378 Prof. J. Trowbridge on the Electrical 



mnnganin wire in our tests ; but in order to be sure that our 

 fine short wires had not been overheated, we constructed a 

 five-ohm resistance of four strands of coarse manganin wire 

 about 0*25 millim. diameter and 3*5 metres long. This was 

 stretched upon each side of a thin vulcanite plate to avoid 

 self-induction, but it allowed essentially the same number of 

 oscillations to pass as did the short fine wire. A short german- 

 silver wire, with a very high temperature- coefficient, showed 

 a conductivity only a very little less ; thus the error from the 

 heating of the wire may be neglected. 



In order to show that common electrolytic polarization does 

 not interfere with the accuracy of our method, we measured 

 with the help of our 20,000 volt storage-battery and conden- 

 sers the resistance between two bright platinum plates similar 

 in size to the copper ones described above in a cupric sulphate 

 solution. This was found to be four ohms, and after plating 

 the electrodes with copper the resistance remained unchanged. 

 Kohlrausch's method gave no satisfactory result with both 

 electrodes free from copper, but when both were plated it 

 indicated a resistance of 3*9 ohms. 



Our method may therefore be a useful one for the approxi- 

 mate determination of conductivities in cases where impurities 

 or polarization render Kohlrausch's method unsatisfactory. 

 For accuracy, of course pains must be taken to develop all 

 the photographs in the same fashion, and in general to arrange 

 the conditions of the exposure alike in all cases. 



Our conclusion that the conductivity of electrolytes is not 

 greatly affected by great changes in current- strength only 

 emphasizes all the more strongly the conclusion of our last 

 paper, that the conductivity of gases is very much affected by 

 changes in the current- strength. 



Harvard University, 

 March 8, 1897. 



LI. The Electrical Conductivity of the jEther. 

 By John Trowbridge*. 



THE electrical conductivity of the sether has been main- 

 tained by Edlund and has been apparently disproved by 

 various recent investigations — notably those of Prof. J. J. 

 Thomson f. The latter writer, in his treatise entitled ' Recent 

 Researches in Electricity and Magnetism/ also remarks, 

 p. 9g : — "Again, if we accept Maxwell's Electromagnetic 



* Communicated by the Author. 



t Roy. Soc. Proc. vol. xlv. 1888, p. 290. 



