ATTRACTION-, ETC.] 



UNDULATORY FORCES. ELECTRICITY. 



175 



every part, to prevent the loss of electricity. 

 Fig. 11. 



For the 



oppos'te reason, lightning conductors (of which we shall 

 speak fully hereafter) have their extreme end, towards 

 the sky, always pointed. A very instructive experi- 

 ment, illustrating the property of points, may be tried 

 as follows : 



Experiment 14. The machine being in good action, 

 and affording large sparks between the conductor and a 

 brass knob, place a sod of grass ou the conductor itself. 

 On presenting the knob, no further spark can be ob- 

 tained, because the points of the grass leaves radiate 

 and diffuse the electricity. The grass of the fields thus 

 exercises a wonderful effect on the electrical state of the 

 earth. 



An ingenious arrangement of points, which are fixed 

 to a common centre, and; tree to rotate on a point fixed 

 in the conductor of the machine, exhibits another effect 

 of pointed surfaces ; which is, that a current of air is 

 driven from them on the electric discharge taking place. 

 fig. 12. One of these is represented in 



Fig. 12, in which a a a a are 

 the pointed wires, fixed in a 

 common centre 6. c is the con- 

 ductor of the machine ; and d a 

 pivot on which 6 can rotate. 

 Now, not only will such an ar- 

 rangement afford a current of 

 air at each point which may be 

 tried by holding a lighted candle 

 thereto but the wheel will 

 speedily turn round, owing to 

 the mechanical action which is generated by the electric 

 force against the surrounding air ; and at each point a 

 brush-like flame will be seen. This arrangement has 

 'ermed the "electric windmill." 



Ini'i't \r,. The current of air produced from a 

 pointed body attached to an electrical surface, may be 

 noticed by fixing a pointed wire to any part of the con- 

 ductor. If the machine be then stt into action, ami :i 

 lighted candle be held before the point, the flame will be 

 rapidly driven away. 



The reader, having thus seen the diffusive power which 

 pointed bodies have on electrified substances, will at 

 once perceive the necessity which exists for all the parta 

 of the machine being kept aa free as possible from dust ; 

 because each particle acts precisely in the same way as 

 the pointed wires, etc., of which we have been speaking. 

 By other arrangements, many interesting experiments 

 may be tried with the electric spark, as taken from the 

 human body, Ac. For this purpose, an electric stool 

 will be required ; which is easily made, by fixing into a 

 piece of wood, previously well dried by heat, four rods 

 of glass, to serve as legs, in place of wooden ones. An- 

 other mode is, that of resting a square piece of wood on 

 four glass wine-bottles, which, as bad conductors, pre- 

 vent the escape of electricity from the person standing 

 on the stool. A sheet of gutta-percha, which also is a 

 bad conductor, may be placed on a block of dry wood ; 

 anil such often answers the purpose of an electric stool ; 

 the object being to interpose, between the earth and the 

 individual, gome non-conducting and insulating body. 

 A wire is then connected with the conductor of the ma- 

 chinn. and its other end is held in the hand of the person 

 atan'lin:,' on the stool. The machine is then to be worked ; 

 and the following experiments may be tried : 



riment 16. Instead of presenting a brass ball to 

 the conductor, as mentioned in Experiment 14, hold one 

 near to the hand of the electrified person, and a bright 

 spark will pass. Indeed, the body becomes, for the 

 time, an extension of the conductor of the machine. 

 Experiment 17. Present a metal spoon, filled with 



ether, to a brass ball, held in the hand of the person on 

 the stool. When the spark passes, the ether will be 

 inflamed. 



These experiments may be varied in many ways, 

 and they will be sufficient to show the manner of con- 

 ducting others. We must, however, caution our readers 

 not to take strong sparks from the eyes or head, as such 

 may prove exceedingly harmful. A spark passing from 

 the body of a person electrified by the hydro-electric, or 

 large plate machine, has so powerful an effect on the 

 system, as to produce a shock which would prevent the 

 individual from choosing a repetition of the experiment. 



We must, of course, here confine our attention to the 

 spark produced by frictional electricity. We shall find, 

 when we treat on voltaic and magneto-electricity, that 

 the light, and general luminous effects produced thereby, 

 are much different from those to which we have been 

 referring, owing to causes which we shall then explain. 



ATTRACTION AND REPULSION. 



HAVING chosen first to speak of the electric spark, 

 because it naturally attracts the attention of the ex- 

 perimenter, and paves the way for more intricate and 

 interesting electrical phenomena ; we shall now con- 

 sider two effects which present themselves when bodies 

 are electrified namely, those of attraction and repulsion. 

 It has already been mentioned, that an electrified body 

 has the power of attracting light substances to its sur- 

 face.* Thus, if a piece of dry window-glass be placed 

 at a distance of half an inch from, and over, some pieces 

 of paper or feathers, and its upper surface be briskly 

 rubbed by means of a silk handkerchief, the feathers 

 will dart up, and adhere for a- short time to the under 

 surface, until, being electrified equally with the glass, 

 they will be repelled from it, and fall again to the place 

 on which they had previously rested. We shall i 

 a few experiments illustrating both attraction and re- 

 pulsion, before entering into the laws which govern the 

 phenomena. 



/ iment 18. Hang two small pieces of pith, 

 obtained from the elder-tree, by means of a silk thread, 

 from the conductor of the machine, so that they may 

 rig. n. rest against each other. On 



^^^^^^Mm working the machine, they will 

 ^Tt^HI^BBBm t similarly electrified, and 

 / \ will repel each other so long as 



/ \ they continue in that state. 



/ \ On presenting the knuckle, or 



O O a brass ball to either, they 



will fall together again, owing 

 to their electric equilibrium 

 being restored. The mode of 

 conducting this experiment is 

 illustrated in Fig. 13. 

 Experiment 19. An amusing arrangement is sold by 

 the instrument-maker ; in which a head of hair is em- 

 ployed. It is to be fixed by means of a brass rod, in an 



upright position, 

 Fi(r. H. 



thj conductor of the machine. 

 So long as it is unelectritied, 



the hair rests naturally about 

 the head ; but on working 

 the machine, each hair be- 

 comes similarly electrified, 

 and so repels its neighbour 

 in the manner shown in 

 Fig. 14. 



Experiment 20. Let a per- 

 son stand on the glass stool ; 

 and having combed out the 

 hair of his head, connect 

 him, by means of a wire, 

 with the conductor of the 

 machine. Each hair will se- 

 parate, as in the last experi- 



ment ; and if the machine be in good action, the effect 



produced is often highly ludicrous. 



Experiment 21. Another mode of showing the effect 

 See anit, p. 167. 



