434 S. P. Thompsoti — Resistance of Carbon. 



and by the present writer, in which they had each independ- 

 ently arrived at the conclusion that this alleged effect was due 

 not to any change in the specific resistance of carbon, but to 

 better external contact between the piece or pieces of carbon 

 find the conductors in contact with them. It may be added 

 that an identical conclusion was arrived at in 1879 by Profes- 

 sors Naccari and Pagliani ; and that a similar result was found 

 a little later by Mr. Conrad W. Cooke. In short, with the 

 exception of Professor Mendenhall, all who have investigated 

 the point are agreed in their verdict. 



As Professor Mendenhall expressly disclaim/, all acquaintance 

 with the experiments upon which previous observers founded 

 their conclusions, it is not extfi tils to appre- 



ciate the vital point in experimenting. His carbon disk lay in 

 its usual place in the tasimeter, <>!' which. however, the upper 

 works had been removed for the purpose of his experiments, in 

 which pressure was a; rider brass rod placed 



in a vertical position upon the center of " the upper contact 

 piece." What this upper contact-piece was he does not say. 

 No information is given of the nature of the contacts above 

 and below. No attempt to distinguish between the resistance 

 at the contacts and the resistance of the carbon disk was made; 

 nor does the memoir show that any care was taken to ensure 

 cither perfection or constancy of contact. Under these circum- 

 stances it is difficult to see what grounds these experiments 

 afford Professor Mendenhall for expressing opinions upon the 

 question at issue. In the crucial experiment of Professor Bar- 

 rett, which was made with one of Edison's own buttons of pre- 

 pared lamp-black, there was a perfect contact maintained dur- 

 ing the variations of pressure by covering both surfaces of the 

 button with mercury; and in some of the other cases amalga- 

 mated copper contacts, and electroplated contacts have been 

 employed. I venture to predict that if Professor Mendenhall 

 will repeat his experiment, ensuring at the outset by one or 

 other of these precautions that the disk of carbon shall have 

 uniform and constant contacts during the experiment, he will 

 find a very different result from that which he has announced. 

 The very interesting observation which he has made that the 

 influence of time makes itself felt when pressure is applied but 

 not when pressure is removed, furnishes another argument in 

 favor of the views held by the majority of experimenters on 

 this question. 



Another point worthy of note is this. Observers have more 

 than once suspected that in carbon there is the peculiarity that 

 the resistance — the true resistance of carbon itself — varies un 

 der dirt-rent electromotive forces. The point is certainly worth 

 investigating. Professor Mendenhall does not state what elec- 

 tromotive force was employed in his investigations. 



