A seismograph is an instrument that detects the presence of an earthquake, measures, records its magnitude, and a seismometer is an instrument used to measure vibrations caused by an earthquake. Seismographs which have two parts, a sensor which detects ground motion which is called the seismometer. The second part of the seismograph is called the seismic recording system which is related to or caused by an earthquake or earth tremor.  Most seismometers are sensitive devices and the basic idea behind measuring ground movement can be demonstrated by using a simpler physical system that is actually quite similar to some of the earliest seismograph systems. Seismometers are usually made to record signals to discuss instruments based on the range of vibration periods so it is well-located to talk about instruments based on the range of vibration that they can detect. There are two parts of a seismometer. One of the parts of the seismometer is called the inertial system which is a movable mass, coupled to a rigid case by a spring and a damping device. The second part of the seismometer is a transducer system that produces an output signal, corresponding to the relative motion of the case with respect to the mass, serving as a fixed reference point, because of its great inertia. Advances in seismograph technology have increased our understanding both earthquakes and the Earth itself.