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# Thermometry

## temperature, scale and time

THERMOMETRY, the art of measuring temperatures. The "measurement" of temper ature is quite a different thing from the meas urement of a time, or a length, or a mass, and it consists merely in assigning to each temper ature that may come up for consideration a definite place upon some sort of a numerical scale. The scale itself may be perfectly arbi trary, so that an interval of temperature upon one part of the scale cannot be said to be "equal," in any physical sense, to an interval on some other part, even though the two are ex pressed by the same number of °degrees." The chief essentials of a practical thermemetric scale are (1) that it shall be perfectly definite, so that when the same temperature is "measured" on several different occasions, the same identi cal result will be obtained each time, at least to a degree of approximation sufficient for the purposes for which the temperature is being determined; and (2) that it shall be possible for two or more different observers, provided with distinct instruments of measurement, to measure the same temperature, and obtain re sults that are identical, at least to the same degree of approximation as noted above. So

long as these essential conditions are fulfilled, we may make use, for the purpose of establish ing a thermometric scale, of any measurable property of matter, which varies in a determi nate way with temperature; the "temperature' in any such case, being defined as proportional to the atttribute that is measured, or to any continuous function of that attribute. We may, therefore, have as many different "scales" of temperature as we please, and any one of these will be just as defensible, and just as "correct" as any other one, although no two of them will be in perfect agreement. In practice it is found that four particular kinds of thermo metric scales are e:-.pecially useful. These are based, respectively, upon (1) the expansion of some substance that is subjected to an unvary ing pressure; (2) the increase in pressure of a gas which is kept rigorously constant in vol time; (3) the variation of the electrical resist ance of a conductor; and (4) the electromo tive force of a thermoelectric couple (see . _