Seismometer

Seismometer (in Greek seismos = earthquake and metero = measure) are used by seismologists to measure and record seismic waves. By studying seismic waves, geologists can map the interior of the Earth, and measure and locate earthquakes and other ground motions. The term seismograph is usually interchangeable, but seismometer seems to be a more common useage.

Improved designs

In 1894, Milne invented a basic, undamped horizontal-pendulum seismometer with a continuous photographic record. He succesfully advocated a system of seismic stations, and the British adopted his seismograph for them.

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In 1895, von Rebeur Paschwitz in Germany used a tiny, 42g horizontal pendulum with optic recording to record the first-ever confirmed Japanese earthquake to be recorded in Germany.

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The expense and fuzziness of photographic seismographs reduced their utility. In 1904 Wiechert of Gottingen, Germany put a 1000Kg mass atop a vertical pendulum and held it upright with weak springs. This gave excellent sensitivity, and permitted a mechanical seismograph with jeweled bearings and conventional paper records to receive distant earthquakes. The inverted pendulum significantly reduces the pendulum length required for a suitably low frequency. This reduces the overall size of the instrument.

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In 1906, Galitizine produced the first electromagnetic seismograph. A pendulum with a magnet induced current in a coil which then drove a galvanometer.

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The Omori seismograph used Zollner's suspension on Milne's horizontal pendulum (Omori was a pupil and colleague of Milne in Japan). It was the prototype of the Bosch-Omori seismograph used worldwide in the early 20th century. It uses two torsion wires or (for the vertical seismometer) a pair of springs for its hinge. Basically, one wire pulls down on the side away from the mass, while another pulls up on the side toward the mass. Bosch added damping that Omori omitted.

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In 1932 Lucien LaCoste invented the zero-length spring. A zero-length spring has a physical length equal to its stretched length. Its force is proportional to its entire length, not just the stretched length, and is therefore constant over a range of flexures (that is, it does not follow Hooke's Law). Theoretically, a pendulum using such a spring can have an infinite natural period. Long-period pendulums enable seismometers to sense the slowest, most penetrating waves of distant earthquakes. WIthin two years, zero-length spring versions of many seismometers were available, and the resonant period of the lowest-frequency seismometers went from 90 seconds to more than 900 seconds.

Related Topics:
1932 - Lucien LaCoste - Zero-length spring - Hooke's Law

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The Wood-Anderson torsion seismometer is one of the most elegant horizontal damped pendulums that was adapted to use zero length springs. A 2-cm pendulum is attached like a flag to the middle of a long, vertical steel torsion wire. A mirror on the pendulum reflects a light beam. A magnet wraps around the pendulum to damp motion by inducing eddy currents in the pendulum. The pendulum and wire are sometimes mounted in an evacuated aluminum pipe with a window to pass the light. This compact, lightweight seismometer is sometimes used with electronic photocells and amplification.

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A practical amateur design was commissioned by Scientific American for their "Amateur Scientist" feature. Basically, the design is a classic small horizontal pendulum (similar to von Rebeur's). The weight is a large sense coil, moving in the magnetic field of a magnetron magnet (cheaply available from microwave oven repair shops). The damper is a one-megaohm variable resistance across the sense coil. The hinges are very thin sheets of brass, held in clamps. The frame is square aluminum tubing. The device senses velocity rather than position, but requires very little care, is very sensitive with modern electronic amplifiers, and it is easy to construct and tune. A special feature is that the pendulum's frequency and damping can be tested remotely by running a pulse of current through the coil.

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The strain seismometer by E. Oddone measures the distance between two piers, which changes when a ground-wave passes the instrument. Oddone specifically wanted to check seismic theory with a seismometer that did not use pendulums.

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The greatest single improvement was the long term drum recorder. A large cylinder is wrapped with paper. The cylinder is rotated by clockwork (or a synchronous electric motor) and, turning on a spiral screw, advances along the axis of rotation. A recording stylus is linked to the proof mass by a series of levers (or uses an electric galvanometer movement), to amplify small relative motions of the mass to drive the stylus. This apparatus collects a recording for an extended period of time (usually a week). Clockwork displaces the recording stylus once per minute to allow time comparisons between charts recorded at different locations. On modern seismometers, two such recorders are coupled to the mass to determine motions in each of two axes.

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