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XLIX. Note on the Correction for the Length of the Needle in 

 Tangent-galvanometers. By G. Johnstone Stoney, M. A. , 

 F.R.S., Secretary to the Queen's University in Ireland. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, Dublin, April 7, 1862. 



PROFESSOR ZENGER, in the Supplement to the December 

 Number of the Philosophical Magazine, which has just 

 been published, couples my name in so pointed a manner with a 

 formula which he criticizes, that I am compelled, though wholly- 

 averse from controversy, to ask you to be so good as to insert 

 the following note in reply. 



I am, Gentlemen, 



Yours faithfully, 



G. Johnstone Stoney. 



Until I read Professor Zenger's remarks*, I was under the 

 impression that the formula 



:=Ktan0(l+ ^\ 2 sin 2 6>V 



which is a necessary result of the known laws of electro-magnetism, 

 had been, either in this or some equivalent form, long adopted 

 by physicists as the expression which furnishes the correction 

 arising from the length of the needle of a tangent-galvanometer 

 of the usual pattern, on the hypothesis that X (the ratio of the 

 distance between the poles of the needle to the diameter of the 

 circular current f) is sufficiently small to sanction our neglecting 

 its fourth power. 



Prof. Zenger does me too much honour in supposing me the 

 author of this formula. I do not know by whom it was origi- 

 nally investigated; but it had been, as I mentioned J at the 

 commencement of the paper which he criticizes, known to, and 

 extended by, other writers before me. What I sought to con- 

 tribute to our knowledge of the subject was an examination from 

 which it appeared that no alteration of the formula becomes 

 necessary when the ordinary tangent-galvanometer is out of ad- 

 justment in that slight degree likely to occur in practice \ whereas 

 the simpler formula for Gaugain's galvanometer requires, under 

 similar circumstances, corrections which it would be difficult to 

 apply. 



* Philosophical Magazine, Supplement for December 1861, p. 52.9. 

 f Professor Zenger seems to misunderstand this symbol. See his defi- 

 nition of X on the top of page 530. 

 % Phil. Mag. February 1858, p. 135. 



