264 On the Electric Resistance of Carbon under Pressure* 



should be made against the carbon itself. Copper wires were 

 then carefully soldered to the copper coating at about 0*5 cen- 

 tim. from the ends. The object of this arrangement was to 

 render possible a comparison between the resistance of the rod 

 when there was merely end-contact — which might be more or 

 less perfect according to the pressure — and the resistance of 

 the rod as measured when there was a perfect contact through 

 the deposited coatings of copper. The rod was then laid in a 

 horizontal frame, where it reposed on two Y-shaped bearings — 

 one end pressing against a lever-arrangement for the purpose 

 of putting on a measurable amount of pressure, the rod being 

 fixed at the other end by abutting against a brass set-screw. 

 A copper piece was introduced between the lever and the ex- 

 tremity of the rod, in order to provide an end-contact; and 

 arrangements were made whereby the rod could be connected 

 up in a Wheatstone-bridge, the connexion being made at 

 pleasure either through the end-contacts or through the 

 copper-plated junctions. 



The resistance of the rod between the copper-plated junc- 

 tions was then measured, the rod being free at both ends. It 

 was found to be 0*56 ±0*007 ohm. The end-contacts were 

 then made to touch lightly (the circuit through them remain- 

 ing open). The resistance through the copper-plated junctions 

 showed no change. Pressure was then applied to the rod lon- 

 gitudinally, and augmented until it began to show lateral 

 distortion, the effective force along the rod being 4150 grms., 

 equivalent (if the area of cross section of the rod be taken as 

 0'18 square centim.) to 23,055 grms. per square centimetre. 

 Yet, even under this pressure, not the smallest change could 

 be detected in the resistance between the copper-plated junc- 

 tions. If there was any, it was certainly less than 0*005 ohm, 

 or less than 1 per cent, of the whole resistance. 



The circuit was now made through the end-contact- by 

 moving the set-screw until the lightest possible contact was 

 obtained, the connexions through the copper-plated junctions 

 having been thrown out of circuit. The resistance thus deter- 

 mined was 1*1 ohm. Pressure was applied as before. The 

 resistance fell to 0*72 ohm when the pressure of 23 kilo- 

 grammes per square centimetre was reached. On releasing 

 the pressure, the resistance again rose until contact became as 

 light as possible. The resistance attained 1*08 ohm, when it 

 rose abruptly to infinity as the set>screw ceased to touch the 

 end of the rod. The battery and galvanometer used through- 

 out were the same as described above. 



Nothing could be more significant than these observations. 

 When perfect contact was ensured by electroplating, pressure 



