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LIX. On Sensitive Galvanometers. By R. Threlfall, M.A., 



Professor of Physics, University of Sydney, JV.S.W. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



THOUGH I expect the matter will have been almost for- 

 gotten when this reaches England, I still desire, if 

 possible, to meet some of the criticisms which have been 

 levelled at me by Professor Gray in the Philosophical Maga- 

 zine for February 1890, and by Professor Ayrton at the 

 Physical Society on January 17th. Of Professor Ayrton's 

 criticisms I have only as yet seen a report in ' Nature ' for 

 January 30, but since they appear in the main to coincide 

 with Professor Gray's as nearly as I can judge, I will endeavour 

 to reply to both at once. 



In the first place, then, I must of course plead guilty to 

 having made a mistake in arithmetic (to my shame be it said), 

 and must express my thanks to Professors Ayrton and Gray 

 for pointing it out. It now appears that instead of my success 

 with the Gray galvanometer being a thousand times less than 

 Professor Gray's, it is only about a hundred times less as 

 compared with the performance in 1884, and three times less 

 as compared with his results in 1889. As the matter is one 

 which purely depends on instrument-making and experiment- 

 ing, I can only say that I am very glad to find myself approach 

 so nearly to Professor Gray. With respect to the much graver 

 and more important charge that I have overestimated the per- 

 formance of another type of galvanometer, I can only say that 

 the mistake rests entirely with my critics, though very likely 

 through my own fault in endeavouring to achieve conciseness 

 of statement. That the galvanometer described did actually 

 attain the sensitiveness quoted, viz. 5 divisions to 10 -11 ampere 

 of current, a fresh reference to my notebook amply proves. 

 Neither in my paper on " The Measurement of High Specific 

 Resistances," nor in the following one on the Specific Resist- 

 ance of an Impure sample of Sulphur, have I given any data 

 from which this value was computed. How, then, can Pro- 

 fessors Ayrton and Gray have concluded that I was in error ? 

 Professor Ayrton, as I imagine, and certainly Professor Gray, 

 have obtained their data from the resistance-measurements 

 quoted ; but it is purely gratuitous on their part to assume 

 that these were the data used by me, and as a matter of fact 

 they were not. In writing the paper, which pretended to be 

 an account of an absolute measurement, 1 gave the maximum 

 sensitiveness reached by the galvanometer in order to show 

 that I had a good margin in the actual experiments, and was ; 



