Strain on the Electrical Resistance of Carbon. 443 



myself* ; but this I rather doubt, for the change of resistance 

 produced by pressure on the carbon rod used by him would 

 appear to be considerably greater than any which was ob- 

 tained by myself on carbon rods of a similar kind. In fact, 

 had I not employed a very sensitive arrangement for testing 

 small changes of electrical resistance f , I might have failed in 

 detecting any change whatever. The result arrived at by 

 myself was that a pressure of 1 grm. per square centim. would 

 produce a percentage decrease of only -00000064. So that, if 

 we assume the decrease of resistance to be proportional to the 

 pressure, there would be required a pressure of 22,215 lb. on 

 the square inch to produce a decrease of resistance of 1 per 

 cent. I am not aware of any investigations on the pressure 

 which would suffice to crush carbon, but we may form some 

 rough notion of its amount in the following manner : — The 

 crushing pressure of wrought iron is 36,000 lb., and the value 

 of Young's modulus for the metal is about 1900 x 10 6 grm. 

 per square centim., whilst the corresponding value for this 

 particular specimen of carbon was 267*2 x 10 6 grm. per square 

 centim. We may perhaps say that the crushing pressure of 

 the carbon would probably be somewhere about 



36,000 x 267-2 1U . , 



— - — -.Qflfl lb* per square men, 



i. e. about 5000 lb. per square inch. If this result be at all 

 correct, it would follow that pressure on the point of crushing 

 the carbon would diminish the electrical resistance by less 

 than \per cent. Prof. Mendenhall does not give any data by 

 which the amount of change of resistance observed by him 

 can be calculated ; but, if one may judge from the tenour of 

 his remarks and his arrangements for experimenting, it would 

 seem probable that the ratio of change of resistance to pressure 

 was considerably greater in his experiments than in mine. 



Further, Prof. Mendenhall seems to credit me with doing 

 what I have not done ; for, after describing his own experi- 

 ments on the effect of pressure exerted in the same direction 

 as the current, he goes on to say, " It was also found that 

 compression at right angles to the direction of the current 

 produced a similar effect, but less in magnitude. These facts 

 had been already announced by Mr. Tomlinson." I have 

 made no experiments on the effect of compression at right 

 angles to the direction of the current, but I think it probable 

 that if I had I should have found increase of resistance, and 



* Phil. Trans., Part I. 1883, p. 65. 



t With the carbon I was able not only to detect but to measure a 

 change of resistance of about 1 in 100,000. 



