ELECTRICITY. 



and the ball presented to the conductor, 

 taking care not to bring it sufficiently 

 near to draw a spark, the balls will be 

 but little affected, and their divergence 

 will continue for a considerable time. 

 But if the rod terminate in a sharp point, 

 instead of a ball, and the point be pre- 

 sented to the conductor at the same dis- 

 tance as the ball was in the former case, 

 the electroscope will immediately col- 

 lapse, showing that the electrical charge 

 has entirely disappeared : it has, in 

 fact, been rapidly drawn off by the 

 pointed rod. It is quite immaterial to 

 the success of the experiment whether 

 we affix a point to the conductor itself, 

 or whether we present to it a point held 

 in the hand ; the escape and dispersion 

 of the electricity being equally pro- 

 moted by the presence of a point, whe- 

 ther the fluid be given out or absorbed ; 

 for it is scarcely necessary to remark 

 that the very same kind of reasoning 

 applies equally to both the positive and 

 negative conditions of electricity. 



(95.) Currents of air always accom- 

 pany the discharge of electricity, whe- 

 ther positive or negative, from pointed 

 bodies ; for each particle of air, as soon 

 as it has Deceived its electricity from 

 the point, is" immediately repelled by the 

 body. These currents tend powerfully 

 to increase the dissipation of the elec- 

 tricity, by bringing in contact with the 

 point a continued succession of particles 

 of air, that are not yet electrified, and 

 are, therefore, ready to receive a charge. 

 Many amusing experiments are founded 

 on this principle. Let two cross wires, 

 (/?g-. 21.) the ends of which terminate in 



Fig. 21. 



in a direction opposite to that of the 

 stream ; and this taking place at all the 

 four points, the whole system will 

 revolve backwards with considerable 

 rapidity. 



The following is another form in 

 which this experiment may be made. 

 Two wires, (Jig. 22.) are stretched in 



Fig. 22. 



points, bent in a similar direction with 

 respect to the axis, be supported by 

 means of a cap upon a fine point, and 

 electrified by being placed upon the 

 prime conductor of a machine. Each 

 of the points will give off a stream of 

 electricity : this will remove a part of 

 the pressure which the fluid would have 

 exerted on that side if no efflux had 

 taken place ; but as the pressure of the 

 fluid on the opposite side of the wire, 

 in the opposite direction, still operates 

 in full force, the wire will be impelled 

 in the direction of that] force, that is, 



the direction of a plane, slightly in- 

 clined to the horizon, between four in- 

 sulating pillars. Across these wires, 

 another wire is made to rest, termi- 

 nating by small balls at each end, and 

 having a cross wire fixed to it at right 

 angles, with two bent points, as in the 

 former experiment. When this system 

 is electrified, the dispersion of the elec- 

 tricity from the points produces a re- 

 volution of the bars, which makes the 

 transverse bar roll up the inclined plane. 



An apparatus consisting of wires ter- 

 minating in points, and having balls 

 annexed to them to represent the planets, 

 may be constructed so as to revolve 

 when electrified ; and thus to imitate the 

 planetary motions. Such an apparatus 

 has been called an electrical orrery. 



(96.) It should be observed, how- 

 ever, that a point loses its power of 

 concentrating and dispersing electricity 

 when it is surrounded by other parts 

 of the conducting body which are 

 equally prominent ; as when it is placed 

 between two balls, or inclosed in a tube, 

 or when it does not rise above the ge- 

 neral surface of the body. The effect 

 of one point is much diminished even 

 by the vicinity of another point ; so that 

 if several points placed near each other 

 be presented to the conductor, the elec- 

 tricity is drawn off much less rapidly, 

 and will be transferred by sparks in- 

 stead of forming a continued stream. 



(97.) When the transfer of electri- 

 city takes place between smooth surfaces 

 of a certain extent, no difference can be 

 perceived in the nature and appearance 

 of the spark, whichever be the position 

 of the negative surface. But in the 

 passage of electricity through points, 



