Number of Electrostatic Units in the Electromagnetic Unit. 301 



in the numbers 0*904187 and 0-03438 (p. 433, Phil. Mag. 

 Dec. 1880), in which the decimal point should stand on the 

 left of the first zero, I trust you will take it for granted that 

 it is a misprint. 



I have the honour to be, Gentlemen, 



Your most obedient Servant, 



R. Shida. 

 Japanese Legation, London, W. 

 September 4, 1881. 



XXXIX. Answer to Dr. Wright's Remarks on Mr. Shida's 

 Measurement of the Number of Electrostatic Units in the 

 Electro-magnetic Unit of Electromotive Force. By Andrew 

 Gray, M.A., Assistant to the Professor of Natural Philo- 

 sophy in the University of Glasgow. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



PERHAPS I, as one to some extent cognizant of the experi- 

 ments the accuracy of which is called in question by Dr. 

 Wright, may be allowed to say a few words in reply to his 

 letter printed in your last number. 



The discussion originated (as Dr. Wright states in that letter) 

 in his remark, in the Philosophical Magazine for May 1881, 

 that the assumption of the equality of the E.M.F. of a Daniell's 

 cell when generating a current, with that of the same cell 

 when its poles are insulated, would introduce an error in the 

 final result of Mr. Shida's research. That this assumption 

 was not made has been already explicitly stated by Mr. Shida; 

 but perhaps, in view of Dr. Wright's account of the matter 

 above, it may not be amiss to sketch shortly the method of 

 investigation which Mr. Shida employed. 



For a reason which will presently appear, he proceeded in 

 the following manner: — (1) He compared by means of a 

 quadrant-electrometer the E.M.F. of a battery of tray Daniells 

 with that of the cell chosen; (2) measured by means of a 

 Thomson's absolute electrometer the electrostatic value of the 

 E.M.F. of the same tray battery ; (3) repeated the quadrant- 

 electrometer comparison. (Dr. Wright is therefore in error 

 in stating in his July Note, and again in his letter of last 

 month, that the quadrant-electrometer comparison was made 

 before and after the electromagnetic valuation. Yet Mr. 

 Shida's statement on this point is perfectly free from ambi- 

 guity.) These experiments were several times repeated, and 

 afforded plainly a means of measuring accurately, more accu- 



