thermograph (thermograf)

[ cording pen. To an iron frame (see the cut) are fastened the thermometer-strips, the clock, the adjustments of the recording lever, and the perforated protecting case. The clock rotates a metallic disk once a week. A paper chart is fastened to the disk and rotates with it. The chart is divided into fourteen equal spaces, the dark spaces indicating night-time. These spaces are subdivided to indicate hours. The recording lever traces with an ink pen a line upon the paper chart, according as the metallic thermometer bends as affected by the heat or cold. The IMG:content-0247.png:[merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors] PS SF Thermograph. A, clock-arbor; C, clock-box; D, ink pen; F, F, ares; L, recording lever; N, N, metallic thermometer-strips; P, P, platinum wires; R, piece for holding thermometer-strips to frame: S, S, screws for adjusting recording lever; W, winding arbors of clock; X, X, screw holes for fastening instrument in place or in packing-box. thermometer is composed of two strips of metal of different expansibilities. The curve thus traced over the concentric lines of the paper chart which indicate degrees thermograph enables the temperature at any time during the week and the rate of variation to be accurately determined. (c) In the electric-contact method a mercurial thermometer having a large bulb and an enlarged stem has the upper end of the tube left open, and a fine platinum wire is made to descend in the tube by clockwork at regular intervals. When the wire comes in contact with the top of the mercury, an electric circuit is closed, and the distance is registered which the platinum wire has descended in order to touch the mercury surface. This method is used in the instruments of Hough and Secchi. (d) In the manometer thermograph the actuating instrument is an air or gas-thermometer. The ve el containing air is connected by a fine tube with a registering apparatus, of which various forms have been devised. Changes of temperature produce changes of pre ure in the inclosed gas, and these changes of pre ure are the subject of measurement and registration. The scale of the thermogram is evaluated in degrees either by a theoretical formula or by actual comparisons. The instruments of Schreiber and Sprung be long to this cla . (e) A still further form, not belonging strictly to any of the preceding cla es, is illustrated by the Richard thermograph. Its thermometer is a Bourdon tube filled with alcohol, to which is attached a lever carrying the registering pen. With a rise of temperature the differential expansion produces a change of shape of the tube, accompanied by a corresponding change in pos i tion of the lever and registering pen. A high degree of sensitivene and consequent accuracy is attained by this instrument.