﻿222 Mendenhall — Electrical Resistance of Soft Carbon. 



The disk is sufficiently sensitive to show very slight changes 

 in atmospheric pressure. On closing the open end of the 

 rubber tube, and slightly pressing any part of it between the 

 thumb and finger, the spot of light instautly moved, showing 

 decrease of resistance. A pressure measured by 5 mm of water 

 produced a decided effect. The resistance of the disk, with its 

 mercury and platinum wire connections, under ordinary condi- 

 tions was slightly greater than 6 ohms. A pressure measured 

 by 5 cm of mercury instantly reduced it to less than 3 ohms. If 

 the pressure was maintained, a slow fall of resistance continued 

 for a long time, as found in the previous investigation of the 

 subject. If the initial pressure was small, the recovery would 

 be instantaneous on its removal, but if it was large, so as to 

 greatly reduce the resistance, it was found that the recovery 

 would not be complete on the withdrawal of the pressure, some- 

 times falling short as much as ten per cent, after which a slow 

 rise would take place. This result is not quite in agreement 

 with the statement made in the first paper upon this subject, 

 which was based, however, upon a much less satisfactory series 

 of experiments. 



An examination was made of the effect of the strength of the 

 current upon the resistance of the disk. The weakest current 

 used was a little less than *001 ampere, and the strongest was 

 about *37 ampere, so that one was approximately 400 times the 

 other. Throughout this range no sensible differences in the 

 resistance of the disk were observed, the agreement at the two 

 extremes being within the errors of measurement. Under all 

 conditions the effect of variations of pressure was the same. 



The faces of a soft carbon disk are always smooth and 

 polished ; the surface of hard carbon on the contrary is gen- 

 erally more or less rough and irregular. It would appear, 

 therefore, that, if the reduction of the resistance of soft carbon 

 by increase of pressure is due to better surface contact, this 

 reduction of resistance should be much more marked with hard 

 than with soft carbon. Experiments already described showed 

 that the effect of pressure on hard carbon was very small ; so 

 small, in fact, that the pressure of a few centimeters of mercury 

 would hardly produce a sensible effect. 



The substitution of a disk of hard carbon for the soft, in the 

 apparatus described, ought to show then whether any consider- 

 able part of the resistance variations observed could be attrib- 

 uted to variation of contact between mercury and carbon. A 

 disk of hard carbon similar in dimensions to the soft disk pre- 

 viously employed was accordingly inserted between two simi- 

 larly arranged tubes. The result of this experiment was to 

 show, as had been anticipated, a small decrease of resistance 

 when the pressure was increased. A pressure of about 7 cm of 



