98 BEST FORMS OF CONSTRUCTING THERMOMETRIC PAIRS. 



a, jig. 35, oe a bar of antimony, and b a bar of bismuth ; let them be soldered together 

 along the line c d, and at the point d let the temperature be raided; a current is inmie- 

 diately excited ; but this does not pass round the bars a, b, inasmjuch as it finds a shorten 

 and readier channel through the metals, between c and d, circulating, therefore, as in- 

 dicated by the arrows. Nor will the whole current pass round the -bars until the tem- 

 perature of the soldered surface has become uniform. 



379. An obvious improvement in such a combination is shown in Jig. 36, which 

 consists of the former arrangement cut out along the dotted lines : here the whole cur- 

 rent, so soon as it exists, is forced to pass along the bars ; and because the mass of 

 metal has been diminished at the line of junction, such a pair will change its tempera- 

 ture very quickly. 



380. One of the very best forms for a thermo-electric couple is given in Jig. 37, where 

 a is a semi-cylindrical bar of antimony, b one of bismuth, united together by the opposite 

 corners of a lozenge-shaped piece of copper, c. From its exposing so much surface, 

 the copper becomes hot and cold with the greatest promptitude, and, from its good con- 

 ducting power, it may be made very thin without injury to the current. With a pair 

 of bars three fourths of an inch thick, and a circular copper plate, c, having both sur- 

 faces blackened, I have repeated the greater part of those experiments which M. MEL- 

 LONI made with his multiplier. 



381. The currents which circulate in a steel magnet are to all appearance perpetual. 

 I thought for some time it might be possible to procure similar perpetual currents by 

 compound thermo-electric arrangements. Fig. 38 will serve to show the character of 

 these combinations, and also the cause of their failure. Let a, b, c, be wires of three 

 different metals, soldered together so as to form a triangle. Now, if these metals were 

 selected, so that a and b could form a more powerful thermo-electric pair than a and c 

 or b and c, it might be expected that at all temperatures an incessant current would run 

 round the system. Such, however, will not be found to be the case. In effect, any 

 one of these three serves simply as a connecting solder to the other two, and hence no 

 current is excited ; for the ends that have the third metal between them, although that 

 metal intervenes, are under exactly the same condition as the other ends which are in 

 contact. 



382. Thermo-electric currents, evolved by pairs of different metals, do not appear to 

 differ specifically. As different gases during combustion burn with differently-coloured 

 flames, and as different sources of caloric evolve rays of heat which are absorbed dif- 

 ferently by different media, it might be expected that a pair of wires of copper and pla- 

 tina would give out a current of electricity unlike that of iron and palladium. I have 

 made many trials on this point, adjusting a wire of copper and one of lead to each other, 

 so as to stop equal quantities of electricity flowing from a pair of copper and platina, 

 the galvanometer needles being brought to the same point, whether the long wire of 

 copper or the short wire of lead was employed. But, in the case of every combination 

 which I tried, these two wires acted alike, nor could I ever evolve a current which 

 would pass with more or less resistance along the lead than along the copper. 



