Conductors 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.)... 

Electrolytic conductance, a property of the so called conductors of the second class, is encountered mainly in the case of salts in dissolved, melted and solid state. Among these compounds are sulphates, halides, nitrates, silicates, also many oxides, hydroxides, sulphides and so on. The same group includes also the potential electrolytes, i. e. the substances from which ions are formed only in mutual reaction with a solvent (solutions of acids in basic solvents, solution of bases in acid solvents, further amines and different chlorine derivatives of organic compounds in liquid sulphur dioxide, nitro-compounds in liquid amines etc.). Finally also numerous colloidal solutions (such as proteins and soaps) conduct the current like electrolytes. 

The mechanism of the passage of direct current tlirough a conductor of the second class which is connected with a migration of ions and chemical reactions on electrodes can be most easily explained by an example. Let us imagine a vessel with two indifferent platinum electrodes A and C electrically connected to a source of direct current B and immersed into an aqueous solution of common salt in water (Fig. 1.). 

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. 

Weak effects are produced with three conductors of the second class and without metal. This is not in contradiction with Volta s generalised law or principle ( 48 f.), according to which an electric current is produced by a circuit of at least three different conductors ( 53). We see now wherein the whole secret, the whole magic of galvanism lies. It is nothing but an artificial electricity set in motion by the contact of heterogeneous conductors ( 54). ... 

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 . 

The dotted lines show how many degrees of difference of force exist between the bodies they separate, as estimated by the taste on the tongue, bodies following one another inunediately having one degree, which is very small. When two bodies are separated by a moist conductor of the second class ... 

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... 

The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the apphcation of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the apphcation of an electric field is possible. FIectromigration, the migration of minority elements (ie, impurities, defects) through the bulk soHd under the influence of current flow, is another process by which an atom may be moved between the surface and the tip of an STM. 

The second class of crystalline sensors is based on the easily fabricated, low-resistance, selectively permeable cast-disk and pressed-pellet membranes based on AgaS. Silver sulfide is an ionic conductor in which silver ions are the mobile species. By itself, it can be used to detect silver ions or to measure sulfide-ion levels. The potential-determining mechanism in an AggS electrode is due to the very low solubility product of AgaS [ATgp = 10 ]. The silver-ion activity... 

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). 

From the point of view of electric properties, all substances can be divided into two main classes—conducting and nonconducting an electric current. Metals, their alloys and a small number of chemical compounds with metal character of interatomic interactions relate to the class of conductors. The second class includes other substances and represents the overwhelming majority. Conductivity is defined by the presence of free charge carriers in a substance their absence determines dielectric properties. So, dielectrics are substances in which there are no free charges capable of covering long distances in the substance (in comparison with molecular sizes). 

A further interesting effect discovered in our laboratories is that the addition of low levels of a second component, or dopant ion, can lead to significant increases in the ionic conductivity [6, 30, 31]. Typically these dopant species, for example, Li, OH , and H" ", are much smaller than the organic ions of the matrix, and since the relaxation times characterizing the motion of these ions are more rapid than those of the bulk matrix itself, these materials may represent a new class of fast ion conductor. The dopant ion effect can be used to design materials for specific applications, for example, Li+ for lithium batteries and H /OH for fuel cells or other specific sensor applications. Finally, we have recently discovered that this dopant effect can also be apphed to molecular plastic crystals such as succinonitrile [32]. Such materials have the added advantage that the ionic conductivity is purely a result of the dopant ions and not of the solvent matrix itself. 

As a class, the transition metals behave as typical metals, exhibiting metallic luster and relatively high electrical and thermal conductivities. Silver is the best conductor of heat and electrical current. However, copper is a close second, which explains copper s wide use in the electrical systems of homes and factories. 

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. 

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