8 



Mr. S. Bidwell. 



[Feb. I, 



the battery and the bridge, that the current might be smaller. The 

 results are contained in Table VII, and are substantially the same as 

 before. 



Table VII. — Same arrangement as in Table VI, with a Resistance of 

 300 ohms between the Battery and Bridge. 









Resistance. 







Pressure. 















Grins. 

















Ohms. 



Ohms. 



Ohms. 



Ohms. 



Ohms. 



Ohms. 



"5 



30 1 



31-5 



31 6 



34 -4 



354 



34 1 



3 



24-3 



25 2 



19 -0 



19-6 



19-8 



18-8 



*5 



32 *9 



33 2 



34 -4 



36 4 



35'1 



36 -4 



It will be noticed, however, that the final resistance is almost 

 invariably slightly higher than the original resistance ; but this 

 increased resistance gradually diminished, as may be seen by com- 

 paring the last figure of one column with the first of the next, and 

 perhaps if time were given it would return to its original value. This 

 is probably a thermo-electric effect. 



8. Effects of Seating Contacts. 



Since, as is well known, the resistance of a continuous conductor of 

 carbon is diminished by heat, it would be reasonable to suppose that 

 the diminution which the resistance of carbon contacts exhibits 

 under the influence of increased currents is due merely to the heating 

 effect of the current. Experiments made with the view of testing this 

 hypothesis failed, however, to support it. Carbon contacts of various 

 forms were heated in an air-bath, and their resistance measured at 

 different degrees of temperature. The resistance was in every case 

 found to vary irregularly (probably because, in consequence of the 

 expansion due to heat, the relative positions of the points of contact 

 were slightly altered) but, in general, rise of temperature was accom- 

 panied by a rise in the resistance. Thus, in one case with a pressure 

 of 1 grm., the resistance which at 16° C. was 9 ohms, became at 

 ♦50° 10*7 ohms. Upon another occasion the resistance increased from 

 11*9 ohms at 15° to 15*6 ohms at 36°. And again with a pressure of 

 5 grms., the resistance increased from 4*7 ohms at 17° to 5*7 ohms 

 at 50°. In every case the resistance fluctuated considerably, rising 

 and falling alternately as the temperature was raised, but the general 

 tendency seemed to be towards increased resistance. This may 

 possibly be due to the formation of a non-conducting film by air or 

 gases expelled from the carbon by heat. 



