Question

how does a seismograph work?

Answer 1

Seismographs are designed so that slight earth vibrations move the instrument. The suspended mass remains at rest as the spring absorbs  some of the motion. The stylus (pen) affixed to the mass, records the relative motion between itself and the rest of the instrument, thus recording  the ground motion.

Answer 2

A seismograph has two basic parts, an outer case that moves with the undulations of the earth and an inner part that remains stable and records the earth's movements. Some seismographs are manual but most of the seismographs record data eletcromagnetically. The seismographs is made up of a supporting structure, and a free mass, or seismometer, inside the device. The supporting structure is securely mounted to the surface of the earth. When the ground shakes during an earthquake, the structure moves as well. The seismometer is suspended inside the structure and does not move with the outer structure. This what allows the seismometer to record the strength and undulations of the earth during an earthquake.