TRANSACTIONS OF SECTION A. 



555 



the common chord of the two circles, and the constants given are, in each case, the 

 means of those determined from a paii- of such points. 



The unit used is the millimetre. The quantity .r is the distance from the 

 diameter parallel to the bars, and is the case of the two circles at right angles to 

 their common chord. V = — 1. 



Table I. — Circle. Eadius = 8-43mm. 



In each case those points only are given which are nearest to and furthest from 

 the circle or circles. 



I 



10. On Foteidial clue to CuntacO By S. Lavington Hart, B.A., B.Sc, 

 Scholar of -St. John's Collecje, Cambridge. 



The object -of tlie paper is to illustrate and extend the analogy that exists 

 between the aotion of an ■ electrolytic cell and- the behaviour of two metals in a 

 gaseous medium, and to apply to laoth cases one simple law : — • 



The metal -which, by reason of superior chemical affinity, combines with the 

 electro-negative ion, receives thereby a negative charge, and acquires consequently 

 (relatively to the medium) a negative potential. 



If we take, for instance, a zinc-copper couple in acid or in air, two principal 

 cases may present themselves : — 



(i.) The metals are insidated ; in which case the zinc is negative to the medium, 

 and this negative to the copper. 



(ii.) The metals touch ; they are therefore at the same potential, but the 

 medium is necessarily at a higher potential than this near the zinc, and at a lower 

 in the neighbourhood of the copper. This difference of potential in the medium 

 around the two metals has been determined, and is generally regarded as the 

 diflerence of potential in the metals themselves. 



In the experiments described, however, these two points are made clear, namely, 

 that in air and in dilute acid : — 



(i.) Metals insulated from one another, but in close proximity, are at difi'erent 

 potentials, the more electro-negative being at a negative potential. 



(ii.) Metals in contact are at the same potential. 



The metals employed were iron and mercury, and the points referred to were 

 proved by using an electrometer, one of the electrodes of which was put in connec- 

 tion with an iron wire dipping in mercury held in a funnel, to the neck of which 

 was attached a fine glass jet, the drops from it forming within an iron cylinder put 

 to the other electrode. 



I obtained a deflection of thirty divisions, equivalent to a diflerence of potential 

 of "lo D (D being the E.M.F. due to a Daniell cell) by bringing up u-on close 

 to the falling drops. 



In the verification of the second point a fresh question was raised, for it was 

 found that a diflerence in potential was set up by diflerent degrees of oxidation at the 

 two surfaces of mercury (in the funnel, and at the end of the thread in the jet) due to 



' Publislied in the Philosophical Magaiine, November 1881. 



