By J. A. Anderson and H. O. Wood
A torsion seismometer of the short-period horizontal pendulum type adapted for local earthquakes is shown in perspective. A tungsten wire, T, about sixteen centimeters long and 1/50 of a millimeter in diameter, fastened to and tightly stretched between the upper suspension lug U and the lower suspension lug L, carries attached to it eccentrically near its middle a copper cylinder C to which is fastened a small plane mirror m. (A somewhat enlarged view, showing the arrangement of the cylinder, mirror, and wire is shown in the inset). T is drawn taut by the weight of the brass cylinder, amounting to about fifteen grams, shown just below the clamp of the suspension lug L. The copper cylinder C is about two and five-tents centimeters long and two millimeters in diameter and, with the mirror attached, weighs approximately 0.7 gram. As the inset drawing shows, the wire T is attached to the surface of the cylinder parallel to its axis. When the wire T is adjusted exactly in the vertical the system forms a strictly horizontal pendulum with a restoring force due wholly to torsion, as explained above. When disturbed by horizontal earth motion having a component at right angles to the plane of equilibrium, rotation of the eccentric system takes place about the wire as an axis in a manner similar to other horizontal pendulums.
But, in addition, if the earth acceleration is sufficiently strong, a horizontal vibration, of relatively very short period, like that of a "violin string" is set up. This is a defect of the Zollner suspension. This effect may be prevented, or minimized and damped out very quickly, by passing the wire T through two small drops of castor oil held in the oil dampers DD, one above and one below the inertia-mass. The wire T passes through two small films or discs of oil, held by surface tension in D and D, without touching anything else. The viscous restraint exerted by these discs or drops of oil almost completely eliminates the undersirable "violin string" vibration, but, as stated above, this exerts no perceptible damping on the rotational oscillation produced by horizontal earth disturbance at right angles to the plane of equilibrium, or the torsional reaction.
Anderson, J. A., Wood, H. O., Description and theory of the torsion seismometer, Bulletin of the Seismological Society of America, vol. 15, p. 1-72, 1925.
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