[ 358 ] 



XLIII. On the Electrical Resistance of Soft Carbon under 

 Pressure. By T. 0. Mendenhall.* 



A PAPER by the writer on " The Influence of Time on 

 the Change in the Resistance of the Carbon Disk of 

 Edison's Tasimeter," was published in this Journal in July 

 1882 [Phil. Mag. for August, p. 115]. The object of the 

 paper, as its title indicated, was to present the results of some 

 experiments with the carbon disk w T hich appeared to show 

 that, when pressure was applied, the entire diminution of 

 resistance did not take place at once, but that the reduction 

 continued with diminished rapidity through a considerable 

 period of time. At the conclusion of the paper brief refer- 

 ence was made to investigations of the same subject by Mr. 

 Herbert Tomlinson and by Professors Sylvanus P. Thompson 

 and W. F. Barrett. 



Only the conclusion reached by some of these physicists 

 was at that time known to the writer, their verdict being that 

 the observed diminution of resistance was really due to the 

 better surface-contact of the electrodes, and not to any actual 

 change in the specific resistance of the carbon itself. 



The last paragraph in the paper contains the following : — 

 " Without knowing anything about the nature of these experi- 

 ments, the writer desires to record his belief that this theory 

 does not entirely account for the facts stated above." 



This, certainly not too rash, declaration of belief in a true 

 pressure-effect was the subject of decidedly unfavourable criti- 

 cism in the columns of one or two European scientific journals ; 

 and in this Journal of December 1882, Professor Sylvanus P. 

 Thompson published an article entitled " Note on the alleged 

 Change in the Resistance of Carbon due to Change of Pres- 

 sure," which was an exceptionally severe criticism of the pre- 

 vious paper by the writer. In this article Professor Thompson 

 refers to the iuvestigations of Mr. Tomlinson, Prof. Barrett, and 

 himself, and also to experiments made by Professors Naccari 

 and Pagliani and Mr. Conrad W. Cooke, and he declares that, 

 with the exception of Professor Mendenhall, all who have in- 

 vestigated the point are agreed in their verdict " that this 

 alleged effect was due not to any change in the specific resis- 

 tance of carbon, but to better external contact between the 

 piece or pieces of carbon and the conductors in contact with 

 them/' The truth of this statement is the question at issue. 

 It may be well to remark, however, that although Professor 

 Thompson makes this assertion in December, Mr. Tomlinson 

 had shown, nearly a year earlier, in a paper presented to the 

 Royal Society, on the 26th of the previous January, that the 

 * From Silliman's American Journal for September, 1886. 



