Seismograph

A seismograph is an instrument, built around a seismometer, that responds to the ground displacement and shaking caused by earthquakes, volcanic eruptions and explosions, and records that motion as a seismogram used to locate quakes and study the Earth's interior. Its basic principle is inertia: a weight is suspended from a spring on a frame that moves with the ground, and because the weight resists that motion, the relative movement between the stationary weight and the moving frame reveals how strongly the ground is shaking; early instruments scratched this onto paper with a mechanical pen, while modern ones use electronic sensors recording motion along several axes. The first such device was built in China around AD 132 by the polymath Zhang Heng, a bronze instrument called the Houfeng Didong Yi said to detect the direction of a distant earthquake, but the modern seismograph dates to 1880, when John Milne, James Alfred Ewing and Thomas Gray built the first horizontal pendulum seismometer while working in Japan. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/