Chap. xiv. j THERMO-ELECTRIC CURRENTS. 143 



junction from copper to bismuth. Now this current 

 may be obtained wherever a circuit is formed of two 

 dissimilar metals ; and it is to be noted that it is 

 the difference of temperature that is the cause of the 

 current. The difference may be caused by cooling 

 instead of heating one junction. If the same junction 

 be cooled, the needle is deflected in an opposite direc- 

 tion to that obtained by heating ; but again the 

 current is from the hot to the cold junction. 



The thermo-electric series consists of the fol- 

 lowing metals so arranged that each metal is positive 

 to those before it in the list, and negative to those 

 after it : bismuth, German silver, nickel, cobalt, 

 mercury, platinum, gold, brass, copper, tin, alumi- 

 nium, lead, zinc, silver, iron, antimony. 



While, however, any dissimilar metals will yield 

 a thermal current, all do not yield it of the same 

 strength. Bismuth and antimony, for instance, give 

 a stronger current than bismuth and copper, in a 

 direction across the junction from, bismuth to anti- 

 inoiiv. 



V 



The positive metal of a thermo-electric couple is 

 the metal from which the current proceeds across the 

 hot junction. 



The electromotive force of a thermal cur- 

 rent is, within limits, proportional to the difference 

 of temperature between the two junctions. This rule 

 indicates a means for the determination of the amount 

 of differences of temperature ; for if one junction be 

 always kept at a standard temperature, the extent of 

 the deflections of a needle would depend entirely on 

 the temperature of the other junction. Therefore, by 

 the amount of deflection is given a means of ascertain- 

 ing the temperature of the other junction, and so of 

 any body in contact with it. 



The therano-electric pile is a means of in- 

 creasing the effects and producing a stronger current. 



