On Contact Electricity of Metals. 117 



terminals into 100 equal parts. The potential applied was 

 usually that given by a Daniell cell of Lord Kelvin's gravity type, 

 and was tested frequently by comparison with a standard Clark cell, 

 or by means of a standard resistance and a current balance. 



7. The permanent connections were as follows : (1) The lower 

 plate to the insulated pair of quadrants of the electrometer; (2) 

 the terminals of the Daniell cell to the ends of the divider (there 

 was a reversing plug in this circuit) ; (3) one end of the divider to 

 the upper plate and uninsulated quadrants of the electrometer. 



The temporary connection ( 5) was from the sliding contact piece 

 of the divider to the lower plate. 



8. Each experiment was as a rule begun by polishing a metal 

 plate on clean glass-paper or emery cloth. Its contact potential 

 with a standard plate, generally of electrolytically deposited gold 

 washed some hours previously with alcohol, as used by M. Pellat, 

 was then measured. The plate was next subjected to some particular 

 treatment ; for instance it was filed, or burnished, or polished on 

 leather or paper, or washed with water, alcohol, or turpentine, or 

 heated in air and oxidized, or not oxidized, or exposed to steam, or 

 oxygen, or fumes of iodine or hydrogen sulphide, or simply left to 

 alter under the influences of the atmosphere and its own molecular 

 forces. Its potential with the same standard plate was again 

 measured, and the change due to the treatment its surface had 

 undergone noted. 



9. The metal which requires to be joined to the zinc end of the 

 battery, in order to effect a balance, will be called positive. Thus 

 aluminium is positive to zinc and zinc positive to copper. Also, 

 when a plate becomes more positive its potential will be said to rise ; 

 when more negative, to fall. When the " potential " of a plate is 

 mentioned, without other qualification, its contact potential- difference 

 with a standard plate ( 8) is meant. 



10. The results given are not in the order in which they were 

 obtained, but are classified in such a way as to show more clearly 

 their true import, and those of many experiments, which are not 

 mentioned in the text, will be found in the tables. 



III. Effects of different Methods of Cleansing the Metallic Surfaces. 



11. It seems probable that the very conflicting results obtained 

 by different experimenters for the potential-difference of any given 

 pair of metals, in air, must be due, to a large extent, to differences 

 in the methods by which the surfaces have been prepared. In order 

 to obtain a metallic surface as free as possible from all contamination 

 it is clear that a hard polishing agent, such as a clean steel file, or 

 emery-cloth, or glass-paper, must be used, as a softer material leaves 



