DIFFERENTIAL THERMOMETER, an instrument for determining the difference of temperatures between two points or places. That of Sir John Leslie is one of the best known, and will be found described under THERMOMETER. His invention is now used only as a thermoscope. Howard's differential ther mometer is the most sensitive of this class of instruments. It consists of a U-shaped tube; the right-hand branch of the U being about twice as long as the other, and each arm ter minating in a closed bulb. A scale is attached to the long arm. The liquid used is ether, and the apparatus is so constructed that air is wholly excluded, the gaseous element above the level of the ether being the vapor of ether. Rumford's differential thermometer is a very flat U-tube, with closed bulbs on both of its short arms. The entire tube is filled with air except a very short section of the horizontal part of the U, where there is a ((block" of sulphuric acid. The scale is attached to this horizontal part, and differences in temperature between the two bulbs are marked by the travel of the sulphuric acid along the scale. A pair of
thermoelectric junctions (see THERMO-ELEC natcrrY) is generally preferable to any other kind of differential thermometer. One of the junctions is put at one of the points and the other junction at the other point and a galva nometer is introduced in the usual way into the circuit, great care being taken to keep all the other junctions of whatever kind that occur in the arrangement strictly at the same tempera ture. The strength of the current generated, which is measured by means of the galvanom eter, is, with the limitation that will be found explained under the article just alluded to, pro portional to the difference of temperature be tween the two thermo-junctions; and with the assistance of a previous experiment at known temperatures on the thermo-junctions employed, the absolute difference of temperature between the two points at which the junctions are placed may be determined with great exactness.