seismographs [1]
Instruments that record seismic wave amplitude at a given point on the Earth’s surface.
mass 
Independent of ground movement, it remains stationary during an earthquake, thus serving as a reference for measuring the amplitude of tremors.
rotating drum 
Secured to the ground, it rotates under the pen, recording ground movements on paper.
seismogram 
Graphic representation produced by a seismograph; the stronger the tremors, the greater the oscillations on the paper.