In its new form, thermodynamics is based on the two following laws: Law I. (Davy and Joule.) When equal quantities of mechanical effect are produced by any means whatever from purely thermal sources, or lost in purely thermal effects, equal quantities of heat are put out of existence, or are generated.
Law II. (Carnot and Clausius.) If an engine be such that, when it is worked backward, the physical and mechanical agencies in every part of its motions are all reversed, it produces as much mechanical effect as can be produced by any thermodynamic engine, with the same temperatures of source and refrigerator, from a given quantity of heat.
The proof of this second law differs from that of Carnot (already given as regards reversible engines) by being no longer based on the supposition of the materiality of beat, but on the following axiom, in some of its many possible forms: It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding ,objects. It will be easily seen that the pair of engines (one reversible) before mentioned would, if worked in combination, form a perpetual motion; and, besides, would constantly trans fer beat from a colder to a warmer body.
One of the immediate and most important deductions from these principles is—that only a fraction of the heat employed in any engine is converted into useful work (the remainder being irrecoverably lost). This fraction was shown by Thomson to be capa ble of expression as S—T S ; where S and T are the temperatures of the source and condenser, measured from the absolute zero of temperature. See HEAT. Thus, an air-engine, in which a far greater range of temperature can be safely used than in a steam-engine, employs effectively a much larger portion of the heat supplied to it; and there is no doubt that air-engines would supersede steam-engines, if we could get a material capable of enduring the great heat required.
treats of the currents that arise from heating the junction of two heterogeneous conductors. Such currents can be obtained in many ways, but we shall here simply indicate the more important.
Thermal Currents with one a copper wire, cut it in two, and fix each ball in one of the bindinn. screws of a galvanometer. Heat one of the free ends to red
ness, and press it again the other, and a current will be generated, passing at the junc tion from the hot to the cold end, as shown by the deflecting needle. In almost all cases where portions of the same metal at different temperatures are pressed together a cur rent is produced, the direction of which depends on the metal, and even on the structure of the same metal.
Currents are also obtained when two portions of the same metal or piece of metal have different structures, and the point where the two structures meet is heated. If, for instance, one piece of wire be hard-drawn and the other part annealed, when the seat of change from the one to the other is heated, a current is produced. Or if the whole be annealed, and one part of it be hammered, the hammering makes the other part harder, and the current, when the junction is heated, passes from the soft to the hard part. The direction of the current differs with different metals in these circum stances. Even the difference of structure introduced by the twisting of a portion of a wire causes a current to flow when the wire is heated in the vicinity of the twist. Thus, when a knot is tied on a platinum wire, or when part of it is coiled into a spiral, a cur rent passes always toward the knot or coil when the flame of a spirit-lamp is directed on a portion of the wire near the knot or spiral. The twisting, in this case, acts as harden ing or hammering would do. By running the flame of a spirit lamp along a metal, it frequently happens, more especially if it be of a crystalline structure, that currents are produced at certain points. These points are supposed to indicate a change in structure. If a bar of fused antimony have its ends connected with a galvanometer, and examined in this way, neutral points are generally found. The flame of a lamp generates a current near these points, always passing toward the point, and changing in direction with the change of the side on which the flame is applied. Bismuth shows. neutral points, but the current always goes from the cold to the hot part across the neutral point. In bars of those metals which are crystallized regu larly and slowly, no neutral points are found.