TERMINOLOGY. - It is the branch of physics that systematically studies phenomena in which heat plays a predominant part. Everyone has some notion of heat tied to the physiological sensations of hot and cold, or even of more or less hot, or more or less cold. With heat thus vaguely conceived, temperature has since ancient times been associated, and often even confused: an error that still occurs not infrequently. It is certain, however, that among physicists the two notions of heat, or rather of quantity of heat and of temperature, were already completely disentangled by the first half of the 18th century. Of fundamental importance, therefore, are the criteria for the measurement of those quantities.
I. MEASUREMENT OF TEMPERATURE
It must first of all be observed that the characteristic which drastically differentiates temperature from the usual quantities of physics — lengths, masses, forces, etc. — consists in this: that whereas each of these is measurable by means of a given standard, for the measurement of temperatures an entire scale is required. And this is precisely what Galileo, and later, better, the academicians of the Cimento, had practically done with their well-known construction of the first thermometers, more than a century before the aforementioned clarification of the notion of temperature.
Today physics has at its disposal many criteria for the measurement of temperature, but the simplest criterion, and still today the most used, is that of utilizing the notably greater thermal expansion of fluids as compared with solid bodies. Galileo and his first pupils used air as the expanding fluid, which was then advantageously replaced by liquid fluids. The devices that immediately followed assumed for the most part the form of today's mercury thermometers. To reduce the arbitrariness of the scale, it was agreed as early as 1694 (Renaldini) to designate as fixed temperatures those of melting ice and of the boiling of water, still in use today, dividing the interval into a certain number of parts. The scales called those of Réaumur and of Celsius or centigrade correspond respectively to the division of the interval into 80 and into 100 degrees.
The scales thus defined did not remain rigorously identical when the liquid employed was varied (mercury, toluene, alcohol, etc.). It was therefore agreed that, for scientific purposes, reference should always be made to the scale of the gas thermometer constructed according to particular norms and with particular accuracy. This scale was called absolute and, also because of its particular agreement with the thermodynamic point of view, thermodynamic scale. Its lowest temperature, absolute zero, corresponds to -273 degrees of the centigrade thermometer; and since in it the interval between the temperatures of melting ice and of the boiling of water was divided into 100 parts, it follows that the values T° of the absolute temperature are expressed by the formula T° = t° + 273.
II. MEASUREMENT OF QUANTITIES OF HEAT
At the basis of this fundamental measurement lies the primitive common notion that in any body a supply or a withdrawal of heat must always bring about a corresponding increase or a corresponding decrease of its temperature. To make this not entirely exact notion less vague, the condition is generally added that the supply or withdrawal of heat should produce in the body in question no other change than the variation of temperature. In that case, if the further condition is added that the variations of temperature must be small, experience confirms not only the qualitative reliability of the notion, but also demonstrates the proportionality between heat and variation of temperature.
Naturally, for the measurement of quantities of heat it is necessary to define arbitrarily, but exactly, a unit; and for this the calorie was chosen (or large calorie or kilocalorie), that is, the quantity of heat required to raise from 14° to 15° C. the temperature of one kilogram of water, which is customarily indicated by Cal.
Evidently the measurements cannot be direct, as for example those of a length by means of the unit meter, but must necessarily be indirect. The numerous devices devised for such measurements under the most varied conditions are given the name of calorimeters.
III. SPECIFIC HEATS
A calorie that, introduced into a kilogram of water, causes its temperature to rise by one degree, when introduced into a kilogram of other substances causes ever greater increases, with very rare exceptions; for example, introduced into a kilogram of copper it causes a rise of over 10°, introduced into a kilogram of mercury a rise of 30°. This makes it necessary to attribute to each substance a so-called specific heat, which physically is equivalent to the quantity of heat that must be given to the unit of mass of that substance to raise its temperature by one degree centigrade. It follows that the specific heat of water is equal to 1, that of copper to 0.095, that of mercury to 0.033, etc.
The specific heats of the elements, multiplied by their atomic weights, called atomic heats, are of great interest to modern chemistry and physics.
IV. THERMAL PHENOMENA AND THEIR APPLICATIONS
Given their great number, it is possible to touch upon them only very briefly. Changes of state: from the solid state to the liquid: fusion, always accompanied by considerable absorption of heat; for example, the fusion of one kilogram of ice requires about 80 Cal.; from the liquid state to the solid, solidification always accompanied by an inverse release of heat (latent in the liquid); from the liquid state to the aeriform, evaporation always accompanied by absorptions of heat that are sometimes enormous; for example, the conversion of one Kg. of water at 100° into one Kg. of steam at the same temperature requires 537 Cal.; conversely condensation accompanied by the release of the equivalent heat (latent in the aeriform). It is interesting to gain a clear understanding of the phenomenon of evaporation and condensation because many important applications are based on it. During evaporation, that is, as long as the vapor remains in contact with some part of its liquid, the vapor is said to be saturated and has particular characteristics that distinguish it from gases: in particular its pressure depends only on its temperature, not on its volume; if one attempts to compress it, it condenses. But as soon as the liquid has completely evaporated, the said vapor behaves like a gas and, like it, approximately follows the well-known Gay-Lussac law characteristic of gases: pressure × volume = R × absolute temperature, with R constant. Furthermore, it must be borne in mind that for every substance there exists a critical temperature above which it cannot exist in the condensed state, no matter how high the pressure to which it is subjected. This temperature for water is 374° C, for carbon dioxide 31.2° (which is why it is dangerous to keep cylinders of that substance in environments that can reach that temperature), for oxygen -116°, for hydrogen -241°. It is these findings that have made it possible, after many vain attempts, to liquefy, by the combined action of pressure and cooling, all gases, even those that were called permanent.
Finally, it deserves to be remembered that there exist thermoelectric phenomena due to interactions of heat and electricity.
Of the greatest importance are the decrease, with temperature, of the electrical resistance of metallic conductors and the analogous variation of the electromotive forces that always manifest themselves at the contact of different metals. These two categories of phenomena have been widely utilized, in particular for the measurement of temperatures, which can be obtained with devices whose sensitivity is adjustable at will, more convenient and easier to use than ordinary thermometers. They are by now used not only for scientific purposes, but have also replaced thermometers in many industrial processes, especially in cases of high temperatures.
V. HEAT ENGINES
They can be divided into machines that use heat produced in various ways to generate mechanical energy, and refrigerating machines.
Among the former, mention is made of stationary and mobile steam engines and of steam turbines, which are used to develop very great powers; internal-combustion engines powered by steam or gasoline, widely used in the automobile industry; the more powerful internal-combustion engines running on heavy oils, also called Diesel engines; the recent and extremely powerful jet engines used in military and civil aviation, which make it possible to reach speeds greater than that of sound.
Refrigerating machines, at first limited to large industrial plants, are now also spreading widely into domestic life. They all operate on the fundamental principles of thermodynamics (v.), for whose rational and practical application a vast technology has by now come into being.
Bibliography2 works
Paolo Stranco