28 SECTIONAL ADDRESSES. 



being the specific heat of electricity. There is no clear evidence that he 

 used the term in anything but an analogical way. To Maxwell the idea 

 of the corporeality of electricity was exceedingly distasteful. He assumed 

 that such a phrase as the specific heat of electricity ' was not intended 

 by Thomson, and must not be understood by up, to imply that electricity 

 either positive or negative is a fluid which can be heated or cooled and 

 which has a definite specific heat.' He shelved the question by talking 

 of change of entropy instead. Maxwell's conceptions in regard to the 

 non-corporeality of electricity almost won the day when Hertzian waves 

 were found to be transmitted in free space where no electricity was. But 

 the bodily nature of electricity came to be a real thing in the years 

 succeeding 1895. Negative electricity was isolated as electrons, while 

 positive electricity has not yet been separated from the rest of the atom, 

 and may consist of all of the atom which is not electrons. There is now 

 no difiiculty in thinking of electricity as a receptacle of energy which may 

 be communicated to it in the form of heat ; that is, it has come fully 

 within the thermodynamical scheme. The question is, which is the best 

 picture that can be given of its position in that scheme ? 



Now, firstly, when an electric current passes across a junction between 

 two metals, say copper and zinc, it is quite certain that no detectable 

 amount of metal is carried by it across the junction. We are not concerned 

 with the formation of brass (as probably Sir Oliver Lodge has already 

 said). The only things that pass are the electrons which are responsible 

 for conveying the current in each metal. These are tolerably free to move 

 under the influence of an electric force. [They are not set free by the 

 force, for otherwise Ohm's Law would not hold good.] The copper or 

 zinc serves merely as a framework through which their motion occurs. 

 The electrons can get across a boundary between the metals, but the 

 fact that heat-changes occur thereat is evidence that they may need to be 

 helped over — there is a rise (or drop) of potential there, though not of an 

 amount equal to the heat entry. It is convenient to think of this 

 potential, V, as a pressure arising from electrical forces, or more strictly, 

 since V refers to unit charge, a pressure divided by the charge on the 

 electron. 



It is clear that thermodynamically we may regard the metal as a 

 solution or binary system, the electrons being the solute. The boundary 

 between the zinc and copper acts as a semi-permeable membrane, since the 

 electrons, and nothing else, can get through it. FitzGerald spoke of the 

 free surface of a solution as the most perfect semi-permeable meml;rane, 

 but the boundary surface between two metals in regard to electrons runs 

 it very close. There is a difference of pressure (or potential) between the 

 two sides. This is an osmotic pressure. The electrons can also escape 

 to some extent from the sides of the wire ; they have a vapour pressure. 

 If the temperature is raised this becomes very conspicuous as thermionic 

 emission. The copper also has a vapour pressure, but much smaller. 

 We have then to deal with a volatile solute dissolved in a practically 

 involatile solvent — at least at moderate temperatures. 



This being so, and thermodynamics being superior to the idiosyncrasies 

 of individual mechanisms, we can at once transfer all that we know about 

 the thermodynamics of solutions to the thermoelectric circuit. 



