444 Stress and Strain on the Electrical Resistance of Carbon. 



not decrease, to follow on compression exerted in the above 

 direction, 



On the whole, then, I am of opinion that my experiments 

 are more in accordance with those of Professors Sylvanus 

 Thompson, W. F. Barrett, and others, than with those of 

 Prof. Mendenhall. 



As regards soft carbon, such as is used with Edison's disks, 

 I cannot speak from experience. I was, however, much in- 

 terested in reading the account given by Prof. Mendenhall of 

 his investigations on the subject, and especially with that part 

 of the account which relates to the influence of time on the 

 change of resistance of the carbon disk when the pressure is 

 varied ; inasmuch as some of my own experiments have shown 

 a similar influence of time on the electrical resistance of the 

 viscous metals zinc and tin in the form of foil, when traction 

 was applied in a direction transverse to that of the current*. 



At the same time I would venture to suggest to Prof. Men- 

 denhall, that his ingenious experiments may not be quite 

 conclusive as to a considerable change taking place in the 

 specific resistance of the carbon. After showing that very 

 considerable change does take place somewhere in the circuit 

 of the mercury in contact with the carbon button and the 

 button itself, he substitutes for the soft carbon a disk of hard 

 carbon similar in dimensions, and finds that a pressure of only 

 7 centimetres of mercury lowers the resistance by nearly 3 per 

 cent.; and he says " There can be little doubt that this small 

 reduction! is due almost entirely! to better surface-contact 

 produced by pressure." From the above it is evident that 

 the surface-contact between the mercury and hard carbon was 

 by no means good ; but, writes Prof. Mendenhall, " The faces 

 of a soft carbon disk are always smooth and polished ; the 

 surface of hard carbon, on the contrary, is generally 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." The above assumption is a very dangerous one 



* Loe. cit. pp. 68 and 69. 



+ Small in comparison to that which ensued with the soft carbon, 

 which was about twenty times as great for the same pressure. — H. T. 



t The words " almost entirely " used here seem to indicate that in the 

 experiments described in the first part of this note Prof. Mendenhall found 

 much greater changes produced in his carbon than I did. Such a pres- 

 sure as that of 7 centimetres of mercury would not have produced any 

 sensible effect even with my arrangement, which must have been much 

 more sensitive than that of Prof. Mendenhall.— H. T. 



