210 Mr. C. Toinlinson's Experiments on the Electrical Fly . 



travel backwards and forwards between the points of the fly and 

 the inner surface of the bell-glass. Brushes are given off by the 

 fly, which now rotates, and a star is seen on the metal point held 

 towards the outside of the bell-glass. 



A charge was given to the bell-glass by holding it in both 

 hands and allowing the inside to rub against a chain hanging 

 from the prime conductor. On lowering the receiver over the 

 insulated fly, it moved slowly for a short time, but on touching 

 the copper disc and also the side of the glass opposite the fly, 

 the motion became very brisk. In this way the fly has been 

 started no less than sixteen times, after having been brought to 

 rest each time (see fig. 2). 



In this case the bell-glass is converted into a Leyden jar : the 

 inside is vitreous, together with the copper disc and the enclosed 

 fly and metal support. The outside of the bell-glass is resinous. 

 On touching the copper disc alone, there is no motion of the fly 

 any more than touching the outside of the glass alone, because 

 in either case the balance between the outer and inner charge is 

 not disturbed. If, however, the copper disc and the outside of 

 the bell-glass be both touched, a portion of the charge is libe- 

 rated, induction can go on between the included air and the 

 points, and the fly is started by the action of the discharging 

 particles. Soon, however, electrical equilibrium is re-established, 

 the momentum of the fly is expended, and the fly is brought to 

 rest. Touching the copper disc and the outside of the bell-glass 

 neutralizes another portion of electricity; the enclosed air is 

 again in a condition to relieve its tension by discharging on the 

 points of the fly, to which motion is thus again imparted. Of 

 course the larger the portion of the outside that is touched the 

 stronger is the action on the fly, and the longer does it continue 

 to spin. 



This experiment is well adapted to the lecture- table, and pro- 

 duces surprise by the facility with which the fly is started appa- 

 rently by a very inadequate operation. It also illustrates the 

 action of the Leyden jar, and how well the coatings may be dis- 

 pensed with. That the copper disc and fly are in the same state 

 as the inside of the jar was proved by means of the gold-leaf 

 electrometer. The experiment was also tried with resinous elec* 

 tricity on the inside with similar results. 



The facility with which a coated Leyden jar may be discharged 

 by presenting a point to the knob is well known. If that point 

 were free to move, it would first be attracted, and then repelled 

 in the act of doing so *. The fly behaves in a similar manner. 



* This effect may be shown by suspending a darning-needle by tying £ 

 filament of silk to the centre of its length ; or if required to be uninsulated, 

 passing the needle through one end of a very narrow strip of tinfoil and 



