168 



♦ KNOWLEDGE 



[March 1, 1886. 



short lengths, say four inclies, of oonimon brass chain, 

 and fi'om the centre a similar bell, E, is suspended by 

 means of a piece of silk thread equal in length to the 

 chains. From E a third chain hangs and is connected 

 with some conductor in contact with the earth or with 

 the rubber of the machine. P, G, are two small con- 

 ducting balls, about half an inch in diameter, susjsended 

 by silk thread, and normally hanging midway between 

 the bells. A hook, made by bending a piece of moderately- 

 stout wire, is soldered Oii to the centre of the suspension- 

 rod, and is intended to be placed over the rod in con- 



A 



Of 



Fig. II. 



uectiou with the [)rime conductor. On the conductor 

 becoming charged, the balls will be seen to oscillate 

 between the bells, striking them in turn. The rationale is 

 not far to seek if we remember that the charge on the 

 conductor (to which the outside bells are electrically 

 connected) is very large in comparison with the capacity 

 of the balls. Supposing the ball F to be inductivelj- 

 electrified, and consequently attracted by the ball C, it 

 will come into contact with that bell, and will at the same 

 instant become similarly charged and necessarily repelled. 

 But being repelled from the similarly charged bell, it is 

 also attracted by the earth-connected and so-called neutral 

 bell E, on coming into contact with which the ball loses its 

 charge, and would then fall back into the vertical 

 position. This, however, it is prevented doing in virtue 

 of the attraction, which is again exerted between it and 

 the bell C. In this way the oscillatiim of F is 

 maintained so long as any charge remains in C. The ball 

 G is actuated by precisely similar influences, the hell D 

 being in the same electrical condition as 0. Were there 

 a given charge imparted to the conductor, and the bells 

 C, D, then F, G, would act as small carriers, and would in 

 time dissipate the whole of the charge. The apparatus 

 can also be used in connection with the Leyden jar (see 

 below), E being connected, directly or indirectly, with 

 the outer coating, and the hook with the insulated inner 

 coating. 



Considerable amusement may be afforded by standing 

 a boy on a dry board of any convenient size, resting on 

 four di'y, in fact hot, tumblers, and connecting him by a 

 wire or chain with the prime conductor. If the hand of a 

 person standing on the floor be passed over the boy's 

 head, his hair will be attracted to the hand, and will, 

 therefore, stand erect. When the experiment is per- 

 formed in a dark room, every tuft of hair will be seen to 

 glow with small brushes of electric sparks, as the charge 

 is conveyed through the hand to earth. Another boy (or 

 girl, for tlie matter of that) can be employed in drawing 

 sparks from the legs, arms, back, or other parts of the 

 electrified boy's body. His linger placed at a short 

 distance (an eighth of an inch or so) over a gas-burner, 

 will emit a spark which, if the tap be turned on, will 

 ignite the gas. 



To a party of young people, few things are more enter- 



taining than a Leyden jar (Fig. 10), which lends itself in 

 a great variety nf ways as a source of amusement and 

 instruction. By its aid all the phenomena of lightning 

 can be illustrated ; but this is only a small portion of its 

 capacity for entertainment. It is a piece of apparatus 

 very simply made. It is to all intents and purposes the 

 same in jjrinciplu as Franklin's pane or plate, which con- 

 sists of a sheet of thin glass, say twelve inches S(|uare, 



Fig. III. 



having gummed on to each face a piece 



The inch and a half margin on each face 



of tin-foil nine 

 inches square. 



should be coated with shellac varnish. One metal sheet 

 should be connected with the earth, and the other with 

 the prime conductor. The charge on the conductor 

 extends to the connected plate, and accumulates there 

 in virtue of the attracti(m exerted by an opposite, 

 but ecjaally powerful, charge, gradually induced and 

 accumulated on the opposite or earth-connected side. 

 On a piece of wire, melted into the end of a stick 

 of sealing-wax (for a handle), being connected at one 

 extremity with the earth-connected face and the other 

 extremity brought near the other face, a more or less 

 intense spark will be observed. Perhaps the Leyden jar 

 is a handier form of "condenser," as this class of 

 apparatus is called. It consists of a glass jar or bottle, 

 of say a quart, or even larger capacity, with a neck at 

 least wide enough to allow the hand to be passed in and 

 out readily. The thinner the glass the more efiicient will 

 the instrument be, but in the absence of the orthodox 

 shape, a French plum jar will answ^er admirably. It is 

 coated, inside and outside, with tin-foil (which can easily 

 be gvimmed on) to within an inch and a half or two 

 inches of the neck, this portion being left uncovered and 

 kept scrupulously clean. A ball, B, an inch to two 

 inches in diameter, either of metal or of wood, coated 

 with tin- foil is attached to a piece of stout wire or rod 

 long enough to reach from the bottom of the jar to two 

 or three inches above the top of the neck, the lower end 

 of the rod being attached to the foil on the bottom 

 (inside) of the jar by a little wax. If convenient, it is 

 perhaps preferable to use a cover or stopper of hard 

 baked wood saturated with parailin wax, and provided 

 with a hole through which the rod should be passed 

 and fixed in it. In this case the rod need only pass an 

 inch or two below the cover, contact with the inner 

 coating of foil being made by means of a piece light 

 brass chain. Before using the instrument it should be 

 thoroughly warmed to evaporate any moisture that may 



