The measurement of a physical quantity implies, generally, the numerical comparison of the quantity with a certain selected quantity of the same kind taken as a unit. Temperature, however, can not be treated as a quantity in the same sense. It is rather to be considered as a state in which matter is found, and all temperature measurements are made by comparing the changes produced by heat in some form of matter. As shown by Lord Kelvin as early as 1848, temperature may be expressed on a scale

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... ich is independent of any particular form of matter, but this thermodynamic scale can not be used in actual temperature measurements, which, in practice, consist in comparing the changes in some particular forms of ma:ter produced by heat.. Certain gases change in volume under constant pressure or change in pressure under constant volume in a nearly regular manner with equal increments of temperature, as estimated on the thermodynamic scale. Gas thermometers have, therefore, naturally been chosen as standards wvith which to compare the changes in various forms of matter, which changes may then serve as a convenient means of temperature measurement. It is probable that all matter exhibits more or less change with even slight temperature variations. Upon no physical phenomenon are human affairs more dependent than upon temperature and its relatively small natural variations. It is owing to the fortuitous circumstance that our planet now continues at about the one twenty-fifth of its probable past temperature that life as we know it can exist. But only in the last century 473 474 NORTHRUP: ELECTRICAL [Miay.30 has the science of thermometry become clearly understood. Its principles are now well known, but it still needs to be further applied in our scientific enquiries and in many of our industries which are largely dependent upon it. Recent developments in producing immense ranges of temperature, from the cold of solid hydrogen, where the last motions have been nearly extracted from it, to 'the all-matter fusing heat of the electric furnace, have called for many new developments in the science and art of thermometry. It is now recognized that many industries, as the making of iron, porcelain, and glass, and many chemicals, can be greatly improved by using methods for accurately measuring temperatures. A knowledge and control of the temperature of storage places, as of grain, tobacco, ice, food-products, and the like, is often required. This puts a demand upon thermometry of measuring the temperature in one or many places, but observing or recording it in a place which is distant from where it is taken. These and many other requirements have demanded and obtained, quite recently, an increase in methods, a refinement in precision, and an extension in range of temperature measurement.