﻿Mendenhall — Electrical Resistance of Soft Carbon. 219 



electrical resistance of hard carbon was diminished by pressure. 

 The amount of the diminution is small, however, and he after- 

 wards expresses the opinion that in such instruments as the 

 microphone transmitter, the greater portion of the observed 

 diminution in resistance is due to variation in surface contact.* 

 Mr. Tomlinson's experiments were made with hard carbon, 

 similar in character to that made use of in experiments to be 

 described presently. 



In the summer of 1884, the writer communicated, to the 

 American Association for the Advancement of Science, a brief 

 account of experiments which satisfied him that the opinion 

 which he had previously expressed concerning the nature of 

 the phenomenon was unquestionably correct. Within the past 

 year the subject has been taken up again, and by means of 

 improved methods and instruments, all doubts seem to have 

 been removed. 



Innumerable experiments made by physicists of many coun- 

 tries have established, beyond question, the fact that the elec- 

 trical properties of matter are modified by stress and strain. 

 In carbon the effect of pressure is to diminish resistance. For 

 hard carbon this was established by the investigation of Mr. 

 Tomlinson. In compressed lampblack, as seen in Edison's 

 disks, the effect is very great and that this is for the most part 

 a true pressure-effect is proved, it is believed, by the experi- 

 ments about to be described. 



In the beginning it was desirable to determine, roughly, the 

 magnitude of this effect in the case of hard carbon. For this 

 purpose a copper plated rod, such as is used in the arc lamp, 

 about 12 cm in length and l*5 cm in diameter, was selected and its 

 ends were ground flat at right angles to its axis. The plating 

 was then removed, except that a band about '5 cm in width was 

 left near each end of the rod. Two cork rings l*5 cm thick were 

 fitted to the rod, after which they were tunneled out on the 

 inside, and a hole was made in each so that when they were in 

 place over the copper bands, and mercury was poured in, it 

 would flow around the ring tunnel and make a contact with the 

 carbon as satisfactory as could be desired. The ends of the rod 

 were protected by thin plates of vulcanite, and it was placed 

 between the jaws of a vise. The current from a battery 

 of two or three gravity cells was passed through the rod by 

 lunging wires into the mercury cups formed by the corks. 



y this arrangement it was possible to apply pressure at the 

 ends of the rod without in any way influencing the contacts 

 through which the current passed. 



The terminals of a reflecting galvanometer whose resistance 

 was about 5000 ohms were also introduced into these mercury 



* Nature, March 16, 1882. 



