CHAP. VIIL] ELECTRICITY AND MAGNETISM. 343 



to the direction of the current. Thus a current which 

 passes through a bismuth-antimony pair in the direction 

 from bismuth to antimony absorbs heat in passing the 

 junction of these metals, and cools it ; whereas, if the 

 current flow from antimony to bismuth across the 

 junction it evolves heat, and the junction rises in tem- 

 perature. 



This phenomenon of heating (or cooling) by a current, 

 where it crosses the junction of two dissimilar metals 

 (known as the " Peltier effect," to distinguish it from the 

 ordinary heating of a circuit where it offers a resistance 

 to the current, which is sometimes called the " Joule 

 effect "), is utterly different from the evolution of heat in 

 a conductor of high resistance, for (a) the Peltier effect 

 is reversible^ the current heating or cooling the junction 

 according to its direction, whereas a current meeting 

 with resistance in a thin wire heats it in whichever 

 direction it moves ; and (b) the amount of heat evplved 

 or absorbed in the Peltier effect is proportional simply 

 to the strength of the current, not to the square of that 

 strength as the heat of resistance is. 



The complete law of the heat developed in a circuit will 

 therefore require to take into account any Peltier effects which 

 may exist at metal junctions in the circuit. If the letter P 

 stand for the difference of potential due to the heating of the 

 junction, expressed as a fraction of a volt, then the complete 

 law of heat is 



H = 0-24 x (C 2 R/ PO) 



which the student should compare with Joule's law in Art. 367. 

 The quantity called P is also known as the coefficient of the 

 Peltier effect ; it has different values for different pairs of metals, 

 and is numerically equal to the number of ergs of work which 

 are the dynamical equivalent of the heat evolved at a junction 

 of the particular metals by the passage of one weber of electricity 

 through the junction. 



381. Thermo-electric Laws. The thermo-electric 

 properties of a circuit are best studied by reference to 

 the simple circuit of Fig. 142, which represents a 



