A seismograph is an instrument used to detect and record ground motion caused by seismic waves.
A seismograph typically consists of three components – a sensor (seismometer), an amplifier or digitizer, and a recording device. Although seismographs have evolved over the years, all operate on the same basic principles by measuring ground motion relative to something that remains fixed or unaffected by the shaking. Early seismographs usually consisted of a mass suspended by a spring within a case. The mass would remain still while the case shook. The relative movement was usually recorded on paper by a pen or light source. In most modern seismographs, the mass has been replaced by a permanent magnet. Ground shaking causes the magnet to generate electric signals in the wires that are wound tightly in the surrounding case. The ground motions are digitally recorded and either stored on site or transmitted to another site for archiving. The record showing the recorded...
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