Conductors, of the first class

Electronic conductance is characteristic for the so called conductors of the first class, i. e. for metals (both in solid and fused state) and some metal oxydes, carbides, sulphides, phosphides and borides and it can be explained by assuming the existence of free electrons which act in solid matter as anions. Under the influence of the external electric field these easily movable electrons start an ordered motion while the atoms deprived of their electrons, which are in fact cations, take practically no part in the current conduction and, apart from their vibration within the mean equilibrium positions, remain practically immobile. The passage of the current does not manifest itself by a chemical change of the... 

Volta says Valli s experiments had convinced him that it is possible to obtain effects with a very sensitive prepared frog by means of contacts of humid conductors alone and without conductors of the first class, which he formerly thought were necessary. Contact of dissimilar bodies, however, is essential. Volta made experiments with non-metallic contacts and concluded that if only the conducting arc is composed of three conductors of different kinds there is always a medium, weaker, or very weak motion of the electric fluid. The force with conductors of the first class (metals) is much stronger and they not only set the electric fluid in motion but excite it. 

The contraction of frogs, taste on the tongue, and the sensation of light in the eye are produced when (i) two different metals or conductors of the first class touch one another on one side or form a heterogeneous metallic arc, whilst their opposite ends touch or enclose between them a conductor of the second class which forms the other arc or (ii) with one metal and two different moist conductors, different liquid pairs being most active with different metals. 

The first theory of electrolysis was developed by Grotthuss in 1805 and Davy in 1807 isolated alkali metals by electrolysis. Volta, thereafter developed the electrochemical series (he called them the contact series) and in 1811, Berzelius extended the electrochemical series to include non metals, and correctly established some of the scientific basis of inorganic chemistry. Volta not only produced the world s first battery and the electrochemical series, but he qualitatively differentiated between groups such as metals, carbon and some sulphides which conducted electricity without undergoing chemical change, this he called conductors of the first class and others such as salt solution which is decomposed by electric current, which he called conductors of the second class. Volta s electrochemical series, correctly identified the flow of current from the more electropositive to the less electropositive metal in his series, when connected with each other and a conductor of the second class . 

According to the nomenclature introduced by Faraday, two basic types of conductors can be distinguished, called first and second class conductors. According to contemporary concepts, electrons carry the electric current in first class conductors in conductors of the second class, electric current is carried by ions. (The species carrying the charge in a given system are called charge carriers.)... 

The tetrachalcogenotetracenes comprise another important class of tetrasubsti-tuted tetracene compounds and one of the first classes of materials studied as organic conductors [66]. The myriad of structures and electronic properties of these materials have been reviewed in detail in a number of papers [67]. However, two keys to the remarkable electronic properties of this class of compounds are the ability of the chalcogen substituents to stabilize the oxidized form of the aromatic ring (making these materials powerful electron donors, to the point that they are highly susceptible to air oxidation) and the jr-stacked nature of the molecules in... 

Whenever in a complete circuit of conductors, either one of the second class between two different ones of the first class. . ., or one of the first class between two different... 

Volta gave a plate of 21 diagrams of different combinations of conductors of the first and second classes and shows in detail how each would behave on the basis of his law or principle. With tin alone, between water and an alkaline liquid (as in the experiment described) almost the same action is obtained as with the most electrically different metals (silver and zinc) with an aqueous conductor between them, and it may be equalled or even exceeded with iron alone between water and nitric acid, or silver alone between water and liquid alkali sulphide ( 23). Volta made experiments with conducting bridges formed between cups of liquids by moist leather or card, a piece of juicy flesh, a sinew or gristle, or a slice of cucumber or melon, or anything which allows an electric current (corrente elettrica) to pass ( 29). 

Metals with large ions of low charge form saline halides with three-dimensional ionic lattices. The salts have high m.p. and b.p. and are good conductors when fused. Most of the halides of the first three A sub-groups belong to this class. 

Twenty years ago, a new class of organic polymer materials was discovered with the new property of electronic conductivity comparable to metallic conductors [55]. The first representative was the polyacetylene followed in the subsequent years by several other polymers, such as polyaniline, polypyrrole, and polythiophene (see Fig. 7). The neutral structure is shown in Fig. 7. This structure has properties comparable to a semiconductor. The metal like conductance is obtained by chemical or electrochemical oxidation (shown for polypyrrole in Fig. 8). In this example, up to 30% of the pyrrole rings can be oxidized. The positive charge of the heterocyclic ring... 

Figure I.6a also reveals the timeline of milestones in fuel cell design. The leftmost curve is the performance curve of the first practical H2/O2 fuel cell, built by Mond and Langer in 1889 (Mond and Langer, 1889). The electrodes consisted of thin porous leafs of Pt covered with Pt black particles with sizes of 0.1 lam. The electrol)de was a porous ceramic material, earthenware, that was soaked in sulfuric acid. The Pt loading was 2 mg cm and the current density achieved was about 0.02 A cm at a fuel cell voltage of 0.6 V. The next curve in Figure I.6a marks the birth of the PEFC, conceived by Grubb and Niedrach (Grubb and Niedrach, 1960). In this cell, a sulfonated cross-linked polystyrene membrane served as gas separator and proton conductor. However, the proton conductivity of the polystyrene PEM was too low and the membrane lifetime was too short for a wider use of this cell. It needed the invention of a new class of polymer electrolytes in the form of Nafion PFSA-type PEMs to overcome these limitations.

However, to make these photonic devices some method of controlling light is required so that it can be manipnlated for a particular application. In other words there is a need to be able to trap a photon of a particular wavelength, and then release it only as reqnired. This is the photonic equivalent of the semi-conductor which controls the flow of electrical cnrrent in electronic devices such as transistors. These light manipnlating materials wonld have a photonic band gap that performs an equivalent role for photons as do electronic band gap semi-conductors for electrons. This new class of materials, known as photonic band gap crystals, was first proposed in 1987, and the constrnction of these artihcial crystals has been an area for intensive research since the mid-1990s. ... 

Extrusion-Applied Insulations. The polymers used in extrusion applications can be divided into two classes low-temperature applications and high-temperature applications. Polymers in the first category are poly(vinyl chloride), polyethylene, polypropylene, and their copolymers along with other elastomers. Polymers in the second category are mainly halocarbons such as Teflon polytetrafluoroethylene (which requires special extrusion or application conditions), fluoroethylene-propylene copolymer (FEP), perf luoroalkoxy-modified polytetrafluoroethylene (PFA), poly(ethylene-tetrafluoroethylene) (ETFE), poly(vinylidene fluoride) (PVF2) (borderline temperature of 135 °C), and poly(ethylene-chlorotrifluoroethylene). Extrusion conditions for wire and cable insulations have to be tailored to resin composition, conductor size, and need for cross-linking of the insulating layer. 

Classification of the Elements.—Berzelius was the first to divide all the elements into two great classes, to which he gave the names metals and metalloids. The metals, being such substances as are opaque, possess what is known as metallic lustre, are good conductors of heat and electricity, and are electro-positive the metalloids, on the other hand, such as are gaseous, or, if solid, do not possess metallic lustre, have a comparatively low power of conducting heat and electricity, and are electro-negative. 

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