On Contact Electricity of Metals. 141 



air; below it, little action takes place; above it, combination 

 proceeds vigorously. 



75. I snail now give a specimen experiment. The two standard 

 gold plates gave 



0-02 volt, 



the minus sign indicating that the upper plate is negative. I now 

 heated the upper: when "tepid," i.e., about 35 C., it gave with the 

 gold plate 



-0-045 volt. 



It was next heated farther until " very hot " (57 C.) and gave 



0-06 volt. 

 When it had cooled down to " tepid " (35 C.), it gave 



-0-04 volt. 

 When "cool" (28 C.), 



-0*03 volt. 



Some hours later, when both plates were cold, they gave as at first 



-0'02 volt. 



Hence the potential of gold which has been washed with alcohol 

 and allowed to dry falls temporarily about 0'0016 volt per degree 

 centigrade rise of temperature. 



76. Most of the temperature experiments on copper were made 

 with the hollow plate previously mentioned, which was filled with 

 hot water in which the bulb of a thermometer was placed. In many 

 of the experiments on zinc, and also on aluminium, a thermo-electric 

 arrangement was used, and the results obtained with it do not differ 

 materially from those obtained by the above method ( 69). These 

 and other details are noted on the diagram. 



77. The diagram gives the temperature-variations of all the sub- 

 stances studied. The curves in it are plotted by taking the results 

 for each metal of those experiments which are most free from all 

 complication or cause of doubt. If the curves be prolonged in the 

 direction of lower temperature they appear to meet somewhere below 

 200 C., and probably asymptotically to the line representing O4 

 volt positive to standard plate at 16 C. Within their range they 

 show contact-potential-differences diminishing with lowered tem- 

 perature. The only apparent exceptions are clean copper, and 

 silver coated with glass ; but both their curves are distinctly bent 

 between 16 C. and 50 C., so that probably they are directed towards 

 the same point as the others at lower temperatures. Thus it appears 

 that at about 200 C. the coutact-potential-differences of metals 



