220 Professors W. E. Ayrton and J. Perry on the 



portant difference, that whereas Professor Clifton only removes 

 the plates of a condenser from a distance a apart to a distance 

 b apart, we removed them to an infinite distance apart, and 

 then put them in such a position that the original charge to be 

 measured was doubled ; so that in fact our method was by far 

 the most delicate, and was only limited in sensibility by the 

 natural imperfections of mechanism. All this was clearly 

 shown in the carefully executed drawing that accompanied our 

 paper. The advantages we derived from the superior delicacy 

 of our apparatus are seen if we examine, as may easily be 

 done, the two papers paragraph by paragraph ; for the metals 

 and liquids employed by Professor Clifton being the same as 

 those used by ourselves, in every case that he in 1877 was 

 only able to detect the difference of potentials, we in 1876 

 published not only the sign but also the numerical value of 

 the difference in question (compare pages 301 to 305 of his 

 paper in the ' Proceedings of the Eo}'al Society,' No. 182, with 

 our paper). Considering, too, that the quantities of electricity 

 to be measured are so small, and consequently the slightest 

 loss of electricity is so serious, we fail to see what benefit was 

 derived from using six insulating stems instead of only the 

 two carefully protected rods of our apparatus. 



We observe that Professor Clifton assumes throughout his 

 paper the " summation law of electromotive force," and that he 

 was compelled to make such assumptions in consequence of 

 his inability to measure directly with his apparatus the differ- 

 ence of potentials between two liquids in contact. But if this 

 be assumed, then we might have employed in our research the 

 method of measuring the difference of potential of two liquids 

 in contact that we have often, as early as 1874, employed as 

 a lecture-illustration to indicate this difference. This method 

 consisted in attaching to the terminals of a quadrant-electro- 

 meter two platinum wires, of which the ends were respectively 

 dipping into two liquids separated by a porous diaphragm ; 

 but to make any use of the observations obtained from such an 

 experiment, it must be assumed that the observed deflection 

 of the electrometer represents the algebraic sum of the three 

 contact differences of potentials such as might be measured 

 separately. At first sight, not to assume this might appear to 

 be a refinement of caution on our part ; but in reality it was 

 imperative to prove experimentally that this assumption was 

 true when it was taken in connexion with the statements ge- 

 nerally made by the supporters of Thomson's theory of con- 

 tact. For example, Professor Pleeming Jenkin says, on p. 44 

 of his ' Electricity and Magnetism': — "When a single metal is 

 placed in contact with an electrolyte, a definite difference of 



