Seismometer

A seismometer or seismograph is an instrument that measures motions of the ground, including those  generated by earthquakes, nuclear explosions and other sources. 

Records of seismic waves called seismograms allow us to map the interior of the Earth, and to find and measure the size of those different sources.

The word derives from the Greek word, seismós, a shaking or quake, from the verb σείω, seíō, to shake; and μέτρον, métron, measure. 

The technique of using a seismometer to build up a picture of the Earth's interior is called seismic tomography.Nolet, Guust (ed) 1987. Seismic tomography: seismology and exploration geophysics. Springer Netherlands. ISBN 9789027725837

References

Other websites

The history of early seismometers 
A Java code applet demonstrating the operation of a damped-mass seismometer
Keeping track 

Category:Earthquakes
Category:Measuring tools
Category:Seismology