Galvanic Pile, and Electromotive Forces. 511 



difference between the attractions exercised by M and N upon 

 the same electrical molecule. 



If the contact-surface at ordinary temperature receives a 

 slight elevation of its degree of heat, the attraction-forces 

 which tend to bring the electric fluid to the conductor which 

 exerts the strongest attraction increase ; and this augmen- 

 tation of attraction must be approximately proportional to 

 the increment of temperature if this is not too great. 



We will now suppose that, for example, the conductor M 

 is divided into two, M, and M /y , of which M, has a higher 

 temperature than M 7/ , but the excess is so slight that the ma- 

 terial molecules in the part M, do not on account of it sen- 

 sibly alter their relative position and distances from one ano- 

 ther. M therefore differs from M y/ only in this — that the 

 molecules of the former are conceived to be in stronger vibra- 

 tion than those of the latter ; but this circumstance need not 

 be taken into consideration, because, as above remarked, the 

 action from this becomes, on the whole, equal to nil for the 

 thermoelectric ring. The constant D in the foregoing formula 

 is, in this case, equal to nil ; and consequently, for the contact 

 between the warmer and the cooler portions of M, we get 



Ax-A^O. 



If the excess of temperature possessed by M ; is but slight, 

 then M, and M 7/ do not, as regards their attraction, behave 

 toward an electrical molecule at all like two different metals. 

 Consequently different distribution of temperature in one and 

 the same conductor cannot put the electric fluid in motion. 

 If, on the contrary, M, has a temperature so considerably 

 higher than M /7 that its molecules are caused by it to sensibly 

 change their positions of equilibrium and their distances from 

 one another, the relation may take another form. It may then 

 happen that the attraction exerted by M, upon on electric mo- 

 lecule situated in the vicinity of the contact-surface will be no 

 longer just equal to that which issues from M /y ; and there- 

 fore the part M y will, in respect of its attraction, behave toward 

 an electric molecule in some sort as if it were of a different 

 material from M y/ . 



In a thermoelectric ring, therefore, consisting of two differ- 

 ent metals M and N, of which one of the junctions has a 

 considerably higher temperature than the other, a thermo- 

 electric current must arise with an intensity approximately 

 proportional to the temperature-difference — a result which is 

 known to agree with experiment. When the one junction 

 receives a considerably higher temperature than the other, the 

 vis viva of the aether molecules at the junction is proportionally 



