446 MR. J. E. H. GORDON ON MEASUREMENTS OF ELECTRICAL CONSTANTS. 
paraffin with those of Messrs. Gibson and Barclay, seems to indicate the non¬ 
existence of any change. The difference between my experiments and Dr. Hopkinson’s 
may possibly, by some, be considered an indication that a change takes place. I hope 
a more or less complete investigation of this point will be the subject of a future 
memoir. * 
I must not conclude this paper without expressing my great gratitude to Professor 
Clerk Maxwell for his kindness in assisting me with advice and suggestions from 
time to time during the progress of the work. The original plan of the investigation, 
and the first sketches of the more important parts of the apparatus, are all due to him. 
* Note added June 17, 1879.—Until now all attempts to use the battery instead of the coil have proved 
unsuccessful. No consistent results have been obtained with it. I think the want of success is due to the 
difficulty of discharging the apparatus, and of properly insulating it from the battery when the latter 
is not supposed to be connected to it. Four different commutators have already been tried without 
success. Professor Cornu has, however, had the kindness to devise another one for me, and this is now 
in course of construction; I have considerable hopes that it will answer its purpose. 
September 25, 1879.—Some preliminary experiments made with the Cornu commutator, and a Holtz 
machine have been successful. 
