Page:Collier's New Encyclopedia v. 09.djvu/410

LEFT THERMODYNAMICS 348 THERMOELECTRICITY means of inanimate material agency to derive mechanical effect from any portion of matter by cooling it below the temper- ature of the coldest of the surrounding objects. This is the law on which Car- not's principle is based — a principle which has led to results of the highest con- sequence. The principle is that the effi- ciency of a reversible engine is the great- est that can be obtained from a given range of temperature. Now a reversible engine in Carnot's sense is an altogether unrealizable heat engine, which can be made to go through a complete cycle of operations, either forward or backward. In other words, not merely is the engine able to do work while it transforms a given quantity of heat from the boiler to the condenser, but, by an expenditure of an equivalent quantity of work upon it, may be made to take back the same quantity of heat from the condenser to the boiler. In subjecting such an engine to a cycle of operations, the engine must be brought back to its original condition before any conclusion can be drawn re- garding the relation between the heat which has disappeared and the work which has been done. Seguin, when he assumed that the work done by an expanding heated body was the equivalent of the heat which it loses, and Mayer, when he went to work on the hypothesis that the amount of heat produced in compressing a gas is equivalent to the work done in compres- sion, violated this principle, so that their conclusions were logically untrustwor- thy. It is easily demonstrable on the conservation principle that Carnot's re- versible engine is the most perfect pos- sible engine, and that consequently all reversible engines working between the same temperatures have the same ef- ficiency. For a small difference of tem- perature the efficiency is a function only of the temperature, and this efficiency, divided by the difference of tempera- tures, is called Carnot's function. Thom- son, defining temperature as the recipro- cal of Carnot's function, has construct- ed a scale of temperature absolutely independent of the nature of the ther- mometric substance. Hence if t is the temperature measured according to this absolute scale of the source of heat, and t' that of the refrigerator or condenser, the efficiency may be expressed by the fraction. Now the efficiency is de- t fined as the ratio of the work done to the heat supplied expressed in dynami- cal measure. Hence, if W is the work done, H the heat supplied to the engine, and h the heat given out to the condenser, all expressed in dynamical measure we have W t—t' H t W = H-h h t' whence — = — H t or, in a reversible engine, the heat re- jected is to the heat received as the abso- lute temperature of the refrigerator is to the absolute temperature of the boiler. From the first of these equations it is evident that the heat supplied cannot be wholly transformed into work unless the refrigerator is at absolute zero of tem- perature, a practical impossibility. Con- sequently, in a material system only a part of the intrinsic energy is available for work, and this available portion or entropy is continually diminishing be- cause of the universal tendency of heat to diffuse itself, and reduce the system to a uniform temperature, when of course no work can be produced. This is Thom- son's principle of the dissipation of en- ergy. THERMOELECTRICITY, electricity produced at the junction of two metals, or at a point where a molecular change occurs in a bar of the same metal, when the junction or point is heated above or cooled below the general temperature of the conductor. Thus when wires or bars of metal or different kinds, as bismuth and antimony, are placed in close con- tact, end to end, and disposed so as to form a periphery or continuous circuit, and heat then applied to the ends or junctions of the bars, electric currents are produced. The thermoelectric battery, or pile, an apparatus much used in delicate experiments with radiant heat, consists of a series of little bars of antimony and bismuth (or any other two metals of different heat-conducting power), having their ends soldered together and arranged in a compact form; the oppo- site end of the pile being connected with a galvanometer, which is very sensibly affected by the electric current induced in the system of bars when exposed to the slightest variations of temperatvire. To the combined arrangement of pile and galvanometer the name of thermo- multiplier is given. Two metal bars of different heat conducting power having their ends soldered together, and the combined bar then usually bent into a more or less horseshoe or magnet form for the purpose of bringing their free ends within a conveniently short dis- tance, designated a thermoelectric pair, are much used in thermoelectric experi-