ELECTROMOTIVE FOKCE.j ELECTRICITY 95 hermo- Many measurements of the electromotive force of therm o- lectro- electric couples have been made by Matthiesseu, 1 Wiede- otive maun, 2 E. Becquerel, 3 and others, but the results are of no great value owing to the effect of impurities and the want of sufficient data to determine all the thermoelectric con stants of any one couple (see below, p. 99). Numerical data, such as they are, will be found in Wiedeinann, Flecming Jeukin s Electricity and Magnetism, or Everett s Illustrations of the Centremetre-gramme-second System of Units. It will give the reader an idea of the order of the magnitudes involved to state that the electromotive force at ordinary temperatures of a BiSb couple is somewhere about 11700 C. G. S. 4 absolute units when the difference between the temperatures of the junctions is 1 C. The corresponding number for a CuFe couple is 1600 or 1700. Thermoelectric currents, or at least what may very likely be such, have been obtained in circuits other than purely metallic, e. g., in circuits containing junctions of metals and fluids, 5 metals and melted salts, 6 fluids and fluids. 7 The phenomena in all these cases are complicated, and the results more or less doubtful ; so that no useful purpose could be served by discussing the matter here. The same remark applies to the curious electrical phenomena of flames, 8 of which no proper explanation, so far as we know, has as yet been given. C p eri . The experiments of Magnus 9 have shown that in a jsnts of circuit composed entirely of one metal, every part of which agnus. is in the same state as to hardness and strain, no thermo- electromotive force can exist, no matter what the varia tions of the section or form of the conductor or what the distribution of temperature in it may be (so long as there is neither discontinuity of form nor abrupt variation of temperature). This statement is of great importance, as we shall see, in the theory of thermoelectricity. Its purport will be all the better understood if we dwell for a little on the three limitations which accompany it. The great effect of the hardness or softness and crystalline or amorphous structure of a metal on its electric properties was observed by Seebeck soon after the discovery of thermo electricity. 10 The effect of temper in wires may be shown very neatly by the following experiment due to Magnus. On a reel formed by crossing two pieces of wood are wound several turns of hard-drawn brass wire softened in a number of places adjacent to each other on the reel. The free ends of the wire being connected with a galvanometer, and the parts of the wire lying between neighbouring hard and soft portions being heated, a thermoelectric current of considerable strength is obtained, whose direction is from soft parts to hard across the heated boundaries. Effects of a similar kind were obtained with silver, steel, cadmium, copper, gold, and platinum. lu German silver, zinc, tin, and iron, the current went from hard to soft across tho hotter boundary. iicts ^ r William Thomson made a number of experiments on train, tu e effect of strain on the electric properties of metals.
The results, some of them very surprising, are contained
i m-" 1 m nis . Bakerian Lecture, 11 along with many other things of great importance for the student of thermoelectricity. Two of his experiments may be described as specimens.
- Oalv., Bd. i. 590.
1 Pogg. Ann., 1858. 3 Ann. de Ckim. et de Phys., 1864. That is, roughly, -000117, if we take for our unit the electro motive force of a Daniell s cell. 5 By Walker, Faraday, Henrici, Gore, and others; see Wiedemann, w. i. 6?9, &o. Andrews, Phil. Mag., 1837; Hankel, Wiedemann (1. c.), Gore, fM. Mag., ISiil. 7 Nobili, Wiedemann, Becquerel; see Wiedemann, L c. See Wiedemann, I. c. 3 Pogg , Ann 1851 . Pogg. Ann., 1856. n PWt Trans., 1856. They afford convenient lecture-room illustrations of the subject under discussion. (1.) A series of copper wires A, B, C, D, E, F, G, itc., are suspended from a horizontal peg. A and B, C and D, E and F, &c., are connected by short horizontal pieces of copper wire, all lying in the same horizontal line, and B and C, D and E, F and G, &c., are connected by a series of pieces lying in another horizontal line below the former. An arrangement is made by means of which the alternate wires A, C, E, G, can be more or less powerfully stretched, while B, D, F, &c., are comparatively free. A piece of hot glass is applied to heat either the upper or lower line of junctions. A thermoelectric cur rent is then observed passing from the stretched to the unstretched copper across the hot junctions. This thermo electric current increases with the traction up to the break ing point. But the most remarkable point that comes out iu such experiments is that when we free the wire after powerful traction, leaving it with a permanent set, there is still a thermoelectric current ; but the direction is now from the soft or unstrained towards the permanently strained parts across the hot region. (2.) Iron gives similar results, only the direction of the current is in each case opposite to that in the corresponding case for copper. The following experiment exhibits this in a very elegant manner. One end of a piece of carefully annealed iron wire is wound several times round a horizontal peg, the free end being slightly stretched by a small weight, and connected with one terminal of a galvanometer. The other end of the wire is wound a few times round one side of a rectangular wooden frame, the free end being stretched by a small weight and connected with the other terminal of the galvanometer. The parts of the wire on the peg or the part on the frame is then heated, and weights are hung to the frame. As the weight increases, the deflection of the galvanometer goes on increasing. If we stop a little short of rupture, and gradually decrease the weight, the deflection of the galvanometer gradually decreases to zero, changes sign before the weight is entirely removed, and finally remains at a considerable negative value when the wire is again free. These experiments of Sir Wm. Thomson s were repeated Le ROUJ by Le Roux. The results of the two experimenters are & c - not very concordant. This may be due to differences in the qualities of the materials with which they worked, or to the fact that Le Roux 12 worked at higher mean tem peratures than Thomson. 13 Le Roux also repeated the experiments of Magnus, confirming his general result, but adding the two last qualifications given above. He found, contrary to the result of Magnus, that when a lateral notch is filed in a wire and one side heated, there is in general a thermo electric current, which is greater, up to a certain limit, the deeper the notch. He also found that when two wires of the same metal, with flat ends, are pressed together, so that one forms the continuation of the other, and the wiie on one side of the junction is heated, no current is ob tained ; but he observed a current in all cases where there was dissymmetry, e.g., where an edge of one end was pressed on the flat surface of the other, where the wires overlapped or crossed, or where the chisel-shaped end of one wire fitted into a notch in the end of the other, and the axes of the wires were inclined, and so on. Whether a very abrupt variation in temperature in a continuous part of a metallic wire would produce a thermo- electromotive force is a question which possesses little physical interest, since it is impossible to realize the Abrupt varia- tempera ture. 11 Ann. de Chim. et de Phys., 1867. 13 Wiedemann, Bd. i. 610. It appears from a note at the end of Le Roux s paper (I.e.) that Sir Wm. Thomson has lately repeated
iome of his experiments and confirmed his former results.