GENERAL ACCOUNT OF COMMON PHENOMENA 17 



Belli 1 s machine.* In the form here described there are two 

 metal plates, the field plates A B,bent, as in Fig. 14, like the covers of 

 a book and fixed on insulating pillars. The movable part consists of 

 two circular metal plates, the "carriers'" CD, at the extremities 

 of a glass arm. By turning a handle these can be revolved on a 

 horizontal axis so that they pass through the spaces partially en- 

 closed by the field plates. Projecting from the inside of each 

 field plate is a spring ,9, which each carrier touches as soon as it is 

 completely within the field plates. After contacts with the springs 



Fro. 14. 



are broken the carriers come in contact with the two ends of a wire 

 // //, the "neutralising rod " of later machines, which puts them in 

 electric communication while still within the field plates. To 

 understand the action of the machine let us suppose that to begin 

 with there is a positive charge on A and that the carrier plates 

 start from the position of contact with the wire n H, C being 

 within A and D within B. Then negative electricity is induced 

 on C and positive is sent to D. These charges are carried round 

 until C touches the spring s within B,andD touches that within A. 

 Since the carriers are almost enclosed by the field plates and are in 

 contact with them, their charges, tending to get to the outside, pass 

 almost entirely to the field plates, increasing the + charge on A 

 and giving a charge to B. The carriers then move on to con- 

 tact with the ends of the wire n n, when D will be negatively 

 and C positively electrified by induction. Further rotation brings 

 C again within A and D within B, and on contact with ss the 

 charges go to increase those on the field plates. Thus the charges 

 on the field plates rapidly increase. 



* I)i VTUI iin>,r,i 1,,'iniera di mvcchinsi cllctrica, Ann. Sci. Lomb. Veneto, I. 

 (1831), p. 111. 



