Seismometers are tools which measure motion in the ground including seismic waves generated by earthquakes, volcanoes, and other activities which might create seismic waves. Seismometers measure any movement of seismic activity in the earth and today this is done by electronic means whereby the machine measures any changes in seismic activity in the surrounding areas of the seismometer. The first seismometer is believed to have been invented by Zhang Heng of China's Han Dynasty in 132 AD.