216 Mr. C. Tomlinson's Experiments on the Electrical Fly . 



tion of the force visible, I filled the loop with thick gum-water 

 and saw it dragged out in the direction of the brush (fig. 10). 

 I next fixed the fly on its point with a little bees' wax, and noticed 

 its behaviour in the dark, when a noisy little brush was seen 

 from each of the shoulders (fig. 10) ; the effect of doubling in 

 the points being to transfer the brush from the sharp point to 

 the nearest approach to a point that the fly now presented, 

 namely the kind of rough blunt point formed by the shoulder. 

 And even here a selection was made : the brush proceeded from 

 that part of the shoulder which was not so nicely rounded as the 

 other part, but where an obtuse angle had been formed by the 

 pliers used in bending-in the point. Similarly in the other ex- 

 periments where the point had been tampered with, a brush 

 could always be traced from the shoulder (figs. 7 and 8) and 

 from the edge (fig. 9). In order to get rid of these brushes, I 

 had the balanced disc, fig. 9, nicely constructed by an instru- 

 ment-maker. The disc w r as turned into the shape of a well- 

 rounded ring of wood, which was filled up with shell-lac, and 

 gilt on one side. A screw at the end of the wire entered the 

 wooden rim, and at the other end of the wire was a brass balance- 

 knob, while at the extreme end was a pith-ball ; in this way all 

 points were avoided. When nicely poised on its point on the 

 prime conductor it moved wildly to and fro, as if bewildered by 

 the multitude of objects that attracted it on all sides, but there 

 was no rotation. When the finger was approached towards it, it 

 swung round and acquired a momentum which made it revolve 

 for some turns ; but it soon came to rest, and would revolve 

 either way according to the position of an uninsulated conductor 

 held towards it at the distance of many inches, or even some feet. 

 With respect to the action of flame in reversing the motion of 

 the fly with wax knobs, that seems to be simply a case of repul- 

 sion. Flame and hot air near the prime conductor convey away 

 the same electricity as that of the prime conductor*; so that if, 

 while the fly with wax knobs is rotating with the points forwards, 

 a flame be brought near, we bring up a highly mobile insulated 

 conductor charged +, which acts by repulsion upon the wax 

 knobs as being the largest and most prominent objects on the 

 fly. By this repulsion the fly reverses its course, the brush from 



* The action of a flame, such as that of a spirit-lamp, seems to be a 

 double kind of convection, calorific and electrical. The lines of hot con- 

 ducting air moving from the flame become electrified in the same way as 

 the prime conductor, and extend some feet from the machine. I have 

 been surprised at the distances at which blasts of very hot dry air have 

 swept across my face, producing an uncomfortable sensation, while endea- 

 vouring to trace them by means of a light fly poised on a fine point. That 

 these conducting masses of dry air were in the same state as the prime 

 conductor was proved by means of a gold-leaf electrometer. 



