Page:The New International Encyclopædia 1st ed. v. 19.djvu/258

* THERMODYNAMICS. 214 THERMO-ELECTRICITY. fall; after the volume has increased sufficiently, the cylinder is removed from the tank, and the inclosed substance is allowed to expand under such conditions that no heat-energy can enter, and its temperature falls: when it reaches that of the second tank the cylinder is placed in it, the piston is forced in by external force, thus reduc- ing the volume : if this process is done slowly the temperature will not rise, but heat-energy will flow out into the liquid of the tank; when the volume is sufficiently reduced, the cylinder is removed and the piston is pushed in under such conditions that no heat-energy can leave or enter; if the volume at the beginning of this last step is chosen correctly the working substance will after a sufficient compression be restored to its original condition. There have been four steps, two 'isothermal' and two 'adiabatic' The net result is: No change in the working substance ; external work ( W ) done by the sub- stance; heat-energy Qj withdrawn from the high- temperature tank; heat-energy Qj given out to the low-temperature tank. Therefore, by the first principle of thermodynamics, W = Q, — Q,. This process of Carnot's is perfectly 'rever- sible:' by doing an amount of work W on the working substance it may be made to pass around the cycle in the reverse way; the low- temperature bath losing an amount of heat-en- ergy Qj and the high-temperature bath gaining an amount Qi. The conditions for reversibility are that each 'point' of the whole cycle should be one of equilibrium ; and it is apparent that for this to be satisfied the changes must all be made slowly and that when the substance re- ceives or gives out heat-energy it must be in contact with .a large tank of liquid at the same temperature, within an infinitesimal amount, as it itself is at that instant. (If a gas expands out of a high-pressure reservoir into the open air — e.g. illuminating, gas rushing out of a burner — the process is irreversible ; if in an engine the flame is at a higher temperature than the steam in the boiler — as it always is — the process is irreversible.) It was stated by Car- W not that the efficiency of his process, ^' was the same for all working substances and de- pended alone on the temperatures of the two baths. His proof was, however, erroneoxis, and was corrected by Clausius in 1850, who showed that the statement was correct if one assumed that heat-energy of itself always passes from high to low temperature. The statement that heat-energy of itself does alwaj's pass from high to low temperature is called the 'second prin- ciple of thermodynamics.' Lord Kelvin has shown that this principle is identical in its con- clusions with the assumption that it is impos- sible by any material agency to derive mechanical efl'eot from any portion of matter by cooling it below the temperature of the coldest of the sur- rounding objects. W Since j^ is a function of the temperatures of the two tanks alone, it is possible to give such numbers to the temperatures of these tanks as will be independent of the working substance (i.e. thermometric substance). Kelvin — then William Thomson — suggested that the numbers T, and Tj be so chosen for the temjjeratures of TO T T the tanks that 7=^ = ^. In this case ]Z ^ ^zz Q,— Q, v ^2 "^2 ^1 — T^T — = 7^ and is therefore independent of every- thing except the temperatures ; T, — T^ may be T chosen to have any arbitrary value; ^ is fixed by the above definition as depending upon mak- ing the thermometric substance pass around a Carnot's cycle; and then T, and T. are obtained. It is found by direct experiment that if Tj — Tj is chosen as 100 for two tanks of melting ice and boiling water, the numbers on 'Thomson's absolute scale' are almost exactly the same as obtained in a gas thermometer using the 'abso- lute centigrade scale.' See Thermometry. Since the efficiency increases as Tj becomes smaller, and since the efficiency cannot be greater than unity, there must be a minimum temperature in the universe. This is known as the 'absolute zero;' its value is about 273° ('., as shown by the agreement between the two 'absolute scales' just described. Carnot stated further that the efficiency of an irreversible process could not be greater than that of a reversible one with the two limiting tcmjx'ratures of the tanks; and Clausius proved that this is true if the second principle of thermodynamics is true. It is generally accepted that the efficiency of an irreversible process is less than that of a re- versible one between the same two temperatures, but it cannot be regarded as a deduction from the two principles of thermodjmamics. All pro- cesses in nature — chemical, electrical, etc. — are irreversible : and a natural system will therefore be in equilibrium if all imaginable processes compatible with the existing conditions necessa- rily involve an efficiency equal to or greater than that which corresponds to a reversible process. Tliis general idea forms the basis of the modern development- "^ thermodynamics by Gibbs, Planck, and others. BinLiocRAPHY. Buckingham. Theory of Ther- moiji/naiitics (New York, 1900) ; Gibbs, "Equilib- rium of Heterogeneous Substances," in Trans- actions of the Connecticut Academy of Sei- ences, vols. ii. and iii. : Mach, Die Principien der Wiirmelehre (Leipzig, 1890) ; Planck, Thcr- modynamik (ib., 1897); Magie, Tlic Second Principle of Thermodynamics, Scientific Memoir Series, vol. vi. (New York, 1899) : Ames, The Free Expansion of Oases, Scientific Memoir Series, vol. i. (ib., 1898). THERMO-ELECTRICITY. It was observed by Seebcck in 1S22 tluit. if the two junctions of a closed metallic circuit made up of two differ- ent wires in series were at different temperatures, there was produced an electric current. As the difference of temperature at the junctions is in- creased, the current increases. It was shown, however, by Cunuuing in 182.3, that for any tem- perature of one junction there is one for the other junction such that there is no current: this is known as the temperature of 'inversion.' If in general, then, the temperatures of the junc- tions are made more and more different, the current increases, then decreases, becomes zero, and is finally reversed. The average of the temperatures of the junctions when the current