40 



THE POPULAR EDUCATOR. 



ELECTRICITY. VIII. 



THUNDER HOUSE ELECTRICAL ATTRACTION EPINUS* CON- 

 DENSER CONDENSING ELECTROSCOPE. 



BY means of the universal discharger we may easily pass the 

 shock through many different substances, and thus observe the 

 effects produced. If a lump of sugar be placed between the 

 wires, and the shock from a large jar or a battery be caused 

 to pass, the sugar will be broken, and the pieces will, for 

 a short time, continue to be luminous when placed in a dark 

 room. A similar effect will be produced with pipe-clay and 

 other bodies, while with, a larger battery a piece of wood may 

 be split. Pieces of deal, about half an inch square f 



and an inch or two in length, may be used for this. 

 They should be well dried, and a small hole made 

 at each end to receive the points of the wires. 



The mechanical effects of electricity are, how- 

 ever, seen in Nature on a much grander scale. 

 When the lightning strikes a tree, we usually find 

 that it splits it from top to bottom, and often 

 irives off large splinters. So, too, when a build- 

 ing is struck, the lightning leaps along from one 

 conductor to another, tearing out and displacing 

 any stones that may happen to lie in its course. 



If we examine a building that has thus been in- 

 jured, we shall observe that the electric fluid has 

 often passed from one bar of iron fixed in the 

 wall to another, picking out all metallic bodies 

 that lie anywhere near its course, and leaving its path dis- 

 tinctly marked out by broken and displaced bricks, thus afford- 

 ing a convincing proof of its immense power. A simple ex- 

 periment will show the way in which any intervening body may 

 be removed by the electric fluid leaping from one conductor to 

 another across it. Cut a piece of rather thin wood (Fig. 29) into 

 the shape of the gable of a house, and fix it to a foot, so that it 

 may stand of itself. Now cut out of it at c a hole about an 

 inch square, and make a piece of wood to fit loosely into the 

 opening. From the peak bring a wire carrying a knob, A, at its 

 upper end, and let this terminate at one corner of the aperture, 

 and from the opposite corner let a wire be 

 carried so as to end in a knob or ring at 

 B ; let a wire be also fixed diagonally 

 across the piece of wood which closes the 

 opening c. If the plug be now turned 

 one quarter round from the position shown, 

 the wire will complete the electrical circuit 

 between A and B, and, the exterior of the 

 jar being placed against B, the connection 

 may be made by the discharging rod be- 

 tween A and the interior. The shock will 

 then pass without displacing the plug ; but 

 if we place it as shown in the figure, so 

 that the electricity has to leap across it, 

 and then allow the shock to pass, the plug 

 will be violently thrown out. This experi- 

 ment shows the importance of making the 

 rod of a lightning conductor perfectly con- 

 tinuous throughout ; if there be any break, 

 it is often worse than useless. 



When the shock is passed through a 

 confined volume of air, it causes a mo- 

 mentary increase in its bulk. On this 

 principle the electrical mortar acts. A 

 chamber is turned in a piece of ivory or 



boxwood, and a ball made to fit its mouth. Inside are two wires 

 brought so that their ends may nearly touch, and when the shock 

 is caused to pass between them, the air is so violently shaken as 

 to propel the ball to a small distance. That this effect is due to 

 the concussion of the air, and not to the expansion caused by 

 the heat of the spark, is clearly shown by the apparatus known 

 as Kinnersley's Thermometer (Fig. 30). The glass chamber 

 is filled at the upper part with air ; below is water commu- 

 nicating with that in the small open glass tube at the side. 

 As soon as the spark passes, the air suddenly expands, and 

 thus causes the water to rise in the side tube. It, however, sub- 

 sides almost immediately to its former level, which it would not 

 do if the expansion were caused by heat. When many shocks 

 are passed, a slight increase of heat is produced. 



The peal of thunder produced by the passage of a Cash of 

 lightning through the air is merely another proof of this violent 

 concussion, and arises from the vibrations thus produced. The 

 sharp crack heard on discharging a Leyden jar arises a 1 so. from 

 the same cause. All the varied phenomena of electrical attrac- 

 tion and repulsion may be classed with the mechanical effects 

 of electricity, and described here. We have already seen that 

 bodies similarly electrified repel one another, while those 

 charged with opposite electricities attract, and this general 

 principle will explain all the phenomena we shall notice. 



If we take a piece of cotton wool, and having pressed it 

 slightly between the fingers, place it on the prime conductor, 

 the fibres composing it will, as soon as the machine 

 is worked, repel one another, so that the lump will 

 increase in size and probably fly off the conductor. 

 The same effect is seen if we place in the conduc- 

 tor a wire carrying a bundle of short pieces of 

 thread ; the individual threads will all stand apart 

 as widely as possible, and if the lower ends be also- 

 tied together, they will arrange themselves into a 

 globular form. 



Dolls' heads of hair are frequently used for illus- 

 trating this, and as soon as they are charged with 

 electricity, all the hairs stand on end, and try to 

 get as far as possible away from those next to 

 them, presenting a very strange appearance. The 

 same effect is seen if a person with dry hair 

 stands on the insulating stool, and combs his 



Fig. 29. 



hair while he is highly electrified. 



Spinning sealing-wax by electricity is another interesting 

 experiment, which can be tried with very little trouble. Fix a 

 short piece of the wax to a piece of wire, and insert it in one 

 of the holes of the conductor. Now light the end with a 

 taper, so that it becomes well melted, and when it is ready to 

 drop let an assistant turn the machine while you hold a sheet 

 of paper near the wax. A large number of very fine threads 

 will at once be given off, and will collect upon the paper, 

 adhering slightly to it. They may, however, by a little care, be 

 gathered, so as to show how fine they are. It will thus be seen 

 that the particles of an imperfect conduc- 

 tor like wax repel each other when they 

 become charged with electricity. 



The apparatus known as the electrical 

 sportsman, and shown in Fig. 31, affords 

 a further illustration of the same effects. 

 An ordinary Leyden jar, B, is taken ; its 

 rod is, however, bent over to one side, as- 

 shown at D, and a second piece of wire, c, is 

 fastened to it. From the end of this some 

 small pieces of elder pith or cork, with 

 small feathers fixed to them, so as to repre- 

 sent birds, as at E, are suspended by short 

 threads, a ledge being placed under them 

 so that they may rest upon it. The figure 

 of a man in the act of shooting is now cut 

 out of cardboard and fixed on a stand A. 

 A small ball is fixed at the nozzle of the 

 gun, and a strip of tinfoil passes from 

 this down the figure to a hook on the 

 stand, so that a wire or chain leading 

 from the outside of the jar may be con- 

 nected with it. 



The figure is now placed so that the 

 ball may be within striking distance of 

 D, the circuit being completed through the strip of foil. 



A wire is then brought from the prime conductor to the rod 

 of the jar, and the machine is set in action. The birds, being 

 similarly electrified, repel each other, and therefore rise, and 

 fly further and further apart as the charge increases ; but as 

 soon as the jar is sufficiently charged, the spark passes be- 

 tween the knob and the gun, representing the flash ; the loud 

 snap represents the report of the gun ; and the jar being thus 

 discharged, the birds immediately fall as if shot. 



To complete the experiment, a small cavity is sometimes made 



in the stock of the gun, in which gunpowder may be placed, and 



the tinfoil and wire are so arranged that a spark may pas? 



across this when the jar is discharged. A wetted string is 



i interposed between the stand and the jar to prevent the scat- 



