22 SECTIONAL ADDRESSES. 



by an equal fall from tlie zinc to the lower plate of the electroscope. In 

 this description it is tacitly assumed that the hand brings the metal 

 touched to its own potential, and that that is the earth (or^zero) potential. 



Volta's own explanation was that there existed in'metals an inherent 

 power of separating the two electricities ; that is, that each metal possessed 

 what Helmholtz later spoke of as a specific attraction for electricity. 

 But an alternative explanation made the effect depend upon the accidental 

 circumstance that the rods are in air, that there is incipient or potential 

 chemical action, between the air (or moisture in it) and the rods, which 

 creates a drop of potential (of different amounts) between the air and each 

 rod ; and that consequently when the twin rods are brought together, 

 their potentials being equalised by a flow of electricity between them, 

 there is still a difference of potential between them and the air. It has 

 further to be supposed that the oxidising properties of the fluids on the 

 hand are not very different from those of the air, and consequently the 

 observed drops of potentials are between the metals and the hands, and 

 not between the metals themselves. The two theories are known as the 

 contact theory and the chemical theory. 



The subject from the beginning proved to be a very controversial one. 

 The time was not ripe in Volta's days properly to discuss it. In its 

 more modern form discussion may be said to have begim soon after the 

 acceptance of the two principles of thermodynamics in 1850. The 

 principle of energy and that of entropy put an entirely new complexion 

 upon it. 



Early in the nineteenth century a^second^mode'of obtaining a flow of 

 electricity was discovered by Seebeck (1822) which depended on creating 

 differences of temperature in a circuit of two metals. This was the 

 discovery of the thermoelectric circuit. It was inevitable that the two 

 discoveries should become associated with one another, for the thermo- 

 electric electromotive force might be simply due to the temperature 

 variations of the other. The Seebeck effect is very small. A hundred 

 degrees difference of temperature pro^ddes an e.m.f. of the order of 

 millivolts at most, while the Volta effect for copper and zinc is of the 

 order of one volt. Whether the two phenomena are intimately related 

 or otherwise it is necessary to discuss them both. It is convenient to 

 give first place in the discussion to the thermoelectric circuit. 



Thermoelectric Circuits. 



Two wires of different materials are joined so as to form a loop with 

 the two junctions at different temperatures, Tj and T.^. The elementary 

 facts about such a circuit are, in general : — 



