Seismograph
Seismograph, an instrument designed to measure and record vibrations in the earth. Seismographs are used to study earthquakes, to detect and measure the intensity of nuclear explosions, and to analyze vibrations caused by detonations used in prospecting for oil. Most seismographs can measure vibrations in one direction only, so at least three (one for up-and-down motion, one for east-west, and one for north-south) are required to describe completely the earth's motion during an earthquake or following a nuclear blast.
To measure a vibration of any kind, it is necessary to have a stationary reference against which to make the measurement. In most seismographs an inertial mass, such as a pendulum bob, serves as the reference. The frame of the seismograph is securely mounted on the earth and vibrates with it. The bob tends to remain motionless, and the resulting relative motion between the frame and the bob is recorded. In the simplest form of pendulum seismograph, there is a writing instrument, such as a pen or stylus, attached to the pendulum bob. The writing tool marks a revolving drum on the frame. Most seismometers today use various electrical measuring devices. With one such device, vibrations of the earth cause a magnet attached to the frame to move with respect to a coil attached to the bob. The motion of the magnet generates in the coil an electric current whose strength is proportional to the intensity of the vibrations.
Seismographs that measure up-and-down motion usually consist of an inertial mass suspended by a spring from a horizontal arm. The relative motion of the frame and the mass is the measurement actually made.