208 Mr. C. Tomlinson's Experiments on the Electrical Fly, 



the action, if I read him correctly, appeared to be this : — When a 

 metal point projects from an insulated charged conductor, the 

 particles of air at and about the point become more highly polar- 

 ized than at the parts of the conductor where the lines of induc- 

 tive force cannot so readily accumulate. The particle of air next 

 the last particle of metal on the point thus becomes so highly 

 charged that it soon discharges itself on the point, becomes 

 similarly electrified to the point, is repelled and acts inductively 

 upon surrounding particles to some distance. Meanwhile a 

 second particle, taking the place of the first, becomes so highly 

 charged as, in its turn, to discharge itself upon the point, and 

 becoming similarly electrified, is repelled. In this way a rapid 

 interchange of particles of air is kept up, producing the convec- 

 tive or brush discharge ; and so rapid is the interchange of par- 

 ticles, that the effect is that of a strong wind sufficient by some 

 mode or other to give motion to the point, if free to move. In 

 the case of the horizontal electrical fly, there are four points 

 all briskly engaged in this work of convective discharge, so that 

 the motion of rotation in a direction contrary to that of the points 

 is exceedingly rapid. 



In the following experiments (unless otherwise stated) a fly of 

 brass wire 2f inches in diameter was used ; it weighed 33 grains ; 

 it had four arms, and was poised by its central conical cap upon 

 a steel point 5 inches high rising from a brass support. When 

 placed on the prime conductor, a short steel needle was used as 

 the supporting point. The electrical machine had a plate 2 feet 

 in diameter*. 



2. The Fly in air. 



When the fly on its stand is placed on the table at different 

 distances below and away from the prime conductor without any 

 connexion therewith except through the medium of the dielectric 

 air, and also when held 12 or 15 inches above the prime con- 

 ductor, it rotates briskly. If held in the aura from a blunt 

 point the fly becomes more active, and also rotates some feet 

 above or beyond the aura proceeding from a sharp point. 



In all these cases the fly is uninsulated. If the operator stand 

 on an electric stool and hold the fly near the prime conductor or 

 in the aura, or if the fly be placed on an insulating stand, there 

 is no motion whatever, because in all these cases the operator 

 and the fly are in a polar condition like the air, and, like it, also 



* I occasionally used other flies ; one with only one arm, a small ivory 

 ball on the other side of the cap being used as a counterbalance. Also a 

 fly with two arms. This and the former oscillated too much. I had two 

 flies with four arms, one with five, and one with eight. The last moved 

 slowly, but was too heavy for most purposes. 



