130 



THE POPULAR EDUCATOK. 



by the friction of sealing-wax, and that bodies charged with the 

 same kind repel each other, while those charged with opposite 

 kinds are mutually attracted. These two kinds are distinguished 

 as vitreous and resinous, the former being produced by the 

 friction of glass, the latter by that of resinous bodies. The terms 

 positive and negative are sometimes used instead of the above. 



Two different theories have been proposed to account for these 

 effects, and as they help us to understand the explanations 

 usually given of electrical phenomena, we must just refer to 

 them here. The first, started by Franklin, supposes that 

 electricity is a subtle fluid, of which all bodies possess naturally 

 a certain amount. If this normal state be disturbed, and the 

 body possess a larger amount, it is positively electrified ; if it 

 possess less, it is negatively electrified. The fluid is further 

 supposed to be powerfully self -repelling, but to attract particles 

 of matter not charged with it. 



The other theory is, that there are two different kinds of 

 electricity. When these are associated together, they fully 

 neutralise one another, and in this state pervade all bodies. If 

 by friction, or in any other way, this fluid be decomposed, one 

 kind or the other will be in excess, and the body will then be 

 charged with vitreous or resinous electricity as the case may be. 

 We need not attempt to decide which of these theories is correct. 

 Most of the phenomena we shall observe may, however, be 

 explained by either ; we shall therefore use the terms positive or 

 vitreous, and negative or resinous indiscriminately, and shall 

 sometimes represent positive electricity by the sign plus ( + ), 

 and negative by the sign minus ( ). 



The pith ball we have already used furnishes us an easy way 

 of telling which electricity a body is charged with. We have 

 Tnerely to communicate a portion of the charge to it, and then to 

 bring an excited glass rod near it. If it be repelled, we at once 

 learn that it, like the glass, is charged with positive electricity; 

 while if it is attracted, we know as surely that the body must be 

 negatively electrified. 



By trying a series of experiments in this way, we shall find 

 that when a glass tube is rubbed with a piece of silk, the silk 

 becomes charged with electricity as well as the glass, but that 

 the electricity is of the contrary kind, the silk being negative, 

 while the glass is positive. This rule holds good whenever two 

 bodies are rubbed together ; both will be electrified, but the cue 

 will be + , and the other . 



When the ball is allowed to touch the excited glass or wax, 

 and has become charged, it will continue to be repelled for some 

 little time. The effect, however, gradually becomes weaker, and 

 then ceases. 



When it is thus charged, touch it with different bodies, and 

 notice the effects. Let it touch the finger, for instance, and 

 the repulsion will then cease, and it will bo attracted again by the 

 glass as at first. If we use a piece of wire, exactly the same 

 effect will be seen ; if, however, we substitute a rod of glass or 

 of sulphur for the wire, a different effect will be observed, the 

 ball continuing to be repelled as before. It is clear, then, that 

 there is a difference between the wire and the glass in the effect 

 they produce on the charged ball, the former appearing to 

 remove all the charge, while the latter allows it to remain 

 intact; and such is in reality the case. There are many bodies, 

 the metals among them, along which electricity can easily pass 

 and escape these are termed conductors; on the other hand, 

 there are nany substances like glass and sulphur, which 

 obstruct its passage and keep it confined, and these are called 

 insulators or non-conductors. It must be remembered that the 

 terms conductors and non-conductors are only relative, for they 

 graduate so slowly into one another, that no well-defined line can 

 be drawn to separate them. By experimenting with rods made 

 of a number of different substances, we shall be able to form 

 an idea of their relative conducting powers. The following table 

 pives us some of the results of a series of such experiments. 

 The best conductors are placed first, while those near the end 

 may be classed as good insulators : 



All the metals. 

 Charcoal. 

 Strong acids. 

 Most minerals. 

 Solutions of salts. 

 Water. 



Moist animal anil vege- 

 table substances. 



Oils. 



Alcohol. 



Paper. 



Dry wood. 



Ice. 



Dry air. 



Glass. 



Fur. 



Silk. 



Gutta percha. 



Resinous bodies. 



Sulphur. 



Amber. 



Shellac. 



The fact of water, and therefore moist air, being a good con- 

 ductor, explains a difficulty that will often be experienced in 

 working an electrical machine. Sometimes the attempt is made 

 to show its power in a room full of people, or where the air is fully 

 charged with moisture, and failure is then almost certain, as the 

 moisture of the breath or the air carries off most of the elec- 

 tricity. Hence in working a machine every part of it should 

 be thoroughly warmed, to avoid the condensation of moisture on 

 the glass. The best effects, too, will be obtained on a frosty 

 evening, as then the amount of watery vapour in the air is very 

 small. 



Having seen that different bodies have different powers of 

 conducting electricity, we understand why it is that electricity 

 does not appear to be produced by the friction of all bodies. 

 If we rub a piece of metal, and hold it near the ball, no effect 

 is produced, the reason being that the electricity is conducted 

 away by the metal as soon as it is generated. If, however, we 

 fix the metal to a rod of glass or some other non-conductor, and 

 then rub it with a piece of silk, we shall find that it attracts 

 the ball in the same way as the rod of glass did. We see, too, 

 now the reason why we used a -piece of silk to suspend the 

 pith ball. 



It is on account of glass being a good insulator that it is so 

 frequently used in the construction of electrical apparatus. To 

 prevent the condensation of moisture on it, it is sometimes 

 coated with sealing-wax, varnish, or shellac. The former of these 

 is very commonly used, both for increasing the insulation and 

 giving a finished appearance to the work. The student may 

 easily make it for himself. Some fragments of red sealing-wax 

 may be put into spirits of wine (methylated spirit is much 

 cheaper, and answers equally well), and dissolved by gentle heat. 

 A quicker plan of making it is to put the spirit into a wide- 

 mouthed bottle, and having lighted the stick of wax, let it melt 

 and drop in. The spirit soon becomes heated and burns, 

 keeping the wax alight and rapidly dissolving it. This varnish 

 may be easily laid on with a brush, and is a great improvement 

 to the look of the apparatus. 



The pith-ball electroscope, already referred to, supplies a sim- 

 ple mode of testing the presence of electricity. If two balls be 

 suspended side by side from the same support, they will, when 

 charged with electricity, repel one another and stand apart ; and 

 thus we have a very easy way of showing the intensity with 

 which any body is charged. Neither of these electroscopes is, 

 however, nearly delicate enough for many purposes, and a different 

 kind of instrument has accordingly been constructed to indicate 

 the presence, and to some extent the intensity, of a charge. 



This instrument, which is represented in Fig. 2, is known as 

 the gold-leaf electroscope. A glass vessel, shaped like a bottle 

 without a bottom, is taken and fixed to a stand, and a cap or 

 cork, with a piece of glass tube passed tightly through it, 13 

 fitted to its mouth. A circular disc of brass about three or 

 four inches in diameter is now screwed on a piece of wire, the 

 lower end of which is slightly flattened out on an anvil. This 

 wire is then passed through the tube, the space round it being 

 filled with shellac, and two small strips of gold leaf are gummed 

 to the flattened end of the wire so as to hf.ng side by side. 

 Inside the cylinder, two strips of tinfoil connected with the stand 

 are pasted, so that, if a strong charge be imparted to the brass 

 plate, the leaves may diverge till they touch these strips, and 

 thus lose their surplus electricity. 



If now the brass plate have any degree of electricity imparted 

 to it, it will at once pass along the wire to the pieces of gold 

 leaf, and these being thus similarly electrified, will repel each 

 other, and stand apart, as shown in the figure. The glass tube 

 and shellac render the insulation nearly perfect, and thus, if the 

 instrument be dry, the presence of even a very trifling amount 

 of electricity will be at once rendered manifest. It is not even 

 necessary for the electrified body to touch the plate, for holding 

 it near will cause the leaves to diverge. With this instrument, 

 too, we can tell whether the electricity is positive or negative. 

 We first touch the plate with an excited tube. The leaves at 

 once diverge, and as there is no means of escape for the elec- 

 tricity, they continue to diverge long after the tube is removed. 

 Now bring the body whose charge we wish to examine near, and 

 if the leaves diverge to a greater extent than before, we see at 

 once that it, like the glass, is positively electrified. If, on the 

 contrary, the leaves have a tendency to diverge less, we know 

 that the body is negatively charged. 



