Seismograph

One of the simplest approaches to the recording of earth movements is simply to attach a pen to the moving element in a seismometer. The pen is then suspended over a rotating drum to which is attached a continuous sheet of graph paper. As the drum rotates at a constant speed, the pen draws a line on the graph paper. If no earth movement occurs, the line is nearly straight. Earth movements that do occur are traced as sharp upward and downward markings on the graph. Since the rate at which the drum rotates is known, the exact timing of earth movements can be known.

In some kinds of recording devices, the moving pen is replaced by a beam of light. Earth movements can then be recorded photographically as the beam of light travels over a moving photographic film. This type of device has the advantage that friction between pen and rotating graph paper is eliminated.