A seismometer measures movements in the ground during an earthquake and creates electric signals that can be read to determine an earthquake's magnitude. Seismometers are contained within a device called a seismograph that is attached to the ground.
When the Earth's surface shifts during an earthquake, a suspended magnet within the seismometer responds to the movements of the ground while coils of wire in the surrounding case remain still. The coils then generate electric signals based on movements of the magnet relative to the case. These signals can be sent to a computer or written on paper to create a seismogram.
Seismographs are very accurate and can pick up seismic waves that a human is unable to feel on his own. However, they can only detect movement in one direction, so many seismograph stations have several seismographs set up to record north-south, east-west and vertical motions. Using several stations over many miles and knowing that seismic waves get weaker the farther they travel from the source, it is possible to estimate the distance, magnitude, direction and type of earthquake that has occurred. This is accomplished by measuring the difference in time it takes for different seismic waves to reach a detector.