Seismographs record earthquakes. They record vibrations in the Earth, and they can detect both large and small earthquakes. Seismographs working together can also pinpoint the location of an earthquake.
Seismographs come in various forms, although modern versions usually work electromagnetically. The general principle in these devices and all other forms of seismograph is the same though. This means it consists of an element to detect the vibrations. This is called a seismometer. It works by having an element that remains fixed, even while the earth is shaking. So in an electromagnetic seismograph, the magnet remains in a fixed position. The case around it has sensors so movements of the case can be measured against the fixed position of the magnet.
The other elements that make up a seismograph are the amplifier and the display panel. They measure the waves that come from an earthquake and can tell the general direction of where the earthquake happened, the magnitude of the earthquake and the character of the wave motion. When this information is combined with readings from seismographs in other locations, the exact epicenter of an earthquake can be determined, usually using triangulation.
Seismographs are sensitive enough to detect earthquakes that are too small to be felt by people.