A self-recording thermometer. The thermometric element is most commonly either a bimetal strip or a Bourdon tube filled with a liquid. In the first case the bimetal element has the form of a helical coil with one end rigidly fastened to the instrument and the other to the recording pen. In the second case the tube is made with an elliptical cross section so that an expression of the liquid caused by a temperature increase will cause the radius or curvature of the bend to increase, thus moving the instrument pen, which is fastened to the tip of the tube. A resistance thermometer and a thermoelectric thermometer may be converted into thermographs if provision is made to record their output. See aspiration thermograph, hygrothermograph, mercury-in-steel thermometer.