TERMOLOGIA. – It is the branch of physics that systematically studies phenomena in which heat plays the predominant role. Everyone has some notion of heat connected with the physiological sensations of warmth and cold, or of greater or lesser warmth, or greater or lesser cold. Heat, conceived in this vague way, has since ancient times been associated with, and often even confused with, temperature: an error that still occurs not infrequently. It is nevertheless certain that among physicists the two notions of heat—or rather of quantity of heat—and of temperature had already been completely disentangled by the first half of the eighteenth century. The criteria for measuring these quantities are therefore of fundamental importance.
I. MEASUREMENT OF TEMPERATURES. – It should first be observed that the feature that drastically distinguishes temperature from the usual physical quantities—lengths, masses, forces, etc.—consists in the fact that, whereas each of these can be measured by means of a given standard, the measurement of temperatures requires that an entire scale be established. And this is precisely what Galileo, and later, more effectively, the academicians of the Cimento, had practically done, more than a century before the aforementioned clarification of the notion of temperature, with their well-known construction of the first thermometers.
Today physics has many criteria for measuring temperature, but the simplest and still most widely used criterion is to employ the considerably greater thermal expansion of fluids than of solid bodies. Galileo and his first pupils used air as the expanding fluid, which was subsequently and advantageously replaced by liquid fluids. The instruments that immediately followed generally assumed the form of modern 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 known as those of Reamur and Celsius, or centigrade, correspond respectively to the division of the interval into 80 and 100 degrees.
The scales thus defined did not remain rigorously identical when the liquid used—mercury, toluene, alcohol, etc.—was varied. It was therefore agreed that, for scientific purposes, reference should always be made to the scale of the gas thermometer constructed according to particular standards and with due precision. This scale was called absolute and, also because of its particular agreement with the thermodynamic point of view, the thermodynamic scale. Its lowest temperature, absolute zero, corresponds to −273 degrees on the centigrade thermometer; and since in this scale the interval between the temperatures of melting ice and boiling water was divided into 100 parts, it follows that the values T^° of absolute temperature are expressed by the formula T^° = t^° + 273.
II. MEASUREMENT OF QUANTITIES OF HEAT. — Underlying this fundamental measurement is the primitive, common notion that in any body, the supplying or removal of heat must always cause a corresponding increase or corresponding decrease in its temperature.
To make this not entirely accurate notion less vague, the condition is generally added that the supplying or removal of heat should produce no other change in the body concerned than a variation in temperature. In that case, if the further condition is added that the variations in temperature should be small, experience confirms not only the qualitative reliability of the notion, but also demonstrates the proportionality between heat and variation in temperature.
Evidently, measurements cannot be direct, as, for example, those of a length by means of the unit metre, but must necessarily be indirect. The numerous devices devised for such measurements under the most varied conditions are called calorimeters.
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TERMOLI, DIOCESE of — Façade of the Cathedral (12th–13th century).
III. SPECIFIC HEATS. - A calorie which, when introduced into one kilogram of water, produces a rise of one degree in its temperature, when introduced into one kilogram of other substances produces increases that are always greater, with very rare exceptions; e.g., when introduced into one kilogram of copper it produces a rise of more than 10°, and when introduced into one kilogram of mercury a rise of 30°. This makes it necessary to assign to every substance a so-called specific heat, which physically is equivalent to the quantity of heat that must be supplied to the unit mass of that substance in order 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, and called atomic heats, are of great interest to chemistry and modern physics.
IV. THERMAL PHENOMENA AND THEIR APPLICATIONS
Given their great number, it is possible to mention them only very briefly. Changes of state: from the solid to the liquid state: melting, always accompanied by considerable absorption of heat; e.g., melting one kilogram of ice requires approximately 80 Cal.; from the liquid to the solid state, solidification, always accompanied by the corresponding release of heat (latent in the liquid); from the liquid to the gaseous state, evaporation, always accompanied by sometimes substantial absorption of heat; e.g., changing 1 kg. of water at 100° into 1 kg. of steam at the same temperature requires 537 Cal.; conversely, condensation is accompanied by the release of the equivalent heat (latent in the gaseous state). It is worthwhile to understand clearly the phenomenon of evaporation and condensation, since many important applications are based upon II. During evaporation, that is, as long as the vapor remains in contact with some portion of its liquid, the vapor is said to be saturated and has particular characteristics distinguishing 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 vapor behaves like a gas and, like a gas, approximately follows the well-known Gay-Lussac law characteristic of gases: pressure × volume = R × absolute temperature, with R constant. It should also be borne in mind that every substance has a critical temperature above which it cannot exist in the condensed state, regardless of how high a pressure is applied to II. This temperature is 374° C for water, 31.2° for carbon dioxide (and it is therefore dangerous to store cylinders of this substance in places that may reach that temperature), −116° for oxygen, and −241° for hydrogen. It is this knowledge that, after so many futile attempts, made it possible to liquefy all gases through the combined action of pressure and cooling, including those once described as permanent.Finally, it should be remembered that there are thermoelectric phenomena arising from interactions between heat and electricity.
Of the utmost importance are the decrease, with temperature, in the electrical resistance of metallic conductors and the analogous variation in the electromotive forces that always appear at the contact of different metals. These two categories of phenomena have been widely utilized, particularly for measuring temperatures, which can be accomplished with devices whose sensitivity can be adjusted at will, more convenient and easier to use than ordinary thermo-
meters. They are now employed not only for scientific purposes, but have also replaced thermo-
meters in many industrial processes, especially in cases involving high temperatures.
V. MACCHINE TERMICHE
They may be divided into machines that use heat produced in various ways to produce mechanical energy, and refrigerating machines.Among the former are fixed and mobile steam engines and steam turbines, which are employed to develop very great power; gasoline internal-combustion engines, widely used in motoring; the more powerful heavy-oil internal-combustion engines, also called Diesel engines; and the recent, extremely powerful jet engines used in military and civil aviation, which make it possible to attain speeds greater than that of sound.
Refrigerating machines, initially limited to large industrial plants, are now also making their way extensively into domestic life. All operate on the basis thermodynamics (v.), for the rational and practical application of which an imposing technology has by now been established.