A Seismograph is a very sensitive instrument which is used to record the magnitude of an earthquake in terms of the shock waves it produces. Usually three seismographs are used when measuring the shock waves from an earthquake. One records the horizontal movements in a north south direction, one those in an east west direction, and another the vertical direction.
One part of the seismograph remains stationary while the rest shakes with the earthquake waves. A weighted boom with a mirror reflects a beam of light onto a sheet of photographic paper on which it traces a record of the earthquake waves.
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