Mr. John Aitken on Dew. 363 



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

 apparatus described, ought to show, then, whether any con- 

 siderable part of the resistance-variations observed could be 

 attributed to variation of contact between mercury and carbon. 

 A disk of hard carbon similar in dimensions to the soft disk 

 previously employed was accordingly inserted between two 

 similarly 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 centim. of mercury reduced the galvanometer-deflection 

 from 36 to 35 divisions of the scale. This indicates a change 

 of less than 3 per cent., resulting from a pressure which with 

 the soft disk lowered the resistance by more than 60 per cent. 

 There can be little doubt that this small reduction is due almost 

 entirely to better surface-contact produced by pressure. 



Throughout all of the experiments with soft carbon, it ex- 

 hibited more or less irregularity in its behaviour. The appli- 

 cation of a pressure very largely in excess of the maximum 

 referred to above would sometimes result in a permanent 

 reduction of the resistance of the disk, indicating that a per- 

 manent set had taken place. By the exercise of care, however, 

 what may be called the " normal " resistance may be main- 

 tained fairly constant for a considerable length of time. 



Conclusions. — When carbon is prepared in the form of com- 

 pressed lampblack, its electrical resistance varies greatly 

 with the pressure to which it is subjected. A small part of 

 this variation is doubtless to be attributed to change in sur- 

 face-contact between the carbon and the electrodes through 

 which the current is introduced, but by far the larger part 

 (provided any effort is made to secure good surface-contact) 

 is due to a real change in the resistance of the carbon itself. 

 The resistance of carbon in this condition is fluctuating and 

 uncertain to a degree that seems to prevent its use as a factor 

 in any device for the accurate measure of pressure. 



XLIV. On Dew. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IN the paper by Mr. Charles Tomlinson, F.R.S., in the 

 September number of your Journal, entitled "Further 

 Remarks on Mr. Aitken's Theory of Dew " there is little of 

 scientific interest, the points advanced being mostly of a con- 

 troversial character. As, however, the whole tone of his 



2B2 



