﻿220 Mendenhall — Electrical Resistance of Soft Carbon. 



cups and enough additional resistance was introduced to make 

 a convenient deflection of the spot of light upon the scale. 

 When all was adjusted and the spot of light was at rest, the 

 pressure was applied by turning the handle of the vise. In 

 every instance the deflection decreased showing diminished 

 resistance. This effect was not due to the heat produced by- 

 compression as experiment proved that cause to be inadequate. 

 It was found to be necessary to make the carbon rod decidedly- 

 warm to the touch in order to lower the resistance by the same 

 amount, besides the effect was not transient as would have been 

 the case if it had been due to the change in temperature. It 

 was also found that compression at right angles to the direction 

 of the current produced a similar effect, but less in magnitude. 

 These facts had been already announced by Mr. Tomlinson. 



These experiments with hard carbon or with other rigid 

 bodies are comparatively easy, as there is no difficulty in apply- 

 ing the pressure independent of the contact surfaces, so that 

 possible variation of the latter need not be considered. Un- 

 fortunately it appears to be quite impossible to secure this 

 arrangement in the examination of soft carbon. It cannot 

 readily be obtained in forms different from the small disk or 

 button in which it originally appeared, and it is so fragile that 

 it requires the most careful manipulation. Under these cir- 

 cumstances the only thing to do is to secure the best possible 

 surface contact of the poles to begin with. Perhaps the ideal 

 arrangement would be a disk with • its two opposite faces 

 electro-plated with copper, through which a contact with mer- 

 cury can be secured. The electro-plating of two opposite faces 

 of a disk of compressed lamp-black is a work of extreme diffi- 

 culty and so far as known to the writer has not yet been ac- 

 complished, although he is greatly indebted to Mr. Edison for 

 a serious and persistent effort to secure this result, none the 

 less appreciated because, owing to the extremely fragile char- 

 acter of the disk, it proved to be unsuccessful. 



It was therefore necessary to depend upon the contact of 

 mercury with the surface of the carbon itself. As this was the 

 contact employed by Professor Barrett in the experiment which 

 Professor Thompson considered " crucial," its use can hardly 

 be objected to in this instance. 



The arrangements for the test of the soft carbon were as 

 follows : two glass tubes about 20 cm in length were bent at one 

 end into a quarter of a circumference, so that when the two 

 were joined and the straight branches of the tube were in a 

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

 the height being about 15 cm . 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 



