CONVECTION, OR CARRYING DISCHARGE — CHARGE OF PARTICLES. 143 



a clean white silk thread, was brought towards it, and it was found that, if the ball was 

 held for a second or two near any part of the charged surface of the boiler, at such 

 distance (two inches more or less) as not to receive any direct charge from it, it be- 

 came itself charged, although insulated the whole time ; and its electricity was the 

 reverse of that of the boiler. 



1565. This effect was the strongest opposite the edges and projecting parts of the 

 boiler, and weaker opposite the sides, or those extended portions of the surface which, 

 according to Coulomb's results, have the weakest charge. It was very strong oppo- 

 site a rod projecting a little way from the boiler. It occurred when the copper was 

 charged negatively as well as positively : it was produced also with small balls down 

 to 0*2 of an inch and less in diameter, and also with smaller charged conductors than 

 the copper. It is, indeed, hardly possible in some cases to carry an insulated ball 

 within an inch or two of a charged plane or convex surface without its receiving a 

 charge of the contrary kind to that of the surface. 



1566. This effect is one of induction, not of communication. The ball, when re- 

 lated to the positive charged surface by the intervening dielectric, has its opposite 

 sides brought into contrary states, that side towards the boiler being negative and 

 the outer side positive. More inductric action is directed towards it than would have 

 passed across the same place if the ball had not been there, for several reasons ; 

 amongst others, because, being a conductor, the resistance of the particles of the 

 dielectric, which otherwise would have been there, is removed (1298.) ; and also, 

 because the reacting positive surface of the ball being projected further out from the 

 boiler than when there is no introduction of conducting matter, is more free therefore 

 to act through the rest of the dielectric towards surrounding conductors, and so fa- 

 vours the exaltation of that inductric polarity which is directed in its course. It is, 

 as to the exaltation of force upon its outer surface beyond that upon the inductric 

 surface of the boiler, as if the latter were itself protuberant in that direction. Thus 

 it acquires a state like, but higher than, that of the surface of the boiler which causes 

 it ; and sufficiently exalted to discharge at its positive surface to the air, or to affect 

 small particles, as it is itself affected by the boiler, and they flying to it, take a charge 

 and pass off; and so the ball, as a whole, is brought into the contrary inducteous 

 state. The consequence is, that, if free to move, its tendency, under the influence of 

 all the forces, to approach the boiler is increased, whilst it at the same time becomes 

 more and more exalted in its condition, both of polarity and charge, until, at a certain 

 distance, discharge takes place, it acquires the same state as the boiler, is repelled, 

 and passing to that conductor most favourably circumstanced to discharge it, there 

 resumes its first indifferent condition. 



1567. It seems to me, that the manner in which inductric bodies affect uncharged 

 floating or moveable conductors near them, is very frequently of this nature, and ge- 

 nerally so when it ends in a carrying operation (1562. 1602.). The manner in which, 

 whilst the dominant inductric body cannot give off its electricity to the air, the in- 



