Quake-Proof Clock

In old-fashioned earthquake recorders there was a heavy mass whose movements, amplified by a system of levers, were transmitted by means of a stylus to a recording drum. In modern instruments the stylus is replaced by a beam of light which makes its zigzag tracks on photosensitive paper. Actually it is not the heavy mass which moves, but everything else--earth, observatory, revolving drum --while the weight, which is freely suspended, remains still because of its inertia.

At the Carnegie Institution's seismograph station on Mt. Wilson in California, Dr. Hugo Benioff has built recorders which work by electromagnetism. The weight is a magnet hung so that its poles are a tiny fraction of an inch from the armature. When an earth tremor twitches the armature, the distance between it and the magnet changes slightly, altering the magnetic field and creating a tiny electric current which is amplified by vacuum tubes. This current fluctuates the light beam which makes the record, also twitches a galvanometer needle. In the Benioff seismograph, earth movements are magnified 200,000 times.

This seismograph is rugged enough to withstand severe quakes close to the observatory--an important consideration in California. An accurate time record of the earthquake waves is also important, however, and pendulum clocks of high accuracy are likely to be thrown out of kilter by strong temblors originating nearby. Dr. Benioff has now devised what he calls an "earthquake resistant time- piece"-- a clock driven by a 1,000-cycle tuning fork, which is in turn driven by a fourstage amplifier. This apparatus is accurate to one-tenth of a second per day and Dr. Benioff was sure last week that it would not be put out of whack by a quake unless the walls of the observatory themselves tumbled.

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