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Thermoscope

temperature, alloys and melting

THERMOSCOPE (Greek, *to show heat"), any instrument for indicating tempera ture. The term is commonly applied, however, to such instruments as indicate one tempera ture only, or a very limited number of temper atures; or to those which are used for indicat ing changes or difference of temperature, with out giving the magnitude of these changes or differences. The forms that have been given to instruments of this kind are so manifold as to be almost past numeration. As a single illus tration, the instrument may be cited, whose in dications depend upon the melting points of alloys. An instrument or device of this sort contains buttons or wires of a number of alloys, whose several melting points are known; and in the observation of temperature by this method, the instrument is exposed to the temperature tinder examination, and a note is made of which of the alloys melt, and of which remain un melted. If T. is the melting point of the least fusible of the alloys that have melted, and T.

is the melting point of the most fusible of the alloys that have not melted, we can then assert that the temperature under consideration is higher than T,, but lower than T.. For many purposes in the arts, it is quite sufficient to know, in this manner, that a temperature is between certain limits. A thermoscope which can only indicate certain limits between which a temperature lies is called a °discontinuous thermoscope." Continuous thermoscopes, which arc capable of affording an actual measurement of any temperature within their range, are now commonly called °thermometers," whether they resemble the ordinary mercury-in-glass ther mometer or not. A thermoscope in which tem perature is inferred by noting the electrical re sistance of a coil of platinum wire, for ex ample, is called a *platinum resistance ther mometer.* See PYROMETER; THERMOMETER; and THERMOMETRY.