142 PHYSIOLOGICAL PHYSICS. [Chap. xiv. 



living body. For instance, the resistance offered by 

 the skin of the body to the passage of a current, and 

 the difference in resistance when the skin was dry or 

 moistened with different solutions might be deter- 

 mined in this way. 



CHAPTER XIV. 



THERMO-ELECTRIC CURRENTS. 



To the methods of producing electricity by friction, 

 by chemical action, and by induction, there remains 

 another method to be added, that is, by heat. 



When two dissimilar metals are placed in circuit, 

 a difference of temperature between the two places of 

 junction causes a difference of potential, and so a 

 current is produced in the circuit, travelling in a 

 direction from the hot to the cold junction. The 

 current is called a THERMO-ELECTRIC current, and was 

 discovered by Seebeck, a professor in Berlin, in 1821. 

 Fig. 74 is an arrangement for showing this pheno- 

 menon. It is a plate of cop- 

 per bent to form three sides of 

 a rectangle, the fourth side 

 of which is formed by a bar of 

 bismuth. In the enclosed space 

 is a freely suspended magnetic 

 needle, the apparatus being 



Fig.74.-Tkermo-Electric , -i ,1 x- -T 



Current. placed in the magnetic meridian, 



so that the axis of the metals 



and the needle coincide. Suppose one of the 

 junctions of copper and bismuth be heated by 

 means of a spirit lamp, the needle is at Once deflected 

 in a way to indicate a current through the copper from 

 the hot to the cold junction. At the hot junction 

 the current is from bismuth to copper, at the cold 



