"Why have I stopped writing? I'd rather be a lightning rod than a seismograph..."
Ken Kesey, writer
During an earthquake the ground moves up and down and from side to side as a result of a release of energy from the Earth's crust. The seismograph is an instrument that continuously records this movement (seismic waves) as a function of time. Crude seismoscopes were invented by the Chinese around 132 C.E. but these merely indicated the direction of the earthquake's epicenter. Later Iranian and Italian instruments containing mercury baths that spilled in measured ways when the earthquake occurred indicated both direction of source and magnitude of movement, but not time of occurrence.
British scientists Sir James Alfred Ewing, Thomas Gray, and John Milne (1850-1913) studied earthquakes and devices to record them whilst working in Japan. This resulted in the invention of Milne's horizontal pendulum seismograph in 1880. The idea of having networks of standard seismographs all over the world was promoted by Milne in the late 1800s.
Seismographs are designed to measure the movement of the Earth's crust during a quake. The main component is a large inertial mass suspended by a spring in a frame attached to the bedrock. When the ground shakes, the frame moves and the inertial mass does not. The varying distance between the mass and the frame is recorded, and this provides details of the quake. Initially the recording was done by pen on a moving paper chart, but today digital records are produced. Seismograph networks led to the discovery of the Earth's liquid core in 1905, the discontinuity between the crust and mantle in 1909, and the small solid inner core in 1936.
