GENERAL SUMMARY. 23,3 



through the wire to . Hence in this thermopolar arrangement 

 the bismuth is the negative metal, and may be compared to the 

 copper in the voltaic cell. If cold instead of heat be applied to 

 c, a current also is established, but in an opposite direction to 

 the former. Similar circuits may be formed of other metals, 

 which may be arranged in the following order, the most power- 

 ful combination being formed of those metals which are most 

 distant from each other in the following enumeration : bismuth, 

 platinum, lead, tin, copper or silver, zinc, iron, antimony. 

 When heated together, the current proceeds through the wire 

 from those which stand first to the last. According tc Nobili, 

 similar circuits may be formed with substances of which the 

 conducting power is lower than that of the metals. 



Several pairs of bismuth and antimony bars may be asso- 

 ciated as in Fig. 23, and the extreme bars being connected 

 FIG. 23. by a wire, form an arrangement re- 



sembling a compound voltaic circle. 

 Upon heating the upper junctions, 

 and keeping the lower ones cool, or 

 on heating the lower ones and keep- 

 ing the others cool, an induction is 

 established in the wire, more in- 

 tense than in the single pair of metals, but still very weak. 

 The conducting wire strongly affects a needle, causing a de- 

 flection proportional to the inequality of temperature between 

 the ends of the bars. Melloni's ther mo-multiplier is a delicate 

 instrument of this kind, which is even more sensitive to 

 changes of temperature than the air thermometer, and has af- 

 forded great assistance in exploring the phenomena of radiant 

 heat (page 32). 



In such a compound bar, also, unequal temperature may be 

 produced, by making it the connecting wire of a single and 

 weak voltaic circle ; whereupon the metals become cold at their 

 junction, if the induction is from the bismuth to the antimony, 

 and hot at the same point if the induction is in the opposite 

 direction. These are the converse of the preceding phenomena, 

 in which electrical effects were produced by inequality of tem- 

 perature. 



13. The friction of different bodies is another source of 

 electrical phenomena. One, at least, of the bodies rubbed 

 together must not be a conductor, and in general, two non- 



