of the Electromotive Forces in a Voltaic Cell. 61 



seeing the position of the contact theorists, lies in their 

 defining potential in a new way not recognized in Electro- 

 statics. Clerk Maxwell's electromagnetic theory is so perfect 

 in itself that no one thinks of attacking it ; and we very much 

 wish that we had the leisure to show, what we feel might pro- 

 bably be shown, that throughout his mathematical work, when 

 he speaks of difference of potentials it is not necessary to 

 assume that he speaks of electrostatic difference of potential. 



Dr. Lodge, in his reply to remarks made during the dis- 

 cussion at the Society of Telegraph Engineers and Electricians, 

 was kind enough to mention the fact that we had drawn Prof. 

 Poynting's equipotential surfaces and lines of flow of energy 

 as they must be drawn in practice, most of them lying like 

 successive sleeves in an infinitely shallow space round all the 

 conductors in a voltaic circuit, if we are to follow Maxwell in 

 assuming a considerable difference of potential between air 

 and metals. As Dr. Lodge has stated, we have also drawn 

 these surfaces and flow-of-energy lines on our own assumption 

 of no air-effect and considerable contact-differences of potential. 

 Dr. Lodge did not, however, refer to the very much more 

 interesting curves which we have drawn as the energy-lines 

 from a primary voltaic or thermoelectric circuit whose current 

 is altering just in such a way as to keep the current in a 

 secondary circuit constant. Suppose it is the E.M.F. of the 

 primary which is altering, it is very interesting to see how 

 the lines of flow of energy sweep over the primary circuit, 

 their parts which cut the secondary remaining unaltered. 

 But now, suppose the total E.M.F. of the primary to be less 

 than 1 volt, and imagine that Prof. C. Maxwell and Prof. 

 Poynting and Dr. Lodge were right in their idea of a consi- 

 derable difference of potential between air and metals, it will 

 be found that no line of flow of energy can connect the 

 primary and secondary circuits, since all the equipotential 

 surlaces of the primary will lie infinitely close to the conductors 

 of which it is composed, and yet it cannot be doubted that the 

 secondary circuit receives energy from the primary. 



For the purpose of evading all difficulties which might be 

 due to our taking different numbers to represent possible 

 contact-differences between the parts of a voltaic circuit and 

 air, let us test Prof. Poynting's deduction from Maxwell's theory 

 in another way. Assume that there is a voltaic circuit com- 

 pletely enclosed in a hermetically- sealed metallic case ; and 

 let us suppose that, just before sealing-up, we set up differences 

 of potentials of many thousands of volts between the voltaic 

 circuit and its case, and that it is in our power at any instant 

 to cause any regular change of current in the voltaic circuit 



