A seismometer is a machine used for measuring earthquakes. It has a pen on a spring, held over a moving piece of paper. As the paper moves, the pen draws a straight line. If the ground shakes, the spring wobbles and the line drawn on the paper gets wiggles in it. The more the ground shakes, the bigger the wiggles. People use the Richter Scale when they talk about how big earthquakes are.

The machine is called a seismometer ("size-mom-ee-ter") and the line drawn on the paper is called a seismograph ("size-mo-graph"). Sometimes they put more than one line on each piece of paper to save paper, or to measure how the ground shakes in different directions.

A seismometer drawing a seismograph.
A seismometer drawing a seismograph.