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LXX. On an Absolute Galvanometer. 

 To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



THE description which I gave in the October Number of your 

 Magazine of an absolute galvanometer has given rise to 

 a communication in the November Number of the Journal de 

 Physique theorique et appliquee, by Professor A. Lallemand. 

 Professor Lallemand states that the instrument described by me 

 is identical with the electrodynamic torsion-balance described by 

 him in the years 1848 and 1851 in the Annates de Chimie et de 

 Physique. At the same time Professor Lallemand describes the 

 more recent modifications which he has introduced into his in- 

 strument. 



It becomes me, therefore, briefly to notice (1) the similarity 

 between Professor Lallemand's electrodynamic torsion-balance 

 as described in the Annates de Chimie et de Physique and my 

 absolute galvanometer, and (2) between the latter and Professor 

 Lallemand's most recent instrument. 



The essential difference between Professor Lallemand's balance 

 and my galvanometer is this, that in the former the current- 

 strength is measured by the repulsion between flat opposed spirals 

 through which the current passes; in the latter, repulsion is ex- 

 erted between electromagnets excited by the current. The law of 

 comparative repulsion is the same; but I believe that the former 

 method (which has been, since Lallemand described his instru- 

 ment, much employed for the determination of electrodynamic 

 constants) cannot give such delicate results as when the form 

 I suggest is adopted. 



In the modification of his balance which Professor Lallemand 

 now describes, he proposes to employ independently excited 

 electromagnets to repel movable solenoids through which the 

 current under examination passes, or to repel the latter by means 

 of semicircular permanent magnets. These latter modifications 

 no doubt increase the similarity between our instruments. But 

 I cannot in any sense admit that the absolute galvanometer 

 lately described by me is identical with the electrodynamic ba- 

 lance described by Lallemand twenty years ago. The latter in 

 all its forms is admirably adapted for the measurement of induced 

 currents. I believe, under correction, the former is more adapted 

 for the absolute comparison of currents, especially of weak ones. 

 Your obedient Servant, 



Frederick Guthrie. 



