360 Mr. T. C. Mendenhall on the Electrical Resistance 



The terminals of a reflecting-galvanometer whose resistance 

 was about 5000 ohms were also introduced into these mercury 

 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 vice. 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 

 applying the pressure independent of the contact surfaces, so 

 that possible variation of the latter need not be considered. 

 Unfortunately 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 

 circumstances, 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 electroplated with copper, through wdiich a contact with 

 mercury can be secured. The electroplating of two opposite 

 faces of a disk of compressed lampblack is a work of extreme 

 difficulty, and so far as known to the writer has not yet been 

 accomplished, 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 

 character 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 centim. 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 



