of Soft Carbon under Pressure, 361 



in a vertical position the appearance was that of the letter "U," 

 the height being about 15 centim. Near the lower end of 

 each a short tube was sealed in, over which rubber tubing- 

 could be passed, and at the lower part of the curve, in each, a 

 platinum 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 partition was formed between the two halves of a "U" 

 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 enclosing 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 

 arrangement being substantially the same as that previously 

 used in the experiments with hard carbon. A pressure-gauge, 

 sometimes of water, sometimes of mercury, was attached to 

 Phil Mag. S. 5. Vol. 22. No. 137. Oct. 1886. 2 B 



