Seismograph

A seismograph is an instrument for detecting and recording motion in the Earth's surface as a result of earthquakes. Such devices have a very long history that can be dated to the second century A.D. when the Chinese astronomer and mathematician Chang Heng invented a simple seismoscope. The term seismoscope is reserved for instruments that detect earth movements, but do not record such movements. In Chang's device, a metal pendulum was suspended inside a jar that held metal balls on its outer rim. When an earth movement occurred, the pendulum swayed back and forth causing the release of one or more balls into the mouths of bronze toads resting at the base of the jar. The number of balls released and the direction in which they fell told the magnitude and location of the earth movement.

Seismographs today consist of three essential parts. One is a seismometer, a device (like the seismoscope) that detects earth movements. A second component is a device for keeping time so that each earth movement can be correlated with a specific hour, minute, and second. The third component is some device for recording the earth movement and the time at which it occurred. The written record produced by a seismograph is called a seismogram.