Electromotive Forces of Contact. 47 



circuit containing an electromotive junction, V 1 — Vis produced 

 by and is equal to E. Hence, at au electromotive junction 

 no work need be done by a current ; in other words, the 

 existence or non-existence of a Peltier effect has nothing to 

 do with the existence or non-existence of a local E.M.F." * 



But although on an open circuit, there are two equal and 

 opposite E.M.F.'s at the junction, and consequently there is 

 no resistance to the flow of electricity in either direction, 

 still the E.M.F. which acts in the direction of the flow will 

 do work upon the opposing E.M.F. 



A mechanical example may make this clearer. If a particle 

 be acted upon by two equal and opposite forces, each of one 

 dvne, the smallest possible force will be able to displace the 

 particle in any direction ; but if the particle be displaced one 

 centimetre in the direction of one of the dyne-forces, this 

 force will do just one erg of work upon the other force. 

 . Xow, at the junction between two chemically inactive con- 

 ductors, the two E.M.F.'s are (1) an E.M.F. E due to a 

 tendency to absorption of heat-energy at the junction, and 

 transformation of the absorbed energy into electrical energy 

 by displacement of electricity across the junction, and (2) an 

 E.M.F. P (=— E) due to difference of potentials between 

 the conductors. 



Hence, if electricity M cross the junction in the direction 

 of the E.M.F. E, heat-energy EM is absorbed in order to 

 drive the quantity M up the step of potential; while if 

 electricity M cross the junction in the opposite direction, 

 energy EM will be given out in descending the step of 

 potential and will appear in the form of heat. 



Returning now to the consideration of the conductors A 

 and B (fig. 2), suppose that they are capable of acting 

 chemically upon one another. In this case the energy re- 

 quired to establish the difference of potentials between A and 

 B may be supplied at the junction either in the form of heat 

 absorbed there or chemical action taking place. For the sake 

 of simplicity let the Peltier E.M.F. be zero, so that we have 

 only to deal with a chemical E.M.F. It will also be simplest 

 to consider one of the conductors to be a solid and the other 

 a liquid, so that any chemical action which may take place 

 will not modify the nature of the subtances in contact. 



For every quantity M of electricity which crosses the junc- 

 tion in the direction of the E.M.F. chemical energy EM will 

 be absorbed, i. e. an amount of chemical action proportional to 

 M will take place. 



This is in accordance with Faraday's Electrolytic Law. 

 * Professor Lodge refutes this by a hydrostatic analogy. 



