﻿Mendenhall — Electrical Resistance of Soft Carbon. 221 



wire was passed through and sealed. The ends of the tubes 

 were ground flat, and they were mounted in such a way that 

 while one was fixed in position, the other could be moved 

 toward or away from it in one plane, and so that the ground 

 ends of the curved parts were always exactly opposite to each 

 other. The movable tube was then taken from its place, the 

 ground edge of its curved end was coated with glue, and it 

 was carefully brought down upon the upper surface of a carbon 

 disk which rested in a horizontal plane. The glue causing the 

 disk to adhere to the tube, the latter could then be secured to 

 its sliding stand, ready to move into place. The ground edge 

 of the fixed tube was now coated with glue, after which the 

 movable tube holding the carbon disk was gently moved up 

 until the disk pressed against the end of the other tube, the 

 glue forming the junction. In this way a carbon wall or par- 

 tition was formed between the two halves of a "IT" tube. 

 When the glue had hardened, mercury was introduced on both 

 sides to a height sufficient to entirely cover the faces of the 

 carbon disk. The current was introduced through the platinum 

 electrodes which plunged into mercury cups on either side. 



In some of the earlier experiments variations of pressure 

 were produced by the addition of mercury to the two branches 

 of the tube, but vastly better than this was the method latterly 

 used in which the pressure of air was substituted for that of 

 mercury. Glass plates were sealed on the open ends of the 

 two upright branches thus inclosing a space on each side, 

 except at the small side tubes to which short pieces of rubber 

 tubing were attached. These were joined by means of a"T" 

 tube so that equality of pressure on both faces of the disk was 

 secured. 



The circuit consisted of the battery, the disk and an addi- 

 tional resistance varying from 3 ohms to 10 ohms for purposes 

 of comparison. The electric ends of the disk and of the resis- 

 tance were joined to a specially arranged key, by means of 

 which either could be connected with the terminals of a reflect- 

 ing galvanometer whose resistance was about 7000 ohms. By 

 means of the deflections of the needle of this galvanometer, the 

 resistances were compared and variations noted, the arrange- 

 ment being substantially the same as that previously used in 

 the experiments with hard carbon. A pressure gauge, some- 

 times of water, sometimes of mercury, was attached to the 

 apparatus to indicate variations in pressure, and these varia- 

 tions were generally produced by blowing from the mouth into 

 a rubber tube about two meters in length. Yery many experi- 

 ments were made, all without exception, showing great diminu- 

 tion in the resistance of the disk by increase of pressure, and it 

 will be sufficient to quote a few of the results. 



