A seismograph is a scientific instrument that detects and records earthquakes. When the ground shakes during an earthquake, even hundreds of miles away, a seismograph captures those vibrations on paper or a computer screen, creating a visual record called a seismogram.

Here's how it works: The instrument has a heavy weight suspended by a spring or wire. When an earthquake strikes, the ground (and the seismograph's frame) shakes, but the heavy weight stays relatively still due to inertia, the tendency of objects to resist changes in motion. A pen attached to the weight draws on a rotating drum or moving paper, and as the ground shakes beneath it, the pen traces out the earthquake's vibrations as a jagged line. Modern seismographs record these movements digitally.

Scientists use networks of seismographs around the world to locate earthquakes, measure their strength, and study the Earth's interior. By comparing readings from different stations, they can pinpoint exactly where an earthquake originated, even if it happened on the opposite side of the planet. The recordings also help scientists understand earthquake patterns and can help with monitoring volcanoes.