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temperature, thermometer and strip

THERMOGRAPH (Greek, "heat-writing)), any form of self-registering thermometer (q.v.), by which an automatic record of varia tions of temperature is kept. Many different types of thermograph have been made, of which the following may be especially noted: (1) Photographic thermographs, in which the posi tion of the mercury thread in an ordinary ther mometer is photographed upon a moving sen sitive film, either continuously or at short in tervals of time; the moving sensitive film be ing actuated by clockwork, so that the time at which any given impression was made can be nicely determined. (2) Metallic-strip thermo graphs, in which a recording pen is actuated by a strip of metal composed of two substances of differing expansibility, riveted or soldered together. When a strip of this kind is heated, one of its sides expands more than the other, and the is that the strip becomes curved by an amount which serves as a measure of the temperature to which the strip has been exposed. The pen which makes the record moves radially on a disc of paper which is caused to revolve at a steady rate by means of clockwork. (3) Electric-contact thermo

graphs, in which a fine platinum wire is caused to descend, at intervals, into the open upper end of the capillary tube of a sensitive mercu rial thermometer. When the wire touches the mercury column, it completes an electricaLcir cuit, and by this means the position of the mercury thread in the thermometer is recorded. (4) The manometric thermograph, in which the pressure in a closed vessel filled with a gas is taken as the index of the temperature; the pressure being recorded automatically, and the temperature being afterward inferred from the recorded pressure, by means of a theoretical formula, or else by direct comparison of the in strument, at different temperatures, with a standard thermometer. (The pressure of an isolated mass of gas of this kind is known to be sensibly proportional to the absolute tem perature of the gas, so long as the volume is kept constant).