which instrument is used to measure the earthquake? and also say the conditions applied in using the instrument.
A seismometer is used to detect the waves of an earthquake. The idea behind a seismometer is fairly simple- it's just a weight attached to a frame. Motions of the ground cause the frame to move, and any movements of the frame relative to the weight are translated into electrical signals. Most seismometers are permanently attached to a recording device; the combination is called a seismograph.
The main condition for a seismograph to function correctly is that it needs to be in direct contact with the ground, and needs to be located in a place isolated from non-earthquake vibrations. For this reason a location far from busy roads and other sources of man made vibration is necessary. Many seismograph stations have been located in old mines, because they provide an ideal setting.
Earthquakes are detected by a seismometer which is connected to a graph (recording instrument). Collectively, the machine is called a seismograph. The seismometer detects movements of the earth and these movements are recorded by the seismograph. A weight attached to a frame accomplishes this.
Correct functioning of the apparatus is when it detects movement and vibrations that are not associated with extraneous movement like from passing cars or trains. Many seismographs are found in very isolated and desolate locations for this reason. The device must be able to sense and record movements that are not produced from normal day to day activities.