CHAMBERS'S INFORMATION FOR THE PEOPLE. 



Fig. ii. 



layers of flannel covered with leather ; and a 

 large flat rubbing surface thus acts on the glass. 



From these rubbers a flap of oiled silk passes on 

 each side of the plate to near the collecting points, 

 to prevent loss of electricity. The attraction of 

 the rubbed plate keeps these flaps in position when 

 the machine is in action ; at other times a split 

 pin, g, is used to keep them up. 



A peculiar feature in Winter's plate machine 

 is a large ring which fits into the ball of the 

 prime conductor. It is apparently a wooden 

 ring, but the core consists of a metallic wire, 

 which communicates with the 

 prime conductor, when it is 

 fitted in its place. The figure 

 (fig. n) shews a section of 

 the ball of the prime conduc- 

 tor with the ring so fitted 

 on. It serves to increase the 

 accumulation of electricity 

 enormously ; the wooden en- 

 velope appears to prevent the 

 discharge into the air, as the 

 silk flap does with the elec- 

 tricity of the plate. 



But we must not omit to 

 notice that the frictional ex- 

 citation of electricity is, in all 

 these machines, very greatly 

 enhanced by covering the sur- 

 face of the rubber with an 

 amalgam, or metallic com- 

 pound. It is in the shape of a fine powder, and 

 is usually made of mercury, zinc, and tin ; it is 

 spread on with a knife, and made to adhere by 

 the aid of a little grease. 



The Electrophorus. This is the least expensive 

 of all electric machines, and forms a useful piece 

 of apparatus when we do not require electricity in 

 great quantity. 



It usually consists of a tin mould filled with 

 shellac, and a movable tin cover with an insulat- 

 ing glass handle, as 

 seen in fig. 12. The 

 shellac is generally 

 mixed with wax, to 

 make it less brittle; 

 but any resinous cake 

 will do, if it be even 

 and smooth on the 

 surface. A round disc 

 of vulcanite forms an 

 excellent substitute for 

 _. the shellac. 



The principle of 

 action of the electro- 

 phorus depends on in- 

 duction, and is easily 

 understood. If the 

 cake be excited, say negatively, by striking with 

 a cat-skin, it will not give up its electricity 

 readily, because it is a bad conductor. Now, 

 this negative electricity will induce positive on 

 the adjacent sides of all objects round the 

 cake. But as shellac is a splendid dielectric or 

 medium of induction, the power will almost all 

 act through it, and so keep positive electricity 

 bound on the face of the mould next the cake, the 

 negative being diffused over the earth. If, now, we 

 put on the cover, the electricity, loath to leave 

 the cake, will act on it by induction, rather than 



264 



Fig. 12. 



pass to it by conduction, and by induction power- 

 fully, because the cover is so near. So, then, if we 

 touch the cover for an instant, the negative induced 

 on its upper side escapes ; but the positive is kept 

 bound by the cake, till we lift the lid by its handle, 

 when a sharp spark passes to the knuckle held 

 near it. By putting on the cover, touching, and 

 then lifting it, we may get any number of sparks 

 from the cake. Once excited, it will keep its 

 power for a long time, sometimes for weeks or 

 even months together. The mould shields it by 

 induction, as the armature of a magnet does its 

 magnetic charge. 



Within the last few years, some machines have 

 been invented by Holtz of Berlin and Tcepler of 

 Riga, which have become very popular. They 

 are generally called double induction machines, 

 and are founded on a principle somewhat anal- 

 ogous to that of the electrophorus. A description 

 of them will be found in any of the recent larger 

 treatises on electricity. 



Experiments with the Electric Machine. (r.) 

 The phenomena of electric attraction and re- 

 pulsion may be illustrated in hundreds of ways. 

 If a common glass tumbler be electrified inside, 

 by a number of sparks from the machine, and then 

 inverted over a few piths on a table, the piths will 

 dance up and down within the tumbler in an 

 amusing manner. They are gradually discharging 

 the electricity of the tumbler, and the dance may 

 be kept up for hours. Small pith figures will 

 dance between two metal plates, the lower of which 

 is uninsulated, and the upper in connection with 

 the prime conductor. 



The repulsion of similarly electrified particles 

 is shewn very nicely by the electric water-pail. 

 This is simply a small tin pail with fine holes 

 pierced in the bottom. If filled with water, it will, 

 when held in the hand, trickle out in drops merely ; 

 but if hung on the prime conductor, the water 

 particles becoming electrified mutually repel, and 

 issue in a multitude of fine jets. The electric 

 wind is a somewhat similar result of the electrifi- 

 cation of the particles of the air. If a pointed rod 

 be stuck in the prime conductor, and the machine 

 worked, the hand placed before the point will feel 

 a sort of wind. The experiment may be made 

 pleasing to the eye, by making the wind drive 

 small models of mills or a small paper orrery. 



This action of points is very curious. So long 

 as the electricity escapes from the prime con- 

 ductor by a point, no spark can be drawn from it 

 Nor, on the other hand, can any spark be got, if a 

 pointed conductor be held at a distance of a few 

 feet from the machine. This is because the elec- 

 tricity escapes silently in each case, and cannot 

 acquire a tension sufficient to give the spark. The 

 snap, heard when a spark is drawn, is due to the 

 disruption or scattering of the air particles, caused 

 when the positive electricity of the machine com- 

 bines with the negative it induces on the hand or 

 the object brought near it. 



(2.) The heating effects of electricity are also 

 very powerful If a spark be drawn from the prime 

 conductor by an iron spoon containing a little 

 ether or alcohol, the liquid will instantly inflame. 

 It will also light a candle newly blown out. A gas- 

 jet issuing from a metal burner is instantly lighted 

 by a spark from the machine, or from the finger 

 of a person standing on a stool with glass legs 

 and touching the prime conductor. 



