[ 202 ] 



XXXVII. Experiments on the Electrical Fly . By Charles Tom- 

 linsox, Lecturer on Physical Science, King's College School, 

 London*. 



[With a Plate.] 



EVERY lecturer on frictional electricity during the last hun- 

 dred years must have made use of an elegant little toy 

 known in England as the " electrical fly/' and in French as the 

 tourniquet elect rique. The explanation of this instrument has 

 changed its position several times on the shifting sands of elec- 

 trical theory, and even in modern works of authority its action 

 is differently explained. Thinking it to be my duty as a lecturer 

 to satisfy myself on all points of scientific doctrine that might 

 arise in my lectures, before I could hope to satisfy the more 

 intelligent of my pupils, I have traced the history of the elec- 

 trical fly and noted the various theoretical opinions respecting 

 it+, and have also contrived some experiments with a view to 

 settle the theory, at least in my own mind. 



1. History of the Electric Fly. 



The electrical fly was invented by Mr. Hamilton, Professor of 

 Philosophy in the University of Dublin, and was first described 

 by Mr. Wilson in 1/60J. He says, "Let a slender brass or 

 iron wire, 5 or 6 inches long and finely pointed at each end, be 

 fitted .in the middle with a brass cap void of angles ; then let 

 half an inch at each extremity be bent in opposite directions till 

 they are perpendicular to the rest of the wire, and in such a man- 

 ner that, when the wire is suspended by means of its cap on a 

 point of metal, it may be in a plane parallel to the horizon §. 

 The pointed metal which supports this wire must be 2 or 3 

 inches long, and have its other end fixed into a small block of 

 wood. Now if this block with a wire suspended be set upon an 

 electrified body, the wire will turn round with a very great 

 velocity, moving always in a direction contrary to that in which 

 the electric fluid issues from its points, without having any 

 conducting substance near it save that of the air; and if the 

 wire be made to turn round by any force in the opposite direc- 

 tion, so that its points go foremost, it will, when electrified, soon 

 be deprived of that motion and made to turn round the con- 



* Communicated by the Author. 



t In the following quotations from different authors, the passages in 

 italics have been underlined by me. — C. T. 



| Transactions of the Royal Society, 1760, "Further Experiments in 

 Electricity," by Mr. Wilson, p. 9( >5. 



§ Some French writers represent the fly according to this description, 

 as consisting of a single wire (see Plate II. fig. 1). English writers usually 

 represent it as consisting of two wires crossed at right angles (fig. 2). 



