A seismic instrument or accelerograph for recording the spectra of strong earthquake shocks or the like, having a series of mechanically- resonant recording units which trace or "scratch" linear records with styli on metallic record plates. No electrical or other power source is needed to make the records, so that the instrument may stand by for many years without attention. The case has, typically, about sixteen separate compartments with a recording unit installed in each. Each unit has a different natural (resonant) frequency, in the range between about 1 to 32 Hz. The record plates are all alike, except for keying or locating grooves, and all the recording units are made to have about the same sensitivity in terms of stylus deflection per unit acceleration. A problem addressed here is that in any simple linear mass- spring system, the static sag (deflection of the mass at 1 g) is inherently related to the natural frequency by an inverse square law. The invention uses different types of recording units for different ranges of natural frequency, providing mechanical magnification or demagnification between the seismic mass and the stylus. Medium- and high-frequency units are of the prior reed and platform types shown in my prior U.S. Pat. Nos. 3,683, 397 and 3,974,504. Low-frequency units use a novel unbalanced torsional mechanism described herein. All units are preferably damped to about 2 percent of critical by an improved air dashpot device.
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