ELECTRICAL THEORIES.] UNDULATORY F OK CES. ELECTRICITY. 



177 



from its source, certain effects of any force which seem 

 to be conveyed on their surface, or by their mass. The 

 relative power of such substances is measured by the 

 rapidity with which these effects are shown when one 

 body is compared with another in that respect. Thus, 

 a piece of wood, six inches long, may be in a rapid state 

 of combustion at one of its extremities, whilst, at the 

 other, not the slightest sensation of heat can be ex- 

 perienced A piece of copper, of the same size, will, 

 however, produce a very different result ; for if one 

 end be heated but moderately, the calorific effect will 

 be speedily evidenced at the other end showing that 

 the principles of heat, or its effects, travel imich more 

 rapidly from one extremity of the metal to the other, 

 than is found in the case of wood. Now, the metal is 

 therefore termed a good conductor, and the wood a bad 

 one ; or sometimes, erroneously, a non-conductor. 



Generally speaking, there is no great difficulty ex- 

 perienced in deciding on the relative electric conducting 

 power of a body. The effects of free electricity can be 

 easily observed by means of electroscopes, <tc. , even 

 when the amount present is but very slight. In the 

 case of heat and sound, our means of detection are not 

 so delicate ; and, therefore, a list of conductors is less 

 to be relied on. On the other hand, the electrical indi- 

 cations rapidly exhibit themselves and pass away, and, 

 for that reason, are sometimes but slightly appreciable at 

 a distance from the source of disturbance. With respect 

 to electricity obtained by friction glass, resins, sul- 

 I hnr gutta-percha, fur, hair, and similar bodies, are in- 

 cluded amongst bad conductors, because they have very 

 little tendency to permit the passage of electricity over 

 ly, for this reason, they most readily ex- 

 electrical effects when submitted to friction ; and 

 hence have been termed electrics. The metals, in the 

 order of gold, silver, copper, platina, brass, iron, itc. , 

 are excellent conductors of electricity, and, accordingly, 

 are employed whenever its effects are required to be 

 shown at a distance from its exciting cause. Another 

 class, standing intermediate between good and bad con- 

 ductors, embraces fluids of various kinds, moisture, itc. ; 

 which, according to circumstances, more or less readily 

 permit of the passage of electricity. We have often 

 spoken of the necessity which exists of keeping every 

 part of electrical apparatus free from moisture ; and 

 this is simply because its presence on the surface of 

 non-conductors, such as glass, etc., actually lias the 

 effect of rendering them, superficially, tolerable con- 

 ductors. 



By way of parenthesis, we may observe, that it is a 

 good plan to cover the legs and supports of electrical 

 apparatus with a solution of shell-lac, or best red seal- 

 ing-wax, in spirits of wine. This forms a coating, which 

 has not so great a tendency to condense moisture at 

 glass ; and, in damp weather, its advantages are greatly 

 felt by the electrician. This resinous coating is a worse 

 conductor of heat than glass ; and it and gutta-percha, 

 for that reason, may at all times be employed for insu- 

 lating purposes. 



With respect to the mode in which electrical effects 

 travel over, or are conveyed by, conductors, we may 

 observe, that, unlike heat, which travels in the mass of a 

 body, electricity seems conveyed only by their outer sur- 

 face. Hence, within certain limits, a large surface of a 

 comparatively bad conductor, may act as efficiently as a 

 small mass of a good conductor. We have thus observed 

 all the ifects of a seven-feet plate machine, or 



of the hydro-electric, entirely dissipated by a damp glass 

 surface ; whilst the same quantity qf electricity, sent 

 i a solid but fine metallic wire, would have in- 

 stantly reduced it to a melted state, or dispersed it into 

 fragments. It is thus of extreme importance, in employ- 

 ing conductors for protecting buildings from lightning, 

 that the double condition of surface and solidity should 

 be attended to. In experiments with large electric and 

 voltaic batteries, we have, for many years, preferred the 

 r strips to that of wire of the same 

 Imt, of course, exposing less superficies. And, 

 at the present day, flat copper bands arc preferred as 



Vi it. I. 



lightning conductors, in the navy, to copper rods which 

 had been previously used. 



The continuity of a conductor, if broken even to the 

 smallest extent, destroys its efficiency. We shall, however, 

 defer considering this element, until we examine the 

 mechanical and other effects of intense charges of electri- 

 city, which will come under our notice when we speak of 

 the Leyden jar. We have already stated our preference, 

 at all times, for the use of wires to that of chains, in 

 connection with electrical experiments ; and this arises, 

 experimentally and theoretically, from the facts which 

 we are here mentioning. As a powerful charge passes 

 over either a brass or iron chain, a spark may be noticed 

 at every link, indicating, that there the continuity of 

 conduction is broken, and that the force has, so to speak, 

 to " leap over" an obstacle. Besides, at each break 

 there is a tendency to diffuse electricity, and hence a 

 decided loss of power ensues. 



The conducting power of bad conductors is modified 

 by heat ; hence, heated glass and air become better con- 

 ductors than when they are at ordinary temperatures. 

 This fact should be borne in mind in warming and dry- 

 ing electrical apparatus ; and their temperature should 

 only slightly exceed that of the surrounding air, so as to 

 prevent the deposition of the moisture, by condensation, 

 on their surface. 



Some very remarkable effects may be noticed when a 

 powerful electric charge passes by a good conductor. We 

 have observed, that the external surface of a metal may 

 be actually fused, whilst the internal mass is entirely 

 unaffected. It often happens that metallic objects, in 

 the pockets, <bc., of persons struck with lightning, have 

 been externally fused ; but this generally occurs where 

 no continuous conductor for either heat or electricity is 

 present. We can only explain this by suggesting, that 

 the development of each force is very sudden, and yet 

 only affects the extreme outer portion of the metal, the 

 mass of which speedily absorbs the intense heat which 

 has been generated. 



Circumstances occur in which a tolerable conductor 

 may become an excellent electric ; and the most extreme 

 case we can point out is that of water. This liquid, in 

 the state of fine particles, spread dew-like over a bad 

 conductor, as we have already seen, improves its con- 

 ducting power ; yet, in exactly the same state at the 

 nozzles of the hydro-electric machine tubes, it affords, 

 by friction, an enormous amount of free electricity. 



From these facts, our readers will at once see, that the 

 idea of conduction and non-conduction can only be sus- 

 tained in a relative or comparative point of view, and 

 that special circumstances can entirely modify, and 

 occasionally change, the character of any substance or 

 liquid. There, however, exists great analogy between 

 heat and electricity in this respect. 



ELECTRICAL THEORIES. 



FOR the sake of simplicity, we have hitherto restricted 

 our observations to such facts as could be explained, 

 iiiuler the idea that electricity, like heat, could be con- 

 veyed to any place ; was diffusible, and, generally, had 

 ;he characteristics of what we may term a single acting 

 :orce. We carefully abstained from mentioning, that 

 certain theories which have been, and are strongly main- 



1, entirely militate against that notion. This 

 course has been adopted because it permits the reader 

 to become acquainted with facts sufficiently plain to 

 explain themselves, and so a kind of foundation 

 has been laid for the more complicated structure of 

 phenomena and laws which we shall now begin to 



i ue. 



The earlier electricians were of opinion, that an elec- 

 trified body differed from a non-electrified one, by con- 

 taining a greater quantity of electricity in a free state. 

 In other words, an electrified object had an excess, 

 whilst the non-electritied had a deficiency, of the electric 

 force. Hence, the terms positive and negative were 



nnally employed, in their strictly mathematical 

 sense, to express such an idea ; and we may, perhaps, 



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