362 Electrical Resistance of Soft Carbon under Pressure. 



the apparatus to indicate variations in pressure, and these 

 variations were generally produced by blowing from the 

 mouth into a rubber tube about two metres in length. Very 

 many experiments were made, all without exception showing 

 great diminution in the resistance of the disk by increase of 

 pressure ; and it will be sufficient to quote a few of the 

 results. 



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 instantly moved, showing 

 decrease of resistance. A pressure measured by 5 millims. of 

 water produced a decided effect. The resistance of the disk, 

 with its mercury and platinum wire connections, under ordi- 

 nary conditions was slightly greater than 6 ohms. A pressure 

 measured by 5 centim. 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, sometimes falling short by 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 was 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 centimetres of mer- 

 cury would hardly produce a sensible effect. 



