The torsion seismometer developed by Harry O. Wood and J.A. Anderson, from an illustration published in the Bulletin of the Seismological Society of America in 1925 (Vol. 15, pg. 10).
Instead of a pendulum, the motion generated when this instrument was shaken by an earthquake came from the rotation of a small, copper, inertial mass (C) affixed to a thin wire under high tension (T), hence the name torsion seismometer. The Wood-Anderson seismometer was designed to be as sensitive and as nearly frictionless as possible. Damping of the torsional motion was accomplished using magnets (M). This was an improvement over previous, mechanically-damped instruments, because the strength of magnetic damping is proportional to the amount of motion in the object being damped -- hence, when the instrument is at rest, the dampers provide no initial resistance to motion of the inertial mass.
Seismograms made using the Wood-Anderson seismometer were "drawn" not with a stylus or needle, but with a beam of light reflected onto photosensitive paper (not shown) from off of a mirror (m) on the inertial mass.Return to the main text