The Thermograph.—The thermograph is both a registering and a recording thermometer. The essential part of the mechanism consists of two thin strips of metal having different coefficients of expansion. The metal strips are brazed or soldered surface to surface, bent to a quadrant or curled into a coil, and annealed. The type used by the United States Weather Bureau consists of a curved flat tube filled with mercury or with alcohol. In either type of thermograph expansion causes a warping of the metal which is communicated to a lever, whose long arm is a recording pen.
The recording part of the thermograph is a drum containing a clock. The clock is geared so as to cause one nearly complete
Thermograph—nigh drum.
revolution of the drum in a week. The slight shortage of a complete revolution is an allowance for the margin of the fastening of the paper on which the record is made.
The paper strips upon which records are made are about 12 inches long. They vary in width according to requirements. Horizontal lines lithographed from engine-ruled plates divide the width of the strip into degree spaces. Arcs of circles, whose radii are the length of the pen, divide the length of the strip into day spaces, each of which is subdivided into two-hour intervals. High drum record sheets are ruled for temperatures varying from −50° to 120°; low drum strips are usually ruled from 0° to 100°.
