ELECTRIC GENERATORS. 105 



producing rotation of the instrument in the opposite 

 direction to that in which the points turn. 



The effect of a stationary point in producing a cur- 

 rent of air is shown in Fig. 34; where the flame of a 

 candle is represented as blown from a point attached 

 to the prime conductor. 



The direction of the air current will be the same 

 whether the charge is positive or negative : since, in 

 either case, the air embraced within a sphere of which 

 the point is the center will have the same potential as 

 the prime conductor; while that outside of this sphere 

 will assume the opposite potential by induction. Hence 

 the air near the 

 point becomes 

 self-repellent, 

 and is also at- 

 tracted by the 

 air outside ; that 

 directly in front 



Of the point Fig 34_ Air Current from "a'print. 



being repelled 



with the greatest force, produces a current in that di- 

 rection, while the air on either side is attracted, and, in 

 its turn, again repelled. 



ARMSTRONG'S HYDRO - ELECTRIC MACHINE. Fig. 

 35 represents a machine invented by Sir William Arm- 

 strong, about 1840, which generates electricity by the 

 discharge of partially condensed steam. 



It consists of a boiler and furnace mounted on glass 

 pillars; the boiler being provided with steam and water 

 gauges, a safety valve, and a condenser inclosing sev- 

 eral small pipes, through which the steam escapes. 



These pipes are surrounded with filaments of cotton, 



