100 Mr R. Adie's Experiments with Thermo- and 



in connection with hard antimony, are not so effective for thermo-bars, 

 as these metals in a soft quiescent state. 



33. When thermo-electrical currents developed in wires or thin pieces 

 of one metal are examined, an opposite order prevails ; for the further a 

 portion of the metal can be forced from its natural density, the better will 

 the arrangement answer as a thermo-couple; a hard and soft piece of any 

 metal form a pair ©f elements, and their point of junction corresponds 

 to an ordinary thermo-joint. A slip of watch-spring with a part in^ts 

 centre hardened by heating and quenching in water, is a good specimen 

 of this kind of thermo coupb; for by heating the point of junction of the 

 two densities only 10° above the adjoining parts, distinct evidence of a 

 thermo-electrical current is given. 



M. Becquerel has shewn, that, when a portion of wire is hammered or 

 twisted, a current of electricity is found passing when the part is heated ; 

 here the density of the metal has been changed, and the galvanometer 

 merely tells, that, in all probability, the texture of the wire is slowly equaliz- 

 ing; however this may be, the experiment suggests an application of the gal- 

 vanometer j ivMch renders it a most delicate test of any change in the density 

 of metals. For this purpose, a wire of the metal which the operator de- 

 sires to test, take for example iron, is repeatedly drawn through the 

 draw-plate to harden it; this hardened wire is then fded into two halves ; 

 the contiguous parts where it has been cut are now tied together with- 

 out twisting, and the necessary connections of the other extremities 

 made with the galvanometer. On heating their points of contact, no elec- 

 tricity is detected. But when one of the halves of the wire has been an- 

 nealed for three weeks at 400°, then again tied to the hard half, and the 

 foregoing experiment repeated, the galvanometer immediately shews the 

 passage of a current from the soft half to the hard, thus proving that 

 the temperature of 400° has effected a change in the iron, which so far 

 as I have been able to test, cannot be detected either by examining the 

 fracture or trying the specific gravity. 



34. The direction of the thermo-electrical current developed betwixt 

 hard and soft portions of the same metal, either in two separate pieces, 

 or, what is more convenient, in one continuous piece, where the nature 

 of the metal examined admits of this, was now tested in bismuth and 7 of 

 the malleable metals, and for all of them found to be from the side of the 

 joint which was losing density to the hard part of the wire or bar. Steel, 

 hardened and made dense by hammering, agreed with the above 8 metals. 

 But steel, hardened by quenching in water, and antimony cast in a cold 

 mould, and formed into couples, with soft steel for the former, and anti- 

 mony cast in a mould heated nearly to redness, for the latter, the direc- 

 tion of the electrical current, with reference to hardness, appeared to be 

 reversed ; yet there is no change, when examined, with reference to the 

 action in the joint, for in the two last cases the hard portions are gaining 

 density y while in the 8 other metals the hard portions lose density. 



The consideration of these experiments with metals in different states 

 of density, together with the experiment described (30), where an ener- 

 getic thermo-current appears to be produced in a vast quantity, from a 



