Contact Potential Phenomena between Metals. 255 



to produce any large change. Those recorded were the 

 largest obtained ; an exposure of a week, in the case of an 

 experiment with copper-brass, produced no change in the 

 final value of the contact potential. The difference in 

 mobility of the hydrogen ions is partly responsible for the 

 difference in shapes of the curves. 



An interesting result was, however, obtained if the hydrogen 

 was prepared from chemically pure zinc and dilute sulphuric 

 acid. Curves (3)* and (4) in fig. 6 show the largest effect 

 obtained for zinc-brass in changing from dry air to this prepa- 

 ration of hydrogen. Curves (5), (6), and (7) in fig. 6 show the 

 exceptional effect in the case of lead. Curve (5) is for lead- 

 brass in dry air. In (6) the impure hydrogen has been 

 passed through for one hour, and in (7) for one day. The 

 effect sometimes became negative, and could always be made 

 as small as shown in (7) for lead only. In the case of pure 

 ^hydrogen the values were constant and could be repeated, 

 but in this case they were uncertain and variable, suggesting 

 chemical action. The presence of H 2 S and S0 2 in small 

 quantities is probably responsible for these changes. It has 

 been shown by Spiers f, " that even in a high vacuum of 

 pure dry hydrogen at a pressure of O'OOOl mm. of mercury, 

 and after four washings in the gas, there is still a large con- 

 tact effect," which he attributes to the fact that there is still 

 enough oxygen present to " completely oxidize the surface." 

 He endeavours to show that the large diminution in the 

 contact potential due to his "heating method" is due to 

 the final removal of this small amount of oxygen. The 

 behaviour of the surface layers in our experiments strongly 

 supports the idea that in cases where the contact potential is 

 diminished we have merely the formation of extra layers 

 which hold back sufficient electrons to minimize the normal 

 contact effects. This would support the opinions expressed 

 by Richardson in a recent paper on the Electron Theory of 

 Contact Electromotive Force J, concerning the experiments 

 of Spiers §, Brown ||, Greinacher H", and Majorana **, which 

 have formerly been interpreted as strong support for the 

 chemical theory. 



* The change in the shape of curve (3) from curve (1) is due to a 

 -change in the charging potential used for the electrometer needle, 

 t Spiers, Phil. Mag. vol. xlix. p. 70 (1900). 

 t Richardson, Phil. Mag. [6] vol. xxiii. p. 263 (1912). 

 § Loc. cit. 



|| Brown, Phil. Mag. [6] vol. v. p. 591 (1903). 

 5] Loc. cit. 

 ** Majorana, Atti de Lincei, vol. ix. p. 162 (1900). 



