346 Contact Difference of Potential of Distilled Metals. 



contact potential on admission of a trace of air is not quite 

 so large as in the case of zinc. 



It may be urged that the platinum in the form of a thin 

 film deposited from a cathode on to a glass disk is not a very 

 suitable standard for measuring the contact potential of 

 distilled metals. From a number of experiments in which 

 zinc was distilled on to a copper disk, there was no reason 

 for believing that the platinum, in the form of a film on glass, 

 was in any way abnormal. A platinum film on glass was 

 found to be rather more convenient than a metal disk. 



It is generally agreed that for the production of the best 

 possible vacuum, the apparatus must be exhausted con- 

 tinuously for several days and maintained at a high tem- 

 perature for a considerable portion of that time. It was 

 inconvenient to run the molecular pump continuously for 

 more than a few hours and impossible to heat the apparatus 

 on account of the wax joints. Hence although the molecular 

 pump is an extraordinarily efficient instrument, it is probable 

 that still better vacua could be obtained than were used in 

 these experiments. A combination of cooled charcoal with 

 a Gaede pump appears to be the most efficient method of 

 completely exhausting an apparatus. However, liquid air 

 was not available when these experiments were made and so 

 the molecular pump alone was used. If the view of the 

 relation of the photo-electric effect to the contact potential 

 is correct, we should expect from Kustner's result, that zinc 

 completely freed from gases would be still more electro- 

 negative than the distilled zinc used in these experiments. 

 It is unlikely that the zinc distilled in these experiments was 

 absolutely free from occluded gases, and so we may expect 

 that further precautions would result in zinc still more 

 electronegative in character. It was not possible to get 

 exactly the same contact difference of potential between newly 

 distilled zinc and platinum in different experiments made 

 under apparently similar conditions. This is probably due to 

 the fact that the vacuum obtained cannot be precisely the same 

 on each occasion, and we have seen how sensitive the contact 

 potential of new surfaces is to traces of air in the vacuum. 



It seems clear from these experiments that the contact 

 potential between metals under ordinary conditions depends 

 to a very great extent upon the gas absorbed in the 

 surface. The results suggest several important and inter- 

 esting questions. Is there any contact difference of potential 

 between metals completely freed from gases and, if so, how 

 will it differ from the accepted values ? Which of the gases 

 in the air is responsible for the change in the contact 



