92 PROCEEDINGS OF THE AMERICAN ACADEMY. 



from this metal back to the first, the case being pretty closely analo- 

 gous to the equilibrium between a liquid and its saturated vapor. A 

 still closer analogy, perhaps, is found in the equilibrium of an isother-' 

 mal atmosphere over some part of the earth. Individual particles 

 in the upper strata are falling toward the earth under the pull of 

 gravity, but their places are taken by equally numerous molecules 

 projected upward by heat energy from the denser layers below. 



The question now to be considered is, whether our "virtual" poten- 

 tial-difference, (F2-F1) for the electromagnetic unit charge, is the 

 e 



same thing as the Yolta potential-difference, which quantity has 

 been the subject of a vast amount of arguing and experimenting for 

 more than a century. In approaching this question let us write 



-(F 2 -F 1 ) = 8V=8 a V+8 c V, (32) 



e 



where 8 a v is that part of - (i*W*i) which depends on the difference of 



specific attraction of the two metals for the electrons, and 8 C V is the 

 part which is due to the difference of electric charge of the two metal-;. 



To give my conception of the Yolta potential difference I shall 

 describe an experiment which, if we could work with perfectly clean 

 metals in a vacuum or in a gas absolutely inert, would be easy to carry 

 out, but which, failing this exceedingly difficult condition, must be 

 regarded as imaginary. 



In Figure 3 let M 2 be a plate of metal connected with two quadrants 



gy u M * 



M. 



fyg.4. 



of an electrometer, E, and with a distant plate, Mi, of another metal, 

 both Mi and the quadrants being grounded. For simplicity let us 

 suppose that the quadrants and all connecting wires are of metal (2). 

 There is, accordingly, no difference of potential between M 2 and the 

 quadrants in Figure 3, but the potential of M 2 exceeds the potential of 

 Mi by the amount, positive or negative, which we call 8V. 



