The frame moves with the ground. The relative motion between the weight and the frame is magnified by using an electromagnetic transducer and an electronic amplifier. The transducer, a coil attached to the weight, moves in the magnetic field created by a magnet attached to the frame. This movement induces an electric voltage in the coil which passes to the amplifier. The amplified voltage is recorded by a computer or by a device that marks the ground motion on a moving sheet of paper.
Some seismographs can detect ground motion as small as one-tenth of a nanometer.
A station in a quiet location may detect several earthquakes every day. Different types of seismographs are used to measure short and long seismic waves. The Benioff linear strain seismograph measures changes in distance between two piers attached to the ground. The strong motion accelerograph records shaking that is too strong for sensitive instruments. There are more than 1,000 seismograph stations located throughout the world.