Seismographs depend on one of the laws of motion developed by Isaac Newton, who lived from 1642 to 1727. The law basically states that any object at rest tends to stay at rest until it is moved.
Seismographs are the instruments scientists most often use to measure and study earthquakes.
A simple seismograph consists of a heavy weight that hangs free and a revolving drum covered in paper on which Earth’s movements are recorded. The drum is solidly fastened to bedrock.
When an earth tremor occurs, the drum moves. The weight, however, remains steady, and a marker attached to it draws on the paper (already marked in segments of time), as the drum jiggles.
When the drum moves, because the bedrock is moving, the marker draws wavelike lines. The more the drum moves, the larger the wavy marks grow.
The most modern seismographs respond to Earth’s natural electrical currents, which are disturbed by tremors, to move the drum. The electrical signals can be amplified to record even the slightest movements.
Lasers and light-sensitive paper replace the markers and regular paper, thereby cutting down on distortion caused by the friction of a marker actually touching the paper’s surface.