what does a seismogragh measure?

Answer 1

It measures the acceleration (back & forth motion) of the ground. The needle is displaced by an amount proportional to how far the ground heaved each time. By analyzing how big each "heave" was and how long it took, and comparing that to simultaneous measurements made by other seismographs far away, seismologists can calculate the strength and location of the disturbance (usually an earthquake) that caused the heaving.