This seismograph records ground movements using an electromagnetic motion sensor. The basic components of the motion sensor are a suspended coil of wire and a magnet that’s attached to the floor. When the floor moves, the magnet moves with it, inducing a small electrical current in the coil. An amplifier then boosts this current so it’s strong enough to move the pen on the revolving drum, creating a graph of the motion—also known as a seismograph.
Seismographs measure not only the seismic waves emanating from an earthquake, but also their precise time of arrival. By comparing the data collected by several seismographs in different places, scientists can determine the true strength of an earthquake and locate its starting point, or epicenter.