What is a seismometer? What are seismographs? How do they work?

A seismometer is the internal part of the seismograph, which may be a pendulum or a mass mounted on a spring; however, it is often used synonymously with "seismograph". Seismographs are instruments used to record the motion of the ground during an EQ--installed in the ground throughout the world and operate as seismographic network. The first one was developed in 1890. The earliest "seismoscope" was invented by the Chinese philosopher Chang Heng in A.D. 132. This did not record earthquakes, however. It only indicated that there was one occurring.

A seismograph is securely mounted onto the surface of the earth so that when the earth shakes, the entire unit shakes with it, EXCEPT for the mass on the spring which has inertia, and remains in the same place. As the seismograph shakes under (in the example below) the mass, the recording device on the mass records the realtive motion between itself and the rest of the instrument, thus recording the ground motion. In reality, these mechanisms are no longer manual, but instead work by measuring electronic changes produced by the motion of the ground with respect to the mass.