Guyton has proposed a pyrometer for measuring high temperatures, in which platina, a metal not fusible even at very intense heats, is employed as the mea sure of expansion. A rod or plate of this metal is placed horizontally in a groove' framed in a mass of hardened white clay ; one extremity of the rod is supported on the mass which terminates the groove ; the other presses against a bended lever of platina, the longest arm of which forms an index to a graduated arc. The expansion, which the rod of metal suffers from exposure to heat, is indicated by the change of position in this index. The mass of clay, being high ly baked, will not introduce any import ant error from its contraction ; and the expansion, which it may suffer during the exposure to heat, will affect only the small distance between the axis of mo tion of the index, and the point of con tact of the plate, so as rather to diminish the effect than to increase it. Platina, hav ing the important advantage of not melt ing by any heat we have to measure, and of not suffering any chemical change from it, is well adapted to the construc tion of a pyrometer.
Besides these, various metallic pyro meters have been invented, capable of measuring low temperatures, by the ex pansion being multiplied by the aid of wheels, levers, or other mechanical con trivances, or being magnified by micro scopes. Such are the pyrometers of
Muschenbroeck; that described by Fer guson ; one invented by Mr. Ellicot, with which he measured the expansions of various metals ; one by Mr. Smeaton, and applied to the same purpose ; Mr. Rams den's, superior to the preceding ones in delicacy and accuracy; Mr. Crichton's, in which advantage is taken of the dill Terence of expansion between a rod of zinc and a rod of iron, to give a curva ture to the bar composed of the united rods, proportioned to the temperature to which they are raised ; by which bend ing motion is given to an index, that, at its other extrimity, where the scale is marked, describes a considerable space; and, lastly, one by Regnier, on a princi ple somewhat similar, of which a report is presented to the French National In stitute. The strict of these in struments may, from the nature of their construction, be regarded as doubtful. It has been found, by El/icoes pyrometer, that the expansion of bars of different metals, by the same degree of heat, is as follows: