544 



* KNOAVLEDGE • 



[June 26, 1885. 



tolerably good conductor of electricity — that is to say, it is 

 a substance which will facilitate the escape of the elec- 

 tricity as fast as we produce it. The object in heating 

 everything used in these experiments is, then, to expel 

 moisture, not merely to heat the bodies. That heat really 

 plays no ]iart in the production of electricity may be 

 proved by holding the glass rod which has been electrified 

 (after having been dried) in a jet of steam as it escapes 

 from a kettle of boiling water. The glass will become 

 covered with aqueous vapour, but it will be hotter than it 

 was before. Maybe it will be too hot to hold, but all the 

 rubbing in the world will fail to produce the slightest trace 

 of electrification, presuming, of course, that in rubbing we 

 do not altogether remove the water. 



Ex. LXXXI. — Get a sheet of stout brown paper, and, 

 cutting ofi' a piece twelve or fourteen inches square, warm 

 it before the fire, making it, in fact, hot. With a good and 

 perfectly dry hog's bristle clothes-brush rub the paper (laid 

 on a hot, dry board) briskly. Lift the paper from the 

 board (by taking hold of the comers), and it will be found 

 to be strongly electrified, having the power of doing any- 

 thing which the electrified sealing-wax or shellac performed 

 (Ex. LXXI. to LXXVIIL). 



Ex. LXXXII. — Hold the electrified brown paper near 

 the wall, or even the ceiling, and it will fly towards and 

 adhere thereto, whether it be the wall or the ceiling. This 

 Is due to the attraction existing between the paper and the 

 boundaries of the room ; but of this more anon. 



Ex. LXXXIII. — Make a sheet of foreign, that is thin, 

 notepaper, hot, and, laying it on a dry board, rub it with a 

 piece of bottle-rubber. The paper will be found to be 

 electrified just as the brown paper was, and similar experi- 

 ments may be performed with it. 



Ex. LXXXIV. — Draw a piece of thin sheet gutta percha 

 — say, three-quarters of an inch wide and twelve inches long 

 —between two dry fingers. The percha will exhibit decided 

 e^ndences of electrification. 



Ex. LXXXY. — A still better experiment is to furnish 

 two adjacent fingers with stalls of india-rubber tubing and 

 draw a piece of silk ribbon — say, half-an-inch wide and 

 twelve inches long — between them. The ribbon will be 

 found to be highly electrified, and will cling tenaciously to 

 any substance, such as the arm, table, wall, tc, which may 

 happen to be near. 



Ex. LXXXVI. — A solid body may be electrified by 

 friction with a liquid or with a gas. A dry tube exhausted 

 of air, but containing a few drops of dry mercury, when 

 agitated in the dark becomes luminous on account of the 

 electrification set up. This experiment, however, requires 

 the use of apparatus which I have chosen to regard as 

 beyond the reach of our young electrician, so I will say no 

 more about it. I shall presently refer to an experiment 

 which he can easily perform, and which will serve the 

 purpose quite as well. 



COyDrCTOE.S AXD IXSULATOES. 



I*K- 2. — Bodies are divided into two classes, viz : — 

 Conductors, or those bodies which freely transmit elec- 

 tricity, and 



Insulators, or those bodies which transmit electricity 

 very feebly ; some of them so feebly as to almost approach 

 infinity. 



_Ai insulator, then, is a bad or non-conductor, and its 

 prim^ function in electrical industrie.s is to serve as a 

 means for preventing the escape of electricity from a 

 condu ;tor. 



Ins ilation and conductivity, the properties of insulators 

 and coiductors respectively, belong, however, to all bodies, 



but in different degrees. Insulation is never perfect, inas- 

 much as the best insulator allows some electricity to pass ; 

 while, on the other hand, conductivity is never so perfect 

 as to present no obstacle or resistance to the passage of 

 electricity. Briefly put, a body which insulates well has 

 high resistance and low conductivity, while a body which 

 has high conducti's'ity has loin resL^tance, and insulates 

 badly or not at all. Conductivity and resistance are, then, 

 the converse one of the other. 



It is found that the relative electrical conductivities of 

 different bodies correspond very closely with their thermal 

 conductivities. 



Ex. LXXXYII. It will be observed that in the fore- 

 going experiments nothing has been said as to the electrifi- 

 cation of metals by friction, for the simple reason that it 

 is impossible to electrify them when held in the hand as the 

 other substances were. This is due to the fact that the 

 hand, body, and metals generally (besides certain other 

 substances) are conductors of electricity, whence, although 

 electricity may have resulted from the friction, it would 

 not be retained on the metal, but would have escaped to, 

 or been neutralised by, the earth ; a sequence which always 

 follows under such circumstances. If, however, we insulate 

 a metal rod, that is, interpose an insulating substance be- 

 tween it and the earth or any other large conductor, we can 

 electrify it. Get, therefore, a few inches of indiarubber 

 tubing of the requisite internal diameter, and slip it over a 

 brass, iron, or other metallic rod or tube, say three-quarters 

 of an inch to an inch in diameter and fifteen inches long. 



Fig. 4A. 



Support the metal (M, Fig. 44) in the hand by grasping 

 the insulating rubber-tube, G, and then rub the rod or tube 

 with a pad, consisting of half-a-dozen layers of flannel. The 

 electrification of the metal will ensue. 



Ex. LXXXVIII. — Procure four good tumblers, and 

 stand them before the fire until they become hot. Then 

 invert them, and lay a rectangular piece of wood on them, 

 placing a glass at each corner of the wood. An improvised 

 insulating stool (Fig. 45) is the residt. Stand a boy on 



this, and beat the back of his coat with a piece of fur or 

 flannel (taking care not to touch him otherwise), and let 

 him present his finger to the suspended pith-ball (Fig. 36), 

 or to a few pieces of dutch metal. Attraction takes place, 

 evidencing electrification. If, now, any one standing on 

 the ground touches him, or if he steps off the stool on to 

 the ground, he is no longer insulated, but the charge 



