"Seismic" pendulum

seismic pendulum

This surveyor's pendulum in mounted in a Plexiglas box that measures approximately 2 inches by 3 inches by 8 feet.  The box is supported as far up into the suspended ceiling as possible.  The pendulum hangs from a thin wire which is slightly less than 8 feet long.   The Plexiglas box has a small door on the front at both the top and bottom.  A fine scale behind the tip of the pendulum allows small motions of the pendulum to be observed.  During times that the displays for the seismometers reveal that earthquake waves are passing through the room, the small motions of the room can be observed by watching the pendulum tip relative to the scale behind it.  The Plexiglas box prevents air motions within the room from disturbing the pendulum.   The photograph below shows the location of the pendulum in the room, between the case of fluorescent minerals (left) and the blackboard (right).