CONVECTION, OR CARRYING DISCHARGE — EFFECTS OF POINTS. 145 



being drawn into elongated forms by the electric forces, combined their effects to 

 produce a band of matter having considerable conducting power, as compared with 

 the oil of turpentine elsewhere, is as yet questionable. 



1572. The analogy between the action of solid conducting carrying particles and 

 that of the charged particles of fluid insulating substances, acting as dielectrics, is 

 very evident and simple ; but in the latter case the result is, necessarily, currents in 

 the mobile media. Particles are brought by inductric action into a polar state ; and 

 the latter, after rising to a certain tension (13/0.), is followed by the communication 

 of a part of the force originally on the conductor ; the particles consequently become 

 charged, and then, under the joint influence of the repellent and attractive forces, 

 are urged towards a discharging place, or to that spot where these inductric forces 

 are most easily compensated by the contrary inducteous forces. 



1573. Why a point should be so exceedingly favourable to the production of cur- 

 rents in a fluid insulating dielectric, as air, is very evident. It is at the extremity of 

 the point that the intensity necessary to charge the air is first acquired (1374.) ; it is 

 from thence that the charged particle recedes ; and the mechanical force which it 

 impresses on the air to form a current, is in every way favoured by the shape and 

 position of the rod, of which the point forms the termination. At the same time, the 

 point, having become the origin of an active mechanical force, does, by the very act 

 of causing that force, namely, by discharge, prevent any other part of the rod from 

 acquiring the same necessary condition, and so preserves and sustains its own pre- 

 dominance. 



1574. The very varied and beautiful phenomena produced by sheltering or en- 

 closing the point, illustrate the production of the current exceedingly well, and 

 justify the same conclusions ; it being remembered that in such cases the effect upon 

 the discharge is of two kinds. For the current may be interfered with by stopping 

 the access of fresh uncharged air, or retarding the removal of that which has been 

 charged, as when a point is electrified in a tube of insulating matter closed at one 

 extremity ; or the electric condition of the point itself may be altered by the relation 

 of other parts in its neighbourhood, also rendered electric, as when the point is in a 

 metal tube, by the metal itself, or when it is in the glass tube, by a similar action of 

 the charged parts of the glass, or even by the surrounding air which has been charged, 

 and which cannot escape. 



1.575. Whenever it is intended to observe such inductive phenomena in a fluid 

 dielectric as have a direct relation to, and dependence upon, the fluidity of the medium, 

 such, for instance, as a discharge from points, or attractions and repulsions, &c., then 

 the mass of the fluid should be great, and in such proportion to the distance between 

 the inductric and inducteous surfaces as to include all the lines of inductive force (1369.) 

 between them ; otherwise, the eflfects of currents, attraction, &c., which are the re- 

 sultants of all these forces, cannot be obtained. The phenomena which occur in the 

 open air, or in the middle of a globe filled with oil of turpentine, will not take place 



MDCCCXXXVIII. u 



