ELECTRO-MAGNETISM. 



of a horseshoe magnet, placed vertically, 

 with the poles uppermost (that is, fixed 

 as in Jig- 50), and a lamp being placed 

 halfway between the poles, each of the 

 circles will revolve by the action of that 

 pole which is exterior to it. 



(309.) It appears from these and 

 other experiments of a similar kind*, 

 that the mutual action of a magnet and 

 a thermo-electric current is subject to 

 the same laws as those of magnets and 

 galvanic currents ; and hence all the 

 phenomena of the attraction, repul- 

 sion, or rotation of conductors conveying 

 galvanic currents, may be exhibited by 

 a thermo-electric current transmitted 

 through the same conductors. 



(310.) The two metals of which the 

 combination and contact produce the 

 most powerful thermo-electric currents, 

 are antimony and bismuth, which we 

 shall accordingly take as the representa- 

 tives of their respective classes. As in 

 the galvanic circuits, where the current 

 of positive electricity is flowing directly 

 from the copper to the zinc (see Gal- 

 vanism, $4), the latter is generally said 

 to be positive with respect to the former 

 (see Galvanism, 73) ; so, in the ther- 

 mo-electric circuit, the bismuth is gene- 

 rally said to be positive with regard to 

 the antimony, because, in the colder 

 portion of the circuit, the electric cur- 

 rent is passing from the antimony to the 

 bismutht. 



(311.) In the Treatise on Galvanism, 

 we have given ($ 73) a list of metallic 

 substances in the order of their oxidabi- 

 lities, or rather in the order of their 

 electrical relations, when united in gal- 

 vanic circuits, wilh interposed acids. A 

 number of experiments have been made 

 by Professor Gumming, for the purpose 

 of determining the comparative thermo- 

 electric relations of the different metals 

 by forming circuits of them taken in 

 pairs. From these the following series 

 has been deduced, descending from the 

 extreme positive, as we have already 

 defined it, which is bismuth, to the ex- 

 treme negative, which is antimony. 



Bismuth, 

 Mercury, i 

 Nickel, } 

 PI at in a, 

 Palladium, 



Cobalt, 1 

 Manganese, / 

 Tin, 

 Lead, 

 Brass, 



* For a more detailed account of these, see Cum- 

 ming's Manual, already quoted. 



t There is still some confusion in the application 

 of the terms positive and negative to the different 

 parts of the circuit, by different authors, and some- 

 times even, by the same author iu different i>Jaces, 



Rhodium, 



Gold, 



Copper, 



Silver, 



Zinc, 



Cadmium, 



Charcoal, T 



Plumbago,/ 



Iron, 



Arsenic, 



Antimony. 



(312.) Tn this series every metal is 

 positive with respect to all those below 

 it, and negative to those above. Hence 

 any two metals occupying situations 

 intermediate between the two extremes 

 will together compose a thermo-electric 

 circuit similar to, though of less power 

 than that formed by bismuth and anti- 

 mony. 



(313.) The order in which the metals 

 stand in the above series does not con- 

 tinue the same at all temperatures: thus 

 gold, silver, copper, brass, and zinc, 

 should be placed below iron in high tem- 

 peratures, though they rank above it in 

 a low heat. 



(314.) On comparing the series above 

 given, which represents the thermo-elec- 

 tric relations of the metals, with that 

 given in the Treatise on Galvanism, 

 73, which represents their galvanic re- 

 lations, it will be evident that there is no 

 correspondence between them. 



(315.) The contact of the metals in 

 these experiments is most completely 

 secured by soldering them together ; but 

 in most cases it will be sufficient, if they 

 are in the form of wires, to twist them 

 closely together. Mercury may be con- 

 veniently employed as the intermedium 

 between other metals : the mercury be- 

 ing previously heated, the extremities of 

 each piece composing the pair being 

 dipped into it at the same moment, and 

 the other extremities being applied to a 

 galvanometer. Even a small fragment 

 of any metallic substance is sufficient to 

 afford indications of its thermo-electric 

 relations, if placed upon a disc of the 

 metal with which it is to be compared, 

 and touched with a hot wire ; the circuit 

 being completed through the wire and 

 the disc. But it is found that the results 

 of experiments so made do not always 

 accord with those obtained by employing 

 larger pieces, for it appears that the 

 effect is much influenced by the relative 

 dimensions of the heated surfaces. Even 

 \vhen the experiment is made by plung- 

 ing the metals to be examined in heated 

 mercury, the direction of the current 

 will be determined, in many cases, by 

 the order in which they have been im- 

 mersed. 



(316.) Considerable diversities take 



