On Contact Electricity of Metals. 115 



small change in the potential, and every increase in thickness of the 

 film is attended by a further change in potential until a limiting 

 value is reached, which is that of a mass of the oxide (see 

 Chap. IV). 



(/) Exposure to the atmosphere at ordinary temperatures does 

 not, as a rule, produce any rapid change in Volta-potential, especially 

 if the air be comparatively dry and free from dust. The 

 ultimate change is usually in the negative direction (see Chap. V), 



(0) I have extended Lord Kelvin's experiments on the effect of 

 temporary immersion of a metal in a gas to the cases of copper, zinc, 

 tin, and silver in oxygen, and find that copper, zinc, and silver 

 become temporarily positive, while tin becomes negative in consequence 

 of this treatment (see Chap. VIII). 



The research, suggested by Lord Kelvin, was carried out in the 

 Natural Philosophy Laboratory of Glasgow University during the 

 Sessions 1893-94-95, and during 1895-96 in the Cavendish Laboratory 

 of Cambridge University. My thanks are due to Lord Kelvin for 

 many suggestions and much valuable advice, both in regard to 

 experiments and to the discussion of results, and to Professor 

 J. J. Thomson for similar kindnesses during my work in the 

 Cavendish Laboratory. A small portion of this investigation, on the 

 effect of Rontgen aj-rays on the contact electricity of metals, was 

 published in the ' Proceedings of the Royal Society ' for March, 1896. 



I have also to thank Professor James Holm, M.A.* and Mr. 

 George E. Allan, B.Sc., for the part they took in the earlier portion 

 of the work. 



II. Method of Experiment. 



3. The measurement of the natural potential-difference between 

 any pair of conductors was usually made, in air, by the null method 

 described very briefly by Lord Kelvin in the Report of the British 

 Association for 1880, p. 494. To make clear the exact circumstances 

 in which the potentials were measured it will be advantageous to 

 describe the apparatus and general method of experiment in detail. 



4. A circular disc of one of the metals, usually about 9 cm. 

 in diameter and O2 cm. thickness, is insulated and in permanent 

 connection with the insulated pair of quadrants of an electrometer. 

 A similar plate of the other metal placed parallel to the first, at a 

 distance of a few millimetres from it, is uninsulated and in connection 

 with the uninsulated pair of quadrants. While in this position they 

 are temporarily connected with one another through a simple form 

 of potential-divider in which a slope of potential is maintained by a 



* When this was written last August, my friend and former fellow- worker was 

 Professor of Applied Mathematics in the South African College, Cape Town. He 

 died in October. 



K 2 



