258 ENERGY OF CURRENTS. 



measuring from the point P, is given (222) by the expression 

 R- 



We can determine the constant Cj by the condition that this 

 resistance becomes equal to R x , for x = - , which gives 



270. ELECTROCAPILLARY PHENOMENA. The preceding experi- 

 ments have shown that any modification of the surface of contact of 

 two bodies brings with it a variation in the electromotive force. 

 This may be considered as a general law, and we must assume 

 a priori that there is a relation between the electromotive forces of 

 contact of two bodies, and any other property dependent on the 

 state of the surfaces. 



If, for instance, we use a surface of mercury as negative electrode 

 to decompose water, the mercury becomes polarized that is to say 

 that the difference of potential at the contact of the two liquids 

 increases with the external electromotive force until the disengage- 

 ment of bubbles of gas begins. The capillary properties of mercury 

 (that is to say its surface tension), depend simply on the state of the 

 surface, and must therefore change with the polarization. 



M. Lippman's experiments have shown that this is the case. 

 The capillary tension of mercury in contact with acidulated water, 

 increases at first with the electromotive force of polarization until 

 this reaches 0.9 of the electromotive force of a Daniell's cell, and 

 then diminishes in proportion as the polarization increases. 



Reasoning and experiment alike show that the converse of this 

 is true. If by any mechanical process whatever, the surface of the 

 mercury is deformed, and therefore the surface tension of contact of 

 the two liquids is made to vary, the difference of potential changes at 

 the same time ; during the deformation the potential varies in such a 

 way that the surface tension which corresponds to it tends to oppose 

 the motion produced. 



