1170 



THE POPULAR EDUCATOR. 



held there a sensation as of cobwebs will be experienced, just as 

 is felt when a person stands on an insulating stool. In the 

 dark such a point becomes beautifully luminous. Not only can 

 the breeze, or aura, as it is called, be felt by the hand, but its 

 effects may be seen on objects held near the point. If a lighted 

 candle be held to it the flame will be perceptibly blown over to 

 one side, just as if a faint stream of air from a blowpipe was 

 passing through it. Wheels fitted with floats after the plan of 

 paddle-wheels have also been turned by it ; it is requisite, how- 

 over, that they should be very light and well balanced, or else 

 the current will be too weak to set them in motion. Instead of 

 the candle being held to a point, it may be placed on the con- 

 ductor, and a pointed wire brought near it ; the flame will then 

 be repelled just as if a negative stream were issuing from the 

 point. 



Another way in which the mechanical effects produced by a 

 point are shown is by means of the electric flyer, which is 

 shown in Fig. 23. A number of pieces of wire radiating like 

 the spokes of a wheel are fixed to a centre cap, and the whole 

 is then carefully balanced so that it may turn freely on a vertical 

 point placed in the conductor. The ends of all the wires are 

 pointed and turned in one direction, and when the conductor is 

 charged the electricity is given off by these, and thus causes 

 the flyer to turn round rapidly, in the same way as the water 

 issuing from the arms in Barker's mill causes it to rotate. 



The experiment is, however, sometimes explained in a different 

 manner. The electricity given off by the points renders the 

 nir around them strongly positive, and it therefore repels the 

 points which are similarly charged, and thus sets the flyer in 

 motion. If it be placed under the receiver of an air-pump, and 

 the air exhausted, no motion will be produced. A simpler mode 

 of constructing the flyer is to flatten a piece of copper wire in 

 the middle, and make in the flattened part a small depression 

 for the point to fit into. The ends of the wire are then pointed 

 and bent round, as shown in Fig. 24. 



The electric orrery is merely a modification of this experiment. 

 A second flyer being placed on a point projecting upwards from 

 the first, small globes made of cork or some other light material 

 are then placed on them to represent the sun and planets, and 

 the reaction of the points sets the whole in motion. 



If these experiments be tried in a darkened room a faintly 

 luminous ring will be seen, caused by the electricity given off in 

 the form of the brush discharge. A good way of showing the 

 flyer, and at the same time illustrating the principle of the 

 Leyden jar, is to use the two together. The knob of the jar is 

 made to unscrew, and the end of the rod pointed so that the 

 flyer may be placed upon it. As soon as the jar is charged it 

 should be placed on a sheet of glass or an insulating stand, and 

 the knob unscrewed. Only the excess of positive electricity 

 will be given off, and the flyer may then be balanced on the 

 point. If now the knuckle be held to its exterior, a series of 

 small sparks will be given off, while a corresponding amount of 

 electricity will be dispersed from the points of the flyer. If the 

 knuckle be withdrawn, the flyer will soon cease to move. It 

 may be mentioned here that it is by no means necessary to have 

 a cover to a Leyden jar. Many electricians fix the rod to a 

 plate at the bottom, or to a thin framework made of wood, 

 covered with tinfoil, and placed within the jar, and this mode 

 seems to be in many respects preferable to the older plan. 



Instead of placing the flyer on the conductor, the latter may 

 be altogether removed, and the flyer balanced on a pointed wire 

 and held near the edge of the silk flap ; the electricity will then 

 be attracted by the points, and will set it in motion. 



If a small metal pail, with an opening in the bottom of such 

 a size that the water can only just drip from it, be suspended 

 from the conductor, the particles of the liquid being similarly 

 electrified will repel one another, and the water will flow in a 

 tine stream instead of in a series of drops. Similarly, the water 

 will drop much more rapidly from a wet sponge if it be hung 

 from the conductor. In both cases, the issuing water will at 

 times appear faintly luminous in the dark, owing to its being 

 charged with electricity. 



Let a piece of camphor be laid on the conductor, and then 

 lighted, it will radiate in all directions as soon as the machine 

 is set in action. 



There is one experiment we must now refer to which hardly 

 comes under this class of effects. Having charged a Leyden 

 jar, take it up by the knob, and trace with the knob a series of 



lines and figures on a cake of resin or vulcanite. Then, having 

 placed the jar on rn insulating stand in order to avoid receiving 

 a shock, take it UD by the knoo and trace a second set of lines 

 on the same surface with the outside. The latter will, of course, 

 be negatively electrified, and the former positively. Now take 

 a mixture of red lead and flowers of sulphur, and sift it evenly 

 on to the resin ; the ingredients will at once separate themselves 

 to a considerable extent, the sulphur settling in a series of tree- 

 like forms along the positive track, while the red lead gathers 

 in a series of stars or spots along the negative. 



The devices thus formed vary, of course, very greatly, and 

 elegant patterns may at times be obtained. They are known as 

 Lichtenberg's figures. 



The chemical effects of electricity are very great and very 

 important ; they are, however, most easily seen by means of 

 voltaic electricity, and must, therefore, be chiefly referred to 

 under that head. We will, however, give one or two illustrations 

 here. If pieces of paper coloured with litmus and turmeric, and 

 moistened with sulphate of soda, be placed on a plate, and a 

 wire leading from the prime conductor be connected with one 

 end, and a second wire connected with the gas or water-pipes 

 be laid upon the other, we shall find on working the machine- 

 that the turmeric paper will be stained brown at the place where 

 the earth wire touches it, showing that the sulphate has been 

 decomposed and the alkaline constituent of it set free against 

 this wire. If the litmus paper be employed a red stain will be 

 found against the wire leading from the conductor, showing that 

 the acid constituent of the salt has been liberated there. We 

 see thus that the acid and alkali appears at the positive and 

 negative poles respectively. The reason of connecting the wire 

 to the gas or water pipes is to ensure a goocl connection with 

 the earth, and thus to let the electricity escape as rapidly as 

 possible. 



If sparks be made to pass over a piece of paper soaked with, 

 iodide of potassium, the iodine will be liberated, and will stain. 

 the paper brown. This effect is, however, sometimes attributed 

 to the nitric acid which is formed in the air by the passage of 

 the electricity, causing the nitrogen and oxygen to unite and 

 not to the decomposing power of the electricity. After a thun- 

 der-storm a very perceptible amount of nitric acid is thus 

 formed, and the influence which lightning has in clearing the 

 air may, in a great degree, be attributed to this cause. 



Water has been decomposed into its constituent gases oxy- 

 gen and hydrogen by the passage of a large number of shocks 

 through a small portion of it ; the experiment is, however, com- 

 plicated and difficult, and the mode of performing it need not 

 be fully described. With this we conclude our glance at the 

 main results produced by the electric spark. 



There is one easy and simple method of obtaining electricity 

 in small quantities, which has not yet been referred to, but will 

 often be found very useful. It is by means of the apparatus 

 known as the electrophorus, and represented in Fig. 25a, I/. A 

 tray, B, about ten or twelve inches in diameter, with an edge about 

 half an inch high all round it, is made of tin or zinc. Any tin- 

 smith can easily manufacture this. A mixture, composed of 

 about equal parts of shellac, Venice turpentine, and common 

 resin, melted together, is then poured in so as to fill the tray up 

 to the level of the edge. 



A cover, A, about an inch or an inch and a-half less in dia- 

 meter, is made of metal, or wood coated with tinfoil, and an 

 insulating handle fixed so that it may be lifted by means of it. 

 The edges of this plate should be carefully smoothed. 



To use this apparatus, the resinous cake is excited, by being 

 well rubbed with a piece of warm dry flannel or cloth, or, 

 better still, by being struck several times with a piece of catskin. 

 The metal plate is now placed upon it, and at first acquires a 

 faint charge of negative electricity by touching the excited 

 resin in places. If, however, it be now touched by the finger- 

 while resting on the resin, and then raised, a bright spark will 

 be given from it to any conducting substance held near. Now 

 replace the plate, touch with the finger and again remove, a 

 second spark will be given off; and in this way we may con- 

 tinue until any number of sparks have been drawn, the original 

 charge not being dissipated or weakened in any appreciable 

 degree. 



The rationale of the process is- simple : the resin is not a 

 perfectly smooth surface, and therefore only touches the cover 

 in a few places, and, being an insulator, does not communicate 





