﻿218 Mendenhall — Electrical Resistance of Soft Carbon. 



Art. XXY. — On the Electrical Resistance of Soft Carbon under 

 Pressure ; by T. C. Mendenhall. 



A PAPER by the writer on "the influence of time on the 

 change in the resistance of the carbon disk of Edison's Tasime- 

 ter," was published in this Journal in July, 1882. The object 

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

 some experiments with the carbon disk which 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 reference 

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

 Tomlinson and by Professors Sylvanus P. Thompson and W. R 

 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 unfavorable 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 pressure," 

 which was an exceptionally severe criticism of- the previous 

 paper by the writer. In this article Professor Thompson refers 

 to the investigations of Mr. Tomlinson, Professor Barrett and 

 himself and also to experiments made by Professors JSTaccari 

 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 



