CONDUCTION, ELECTRIC

when the terminals of a galvanic cell are connected together by a metallic wire, an electric current pa es through the wire, and the wire is said to conduct electricity. The case is analogous to the flow of water in a pipe which connects two cisterns at different levels. The amount of water flowing per second will be greater in proportion to the difference of levels, and will be le in proportion to the resistance to flow, which is offered by the pipe. Similarly, in the case of an electric current, the difference of electromotive forces at the ends of the wire, and the resistance of the wire, are the all important factors. The relationship is expre ed by Ohm’s Law, which states that the amount of current is proportional to the electromotive force acting, and inversely proportional to the resistance, and is ie expre ed by the equation ers where © is the current in amperes, E the electromotive force in volts, and R the resistance. The unit of resistance is known as the ohm, and is defined as the resistance of a uniform column of pure mercury, 1063 centimetres long, and weighing 144521 grams, the measurements being taken at 0°C. The specific resistance of a substance at any given temperature is the resistance, expre ed in ohms, between two opposite faces of a centimetre cube of the substance at the given temperature. The reciprocal of the specific resistance is known as the electric conductivity or conductance. The resistance of metals depends on a number of conditions. Firstly, on the chemical purity of the specimen: in general, small i es of other metals increase the resistance. Secondly, on the temperature: resistance of a pure metal increases nearly proportionately to the absolute temperature, although there is evidence to show that it vanishes before the zero of absolute temperature is reached. Thirdly, on the physical state of the substance: annealing of a metal generally alters its specific resistance. Fourthly, the resistance generally changes when the material is subjected to stre es such as compre ion or torsion. JFifthly, it also changes in certain cases when the substance is exposed to magnetizing forces. Alloys have, as a rule, a greater resistance than any of their constituents, and with increase of temperature/their increase in resistance is much smaller than in pure metals. The pa age of a - current through any conductor is accompanied by the development of heat. Joule’s Law states that the rate at which heat is developed is jointly proportional to the square of the current strength and the resistance; 7.e, if a current of strength C flows through a conductor of resistance Be the heat developed is C2R per unit ime. With regard to conduction in liquids, the chief fact is that when it takes place chemical changes are always produced. For example, if a current be made to pa through a solution of sodium chloride (common salt), chlorine gas appears at the point where the current (according to the usual convention) enters the solution, while the sodium, liberated at the point where the current leaves the solution, is acted on by the water present so as to form hydrogen, which is thus set free at that point. This proce is known as Electrolysis (q.v.), and the species of conduction involved is known as electrolytic conduction in order to distinguish it from metallic conduction referred to above. Both forms of conduction may be satisfactorily explained by the electronic theory which has been brought forward prominently in recent years. According to it, a conductor of pure metal contains atoms which carry a charge of positive electricity and also contains a large number of negatively charged particles, termed electrons, which are free to move among the atoms. The motions of the electrons may be regarded in the same way as that of the molecules in a gas are regarded by the kinetic theory of gases. Taken on the average, they will have a certain mean velocity, will collide with each other and with the positively charged atoms, and between two succe ive collisions each will have traversed a certain mean free path in a certain time. In the ordinary condition of the conductor, when no current is pa ing through it, the velocities of the electrons will be distributed equally in all directions in space; but if the conductor be included in a galvanic circuit; the electromotive force thus applied induces them to travel, on the whole, in the direction of that force. The positively charged atoms are, however, not free to move except through small distances from their mean positions, and hence the electrons move relatively to the atoms. It is this motion of the electrons which constitutes a current of electricity. The theory shows that the conductivity is proportional to the number of electrons in unit volume of the substance and to the length of their mean free path. Apart from the influence—supposed to be small—of the second of these two factors, it follows that good or bad conductors of electricity differ by having more or fewer electrons, respectively, per unit volume. As a rule, substances which conduct electricity well also conduct heat well, and this is quite in agreement with the electronic theory, according to which the ratio of the two conductivities, thermal and electric, should be the same for ali pure metals. Experiment shows that this is the case, at least for all the better conducting metals. For an explanation of the mechanism of conduction in liquids, see ELECTROLYSIS. In ordinary circumstances gases do not conduct electricity. But a gas may easily be rendered conducting in a variety of ways. If it is exposed to the action of Rontgen rays, cathode rays, rays from uranium or radium, ultraviolet light, electric spark discharge, contact with incandescent metals, or if it is mixed with the products of combustion from fiames, it can conduct electricity for some time after the action which produced conductivity has ceased, but this conducting power always diminishes and finally disappears. When in the conducting state the gas is said to be tonised. Conduction in such cases presents certain peculiarities. In the first place it does not follow Ohm’s Law unle the electromotive force is very small. Second, the conducting power may be removed by various methods, such as filtration of the gas through gla -wool, bubbling it through water, pa ing it through a metallic tube, or pa ing a current of electricity through it. These and other facts show that conduction in an ionised gas is due to the presence of particles, that these particles are electrified, and that their electrifications are both positive and negative. _ The principles of electric conduction in metals find their widest application in the distribution of electric energy for the purposes of lighting, heating and power.