1883.] On the Electrical Resistance of Carbon Contacts. 15 



effect may result: that is (after the first great fall of resistance), 

 diminished current may produce increased resistance. Table XIII 

 gives the measurements, with the same currents as before, when the 

 bismuth was first rubbed with the finger, instead of being scraped as 

 had been done in the previous experiments. 



Table XIII. — Experiments after Rubbing the Bismuth with the 



Finger. 



Current. 

 Ampere. 



Resistance of contact witl 



1 pressure of 



- 35 grm. 



•5 grm. 



1 grm. 



'5 



1-30 



1 30 



1-22 



•3 



175 



1-46 



1-28 



1 



1 92 



1-69 



1-29 



•05 



1-83 



192 



1-29 



•01 



1-95 



2-29 



1-29 



•05 



1-89 



2-07 



1-28 



1 



1-87 



1 -98 



1 28 



•3 



1-77 



1 -72 



1-2? 



"5 



1-46 



1-45 



1-27 



I think these effects admit of a simple explanation. When the' 

 surfaces of the bismuth are clean, contact takes place entirely through 

 the metal. The current heats the metal at the points of contact to an 

 extent which depends upon the current strength ; and the resistance, 

 in accordance with the general law, increases with the temperature : 

 strong currents will, therefore, give higher resistance than weak ones. 

 When, on the other hand, the surface is not clean, a film of oxide or 

 some foreign substance is interposed, the resistance of which, like that 

 of carbon, is higher with a weak current than with a strong one. 



It is probable that similar effects occur with metals of lower specific 

 resistance than that of bismuth ; but their observation is very diffi- 

 cult, and requires more delicate apparatus than that at my disposal. 

 Thus, of forty measurements made with platinum contacts, nineteen 

 results were favourable to the theory, ten adverse to it, and eleven 

 neutral. With copper the indications were even more uncertain. 



14. Effects on Resistance of Varying Pressure with Constant Current. 



The resistance of bismuth contacts at various pressures was 

 measured with fixed currents of *1, *01, and '001 ampere, the method 

 used being the same as in the case of carbon. The mean of several 

 series of such measurements is given in Table XIV, which reveals the 

 existence of a more or less definite law, though the results of the 

 experiments did not agree very closely among themselves. 



