134 E. S. Brough — Maximimi Magnetic "Effect of Electromagnets. [June, 



new Si^ecies of Lepidoptera found in the late Mr. Atkinson's Collection, as 

 a separate work in quarto form, to be brought out in fasciculi as funds per- 

 mit. 



Also that on the recommendation of the Library Committee, the Coun- 

 cil had sanctioned the appointment of a special assistant under Mr. Bloch- 

 mann for the preparation of the Library Catalogue. 



The following papers were read : — 



1. On the Diameter of the Wire to he employed in ivinding an Elec- 

 tromagnet in order to produce the Maximum Magnetic Effect. By R. S. 



BEOrGH. 



In 1866 Mr. Schwendler investigated the best galvanometer resistance 

 to employ in testing with Wheatstone's Bridge, a question which was 

 previously involved in complete obscurity, some physicists arguing that 

 since near balance the current passing through the galvanometer approaches 

 the indefinitely small, therefore the number of convolutions, and hence the 

 the resistance, of the galvanometer ought to be indefinitely great.* 



I may here remark that Count du Moncel in a communication to the 

 Academy of Sciences has unjustly criticized Mr. Schwendler' s method of 

 treating the subject of electromagnets. f The latter supposed the dimen- 

 sions of the bobbins (as I, also, do in this paper) to be given, fixed, and 

 immutable ; while the learned Count starts with varying the depth of the 

 bobbin, and piques himself on getting a larger magnetic effect out of it 

 than Mr. Schwendler did. The best thickness of wire to wind on a given 

 bobbin, and the best size and shape of bobbin to employ for a given purpose, 

 are two totally distinct questions. 



While investigating the above problem, the question of the influence 

 of the insulating covering of the wire on the results occurred to Mr. 

 Schwendler, and he went into it in a subsequent paper. J 



Mr. Schwendler attacked the problem from the point of view of the 

 resistance of the bobbin ; but it seems to me that it yields more readily^ 

 and presents a more definite result (the former method gives an equation 

 of the 4th order, which has to be solved by a rather coarse approximation) 

 when we start from the thickness of the wire. This method has also led 

 me to a singularly simple relation existing between the resistance of the 

 electromagnet and the external resistance. 



I shall take the case of an elongated bobbin with straight sides and 

 circular ends, because this is a very common form to give to galvanometer 

 coils, and because the results can at once be reduced to those applicable to 



* Philosophical Magazine, May, 1866. '^ 



t Comptes Eendus, Vol. LXXVI, pp. 368-371. 

 X Philosophical Magazine, January 1867. 



