Definition = A seismograph is an instrument that measures and records seismic waves that move through the earth as the result of an earthquake.
Working = A basic seismograph includes a solid base and a heavy weight suspended from a spring over the base. A pen hangs from the weight and a rotating drum with paper sits below it on the base. The tip of the pen touches the drum. When the earth shakes from an earthquake, the drum rotates, and the weighted pen moves back and forth due to the motion of seismic waves. The pen records the movement on the drum. The paper recording of an earthquake is called a seismogram
The figure below shows the typical seismograph: