16 



THE POPULAR EDUCATOR. 



jar, and bring the other quickly against the knob of the jar, 

 the spark will at once inflame the resin. Unless, however, it 

 be done quickly, the experiment will fail, for the loose fibres 

 of the tow will conduct the electricity or draw it off by their 

 ends, and thus discharge the jar silently, or, at least, too slowly 

 for the resin to be ignited. 



Frequently a model house is fitted up to show this experi- 

 ment, and illustrate at the same time the way in which a house 

 is sometimes set on fire by a flash of lightning. The model is 

 constructed of tin, and painted so as to resemble a house. At 

 one side a glass tube is placed inside a chimney, and down this 

 there passes a wire carrying at its lower end a brass ball ; about 

 half an inch from this is placed a second ball connected with 

 the metal of the house ; one of these is wrapped round with the 

 tow, and then powdered with resin. 



The jar is now placed on a strip of tinfoil, connected with the 

 house, and, by means of the discharging rod, the shock is made 

 to strike the upper end of the wire fixed to the chimney. The 

 glass tube insulates this, and thus the fluid passes down it and 

 between the knobs, setting light to the resin as it passes. 



In just this way the lightning strikes the highest portion of 

 any building, and passes along by the 

 best conductors ; but if any inflammable 

 body happens to be in such a position 

 that the lightning darts to or over it, it 

 is at once ignited. Sometimes a board 

 covered with tinfoil and connected with 

 the inside of the jar is suspended by 

 silk threads over the tin house, so as to 

 represent the clouds, and 

 when a sufficient charge 

 has accumulated, tho 

 spark will strike and firo 

 the house, just as when 

 the clouds are highly 

 charged the lightning 

 darts from them. 



Ether and spirit may 

 also at times be lighted 

 by the electric spark. 

 They should be placed in 

 a warm metal spoon, and 

 sparks taken from the 

 conductor so as to pass 

 through the liquid ; if the 

 sparks only run along the 

 surface, the ether will 

 not be lighted. This ex- 

 periment may be shown 

 well by a person standing 

 on the insulating stool, 

 and holding the spoon in 



his hand, while a second person, standing on the ground, draws 

 sparks with his finger ; or the one on the stool may hold tho 

 spoon to the other. Another way of performing the same ex- 

 periment is shown in Fig. 26, and will be understood without 

 further explanation. For this, it is a great convenience to have 

 the rod of the jar bent as shown. 



If the shock be passed through gunpowder, we shall see a 

 strange result, for tho powder will not be fired, but merely 

 violently scattered. The reason of this is that the shock passes 

 so rapidly that it has not time to heat the powder sufficiently 

 to ignite it ; if, however, we interpose some badly conducting 

 medium, we shall diminish the speed of the current, and thus 

 succeed in firing the gunpowder. Water or wetted string 

 supplies us with a suitable imperfect conductor ; if then we let 

 the shock pass in its circuit through six or seven inches of 

 string, or across a plate of water, the powder will be inflamed. 



Fig. 27 represents the apparatus known as the electric pistol. 

 It consists of a hollow metal cylinder, the mouth of which is 

 closed by a cork. At A is an opening through which passes a 

 glass tube, and inside this a brass rod, terminated at each end 

 in a knob, as shown in the sectional view D'. The cylinder is 

 now filled with a mixture of oxygen and hydrogen gases, in the 

 proportion of two parts of the latter to one of the former, and 

 the cork replaced. If the knob A be now presented to the prime 

 conductor, the spark at once passes, and as the rod on which A 

 is fixed is insulated by the glass tube, a second spark passes 



between B and the inside of the cylinder. This spark explodes 

 the mixed gases, and drives out the cork with a loud report. If, 

 instead of oxygen and hydrogen, the cylinder be about a quarter 

 filled with common coal-gas, by holding it a short time over a 

 gas-burner, the mixture will explode, but with much less violence ; 

 there is also some difficulty in getting the right proportion of 

 gas, for if too large an amount be present, it will not explode. 



The best proof that the heating effects of electricity are only 

 seen when the current passes along an imperfect conductor is 

 afforded by passing the shock from a large jar or battery 

 through pieces of fine wire. It will be found that the finer the 

 wire the more will it be heated ; if a very fine wire be employed, 

 it will be rendered red-hot or even melted. Fine silver wire 

 has been so intensely heated as to be dissipated in vapour when 

 the charge from a very powerful battery has been transmitted 

 through it. The only other experiment of this class we shall 

 refer to now, consists in melting gold leaf or Dutch metal into 

 glass by means of the electric shock. Two pieces of glass of 

 the same size are taken, and a narrow strip of the leaf is placed 

 between them, extending from side to side. They are then tied 

 together, and laid on the table of the universal discharger. 

 The shock from a battery is now passed 

 along the metal, which is at once fused 

 into the glass, usually with such violence 

 that the pieces of glass are broken into 

 shivers. Fragments, however, will be 

 found large enough to show clearly 

 that the gold is actually melted in, and 

 does not merely adhere to the surface. 

 If the pieces of gold leaf 

 be laid on paper or card, 

 they will, in a similar 

 way, be burnt in, leaving 

 only a purple stain upoa 

 tho paper. 



We now pass on to 

 notice some of the me- 

 chanical effects of electri- 

 city ; and here, as before, 

 any attempt at arrange- 

 ment will fail to be of 

 much service: we must, 

 therefore, give a descrip- 

 tion and explanation of 

 those experiments which 

 best illustrate the prin- 

 ciples of the science. 



When discharging a 

 jar, place a card or piece 

 cf paper between the 

 discharging rod and tho 

 knob of the jar, so that 



the spark may pass through it. It will be seen on examining 

 the card that a hole has been pierced through it ; several 

 cards or pieces of paper may be pierced at once in this way. 

 The most remarkable fact about this experiment is, that a 

 distinct burr is raised on each side of the card or paper, and, 

 in the case of several cards pierced at once, on each side of 

 every card. This fact seems to point strongly in favour of the 

 hypothesis that there are two kinds of electricity, for if there 

 were only a single fluid, we should expect to find the burr only 

 on the positive side, or that from which the fluid passed. 



Not only card and paper, but even glass may be pierced by 

 the electric shock. Fig. 28 represents an elegant way of show- 

 ing this. The piece of glass, A, to be pierced rests on a glass 

 cylinder, so that a pointed wire touches its under surface ; a 

 second pointed wire, B, is supported by the two glass rods, and 

 brought exactly opposite to the first ; the shock is then passed 

 as shown. The glass, however, should be thin, and the jar be 

 rather highly charged, in order to ensure success in the experi- 

 ment. 



It is said that a thin glass phial may sometimes be pierced 

 in a similar way. It is first filled with oil, and a pointed wire 

 passed through the cork, and turned up so that the point may 

 touch the side of the bottle. The other end of this wire should 

 be bent over so that the whole may hang from the conductor, 

 and sparks may then be drawn by the knuckle or a ball held 

 opposite the point, and will pierce the glass. 



