330 



MR. W. DUDDELL ON THE RESISTANCE AND 



electrode and the search carbon, the difference for the same movement of the search 

 carbon is 1'64 ohms. The mean of these two results is 1'68 ohms for a movement 

 of the search carbon of 4 millims. If this distance really represented the length of 

 the vapour column between the two positions of the search carbon, and if its 

 resistance is uniform, then its resistance per millim. would be 0'42 ohm. It is 

 probably less than this, owing to the length of the vapour column between the two 

 positions being appreciably longer than 4 millims., due to its distorted shape, 



TABLE V. Search Carbon in Arc. 



Carbons both 11 millims. diameter. Solid " Conradty Noris." 



Arc length 6 millims. Direct current through arc 9 '91 amperes. 



Added alternating current O'l ampere. Frequency 100,000 -- per second. 



Assuming the resistance to average 0'42 ohm per millim., then the resistance of 

 the G millims. of vapour column is only 2 - 5 ohms, as against 5 '2, the measured 

 value for the lamp and arc. There must therefore be a large resistance at or near 

 the electrodes. Calculating its value from each of the three experiments, the resist- 

 ance at or near the positive electrode is 172, T36 and 176 ohms respectively, mean 

 1'61 ohms; and the resistance at or near the negative electrode is I'll, 1'29 and 

 T15 ohms respectively," mean 1'18 ohms. As .there seems no very good reason to 

 suppose that the resistance of the vapour column is very much greater near the 



