A thermograph gives a continuous record of the temperature of the air. The sensitive element of the instrument is a bimetallic coil like invar and bronze or steel and brass welded together and rolled in the form of a thin single strip of several turns. When temperature changes, the curvature of the arc of the sensitive element changes as a result of the differential expansion of the two metals. The movement is transmitted to the pen point and recorded on a chart fixed on a revolving clock drum.
