Seismograph is the instrument that measures the magnitude of the earthquake. A sensor that identifies the vibration of the earth surface is connected to it.

The seismograph is placed firmly on the land. A weight box is hanged on the frame in a spring. The round object on the right side is called drum and it rotates from left to right like the earth. A carbon pencil is kept on the weight box which touches the drum. When the drum rotates, the pencil continuously makes round circle on it. When there is a vibration on earth, the pencil makes the curve on the drum. The length of the curve line is measured with a scale to know the magnitude.