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XIX. Remarks on Amperes Experiment on the Repulsion of a 

 Rectilinear Electrical Current on itself. By Professor Van 

 Breda of Haarlem, in a Letter to James D. Forbes, D.C.L., 

 F.R.S., F.P.R.S.E., Principal of the United College, St. 

 Andrews*. 



[Plate I. fig. 7.] 



To James D. Forbes, Esq., F.R.S.L. $ E. ■ 

 Sir, 



IN the February Number of the Philosophical Magazine for 

 this year, you have published the description of some expe- 

 riments tending to prove that, contrary to Ampere's theory, the 

 contiguous parts of an electric current attract instead of repel 

 each other. And in the April Number of the same Journal, 

 Mr. Croll of Glasgow endeavours to show that the experiment 

 by which Ampere believed he had proved the reality of this repul- 

 sion might be explained in an entirely different manner. The 

 result of experiments, partly old and partly new, which we have 

 made, leads us to differ from you as to the import and signifi- 

 cance of the results which you have obtained; and from Mr. 

 Croll, as to the validity of his explanation. We shall describe 

 these experiments and communicate the reflections which they 

 have suggested, requesting you to publish them in the same 

 Journal if they seem to you to be worth the attention of physicists. 



We shall commence with Mr. Croll's article. We shall not 

 discuss it from the theoretical point of view, nor ask the author 

 how, if he admits that two currents, one of which is directed 

 towards their point of intersection, and the other from it, repel 

 each other when they make any given angle with each other, it 

 is possible to deny this repulsion in the single case in which this 

 angle is equal to two right angles, that is, when the two currents 

 form part of the same rectilinear current. Such discussions 

 appear to us only of relative value ; they can only serve to de- 

 cide a question when this cannot be decided experimentally. We 

 have resorted therefore to experiment to determine the value of 

 Mr. CrolPs experiment. 



At a height of about 8 inches above a wooden trough A B 

 (fig. 7, Plate I), divided into two compartments for Ampere's 



* Communicated by Principal Forbes. The paper by Mr. Forbes, referred 

 to in this communication, appeared in the Philosophical Magazine for 

 February last. An interesting experiment by Professor Tait of Edinburgh, 

 in which the Amperian repulsion was distinctly proved in a homogeneous 

 conductor composed of mercury alone, is printed in this Magazine for April 

 1861, but has probably escaped the notice of Professor van Breda. We are 

 authorized to state that Professor Tait's experiment had already removed 

 all doubt from Principal Forbes's mind as to the reality and energy of the 

 Amperian repulsion. — Ed. 



