments become increasingly critical. Note that trying to increase the period much beyond 12-seconds will begin to spoil the ability of the pendulum to return to its rest point. Our instruments have been successful when adjusted for a periodicity between 8 and 14 seconds.

Once you are completely satisfied with the adjustments, be very careful not to bump or otherwise disturb the instrument throughout the following procedures:

Add damping oil to the container, covering about 1-cm of the damping flag. Once this is done, all evidence of pendulum periodicity will vanish. The purpose of the damping mechanism is to destroy the resonance of the pendulum so that the instrument will respond to a broad range of periodicities. Therefor, do not make any further adjustments to the leveling screws or pendulum mechanism once the flag is in contact with the damping oil. If readjustments are necessary, you must remove the damping oil from contact with the flag.

Using a feather, a very gentle breath of air, or other delicate device, push the pendulum about 1- cm away from its rest point. Release. If the pendulum, upon returning to its rest-point, overshoots the point by greater than 1-mm, add more oil to the damping container, covering progressively more and more of the damping flag. Repeat this procedure as necessary. Do not over-damp. A small overshoot is ideal, but it should not be more than 10 to 15% of the original motion. Use an eye-dropper to remove or replace damping oil as needed as you approach this point of critical-damping.

Cover the seismometer with a cardboard box and try to seal-off any holes or gaps which might allow air currents to enter. If possible, use a second, larger box to cover the first.