Electrical Resistance of Soft Carton under Pressure. 359 



electrical resistance of hard carbon was diminished by pres- 

 sure. The amount of the diminution is small, however, and 

 he afterwards expresses the opinion that in such instruments 

 as the microphone transmitter, the greater portion of the ob- 

 served 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 experi- 

 ments 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 

 experiments about to be described. 



In the beginning it was desirable to determine, roughly, 

 the magitude 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 centim. in length and 1*5 centim. 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 centim. in width was left near each end of the rod. 

 Two cork rings 1*5 centim. thick were fitted to the rod, after 

 which they were tunnelled 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 vice. The current from a battery of two or three gravity- 

 cells was passed through the rod by plunging wires into the 

 mercury cups formed by the corks. By this arrangement it 

 was possible to apply pressure at the ends of the rod without 

 in any way influencing the contacts through which the cur- 

 rent passed. 



* * Nature,' March 16, 1882. 



