226 O. U. VONWILLER. 



curve B O eventually shows a like effect, but there is no 

 doubt that this is due to a rise in temperature caused not 

 so much by the small O 2 R development of heat in the 

 region B O as by heat conducted from the regions round 

 A and D where on account of the big potential differences 

 at the contacts there is a great development of heat on the 

 passage of comparatively small currents. With large 

 currents temperature differences in different parts can be 

 detected by touching the specimen with the fingers. When 

 large currents are flowing measurements are usually made 

 with difficulty as conditions seldom remain steady for long; 

 frequently permanent changes are produced in the contact 

 resistances by the passage of large currents, changes like 

 those produced by heating as described later. 



These results, it is seen, have a bearing on Waterman's 

 conclusion quoted above. Readings with very low currents 

 show a large but finite resistance depending on the nature 

 of the contacts and the maximum resistance occurs not for 

 zero current but for a small current entering at the end 

 where the contact resistance is greater. It is important 

 to distinguish between the resistance measured between 

 points such as A D and points such as B C as the former 

 depends largely on the contact resistances, while the latter 

 depends on the dimensions and on the resistivity of the 

 material only. 



In this specimen the resistance between B and O is about 

 4 ohms, while that between A and D, including the two con- 

 tact resistances, for very small currents is about 237 ohms, 

 the maximum value being 279 ohms for a positive current 

 of about 0'000825 ampere. The thickness and width of this 

 specimen were not recorded. Readings with various speci- 

 mens give in many cases figures of the same order for B O 

 but occasionally very high values are obtained, much higher 

 than can he accounted for by differences in dimensions; in 



