Prof. R. Threlfall on Sensitive Galvanometers. 509 



not using the galvanometer at its limit of sensitiveness. Surely 

 it is not necessary to remind such well-known experimentalists 

 as my critics that an absolute measurement depending on the 

 indications of an instrument at its extreme sensitiveness are 

 always liable to objection. I find in fact from my notebook 

 that the sensitiveness mentioned having been obtained, it was 

 reduced (by lowering the control magnets) to meet the 

 requirements of the work in hand. The resistance of the 

 sulphur turned out to be much less than I had anticipated, 

 for I had got three hundred and sixty small storage-batteries 

 ready for the work, as well as some forty Clark- cells; so I 

 reduced the number of cells and the sensitiveness of the galva- 

 nometer in what appeared to me at the time as the most 

 advantageous manner. On reading the paper, however, I see 

 that I was not so explicit (for the sake of brevity) as I might 

 have been. I welcome a discussion on sensitive galvano- 

 meters, however, though it has been raised somewhat at my 

 own expense, because I believe that the galvanometer-method 

 has several advantages over the electrometer-method in high- 

 resistance measurements. To this, however, I will not now 

 refer, but will instead make some further remarks anent sen- 

 sitive galvanometers. 



My friend Mr. Boys, at the Physical Society some time 

 ago, said that my quartz fibres were too long and thick. 

 Well, that is a matter of opinion. I used the finest fibres 

 that I could manipulate ; and as I saw that I could handle a 

 yard of them almost as well as an inch, I did not see why I 

 should not reap the advantage of length. I saw Mr. Boys's 

 fibres, and I do not think that he mounts much finer ones than 

 I do, though as it is purely a matter of manipulatory skill I 

 daresay Mr. Boys can mount finer fibres than I can. If I 

 can persuade Mr. Gray to use quartz instead of silk as a 

 means of suspension, I am sure he will live to bless me. 

 While I am on this subject I will add, that if Professor Gray 

 will reread my paper he will find he has been unjust to me in 

 stating that I must have used the galvanometer near the limit 

 of sensitiveness as the zero was " always on the move." He 

 will find that the whole sentence runs thus r — "It is practi- 

 cally impossible to get a galvanometer of this degree of sen- 

 sitiveness to work with a silk fibre, the zero being always 

 on the move." Since I gave this as one reason for having 

 adopted quartz, Professor Gray will, I think, see that his 

 argument, in so far as it depends on the galvanometer being 

 at its limit of sensitiveness, is baseless, and that his stricture 

 is unmerited. 



With respect to quartz fibres, 1 may say that I am forward- 

 ing a paper on this subject to you ; but meanwhile I desire to 



