ON ELECTKOLTSIS IN ITS PHTt>ICAL AND CHEMICAL BEARINGS. . 331 



raised lately by Dr. Gore, several papers on the subject being published in the 

 ' Philosophical Mag'azine ' for this year. 



It seemed that if there was such an eft'ect as contact resistance it was impos- 

 sible to obtain the absolute values for the conductivities and resistances of different 

 solutions, and that before proceeding to the measurement of these quantities it 

 was necessary to test for contact resistance. 



For this purpose I filled one of my cells with copper sulphate solution, and 

 determined its resistance value with diderent pairs of electrodes. 



These were — 



(rt) Platinum plates (slightly platinised). 



(b) Platinised copper electrodes. 



(c) Amalgamated copper electrodes, or copper electrodes with freshly deposited 

 copper surface. 



I found that the amalgamated copper electrodes left in a solution of copper 

 sulphate became coated with a film of copper, mercury apparently going into 

 solution ; this of course was only to be expected from the chemical ideas of mass 

 action. 



The values obtained were — 



(«) Platinum electrodes, 469-2 at ]6'9°. 

 (6) Platinised copper, 469 at 17°. 

 (c) Copper electrodes, 471 '95 at 16-7°. 



These same copper electrodes gave after standing in air or copper sulphate 

 solution a value much bigger than that of the platinum plates. 



Another pair of copper plates, which had been prepared some days, gave the 

 values with a different copper sulphate solution of — 



(1) Copper plates, R - 477 at 17-6°. 



(2) Platinum plates. Pi = 465 at 17-4°. 



(3) Platinised copper, R = 465 at 17-4°. 



It appears therefore that the platinum and platinised copper plates give iden- 

 tical values, and it would appear that the value given by copper electrodes with a 

 fresh metallic surface is almost the same ; but that, due to oxidation in air or in 

 the solution, the surface is changed, and as a consequence the resistance value much 

 increased. 



This would tend to show that with clean metallic surfaces, and the same solu- 

 tion, the resistance value is the same, and that there is no contact resistance, or at 

 least that it is the same in aU three cases. To further examine this question I 

 determined to experiment with a solution of zinc sulphate with 



(1) Platinum plates. 



(2) Amalgamated zinc electrodes. 



Two zinc plates were freshly amalgamated, and left over night standing m a 

 solution of zinc sulphate, and when introduced into the cell they gave a value 



R = bl2 atl6-4°. 



X4 = i5iZ ai 



The platinum electrodes gave the value 



R = 280 at: 



16-5°. 



After again standing in the zinc sulphate solution the value for zinc plates was 



R = 318at 16 5°. 



These same electrodes were then washed with dilute sulphuric acid, to dissolve 

 off the film of zinc or oxide, and after careful washing were introduced into the 

 cell ; the value now came down to 



R = 286 at 16-5°. 



In this case too it would appear that with a pure metallic surface the resistance 

 value would be the same, whatever the electrodes used. 



