Electrical Conductivity

Electrical Conductivity, the ability to conduct electricity. According to their ability to conduct electric current, all bodies are divided into two groups—conductors of the first and second kind. Conductors of the 1st kind, represented by metals and therefore also called metallic conductors, possess electronic conductivity: the transfer of electricity in them occurs through the movement of free electrons. In contrast, in conductors of the 2nd kind, electrons can move only together with material particles, in the form of ions. The best example of such ionic conductivity can be solutions of electrolytes. In the case of gases, electrical conductivity is likewise due to gas ions—atoms or molecules of gas carrying positive or negative charges. Regardless of the type of conductivity, electrical conductivity is quantitatively characterized by the magnitude of the resistance offered by the conductor to the electric current: electrical conductivity represents a quantity inverse to electrical resistance. Since resistance is expressed in ohms, electrical conductivity is expressed in 'reciprocal ohms.' Electrical conductivity is directly proportional to the cross-sectional area of the conductor and inversely proportional to its length. The electrical conductivity of a conductor whose length is 1 cm and whose cross-section is 1 cm² is called specific electrical conductivity (κ). The measurement of electrical conductivity reduces to the measurement of electrical resistance. It is performed using a Wheatstone bridge. The principle of its design is that the current (from a battery) passing through the system of conductors branches: one part of it goes through the resistance being studied and a rheostat, the other—through a wire stretched on a measuring scale (rheochord). The wire 'bridge' is connected at one end between the resistance being studied and the rheostat; its other end is connected to a movable contact sliding along the rheochord scale. By moving this contact (as well as varying the resistance), its position is found at which no current passes through the bridge, and therefore the electrical potentials at both ends of the bridge have the same value. Obviously, at this moment the measured resistance w relates to the known resistance W as the resistance of both parts of the rheochord divided by the movable contact: w = W ohms. The electrical conductivity in this case is equal to 1/W reciprocal ohms. When measuring the resistance and electrical conductivity of electrolyte solutions, alternating current is used, because when direct current is passed, polarization phenomena at the electrodes would create additional (polarization) resistance. To detect the presence or absence of current in the bridge, a galvanometer is included in the case of direct current, or a telephone—in the case of alternating current. Among metals, copper and silver are very good conductors of electricity, having approximately the same electrical conductivity. Platinum has lower electrical conductivity, and it is even significantly lower for mercury. Expressed in reciprocal ohms, the specific electrical conductivity has the following value for these metals: copper—5.8×10⁵, silver—6.1×10⁵, platinum—9.1×10⁴, mercury—1.0×10⁴. The electrical conductivity of metals increases with temperature; however, for some alloys, the temperature coefficient of electrical conductivity is practically zero. Such alloys include constantan (60 parts copper, 40 parts nickel), which is therefore used for preparing standard resistances independent of temperature. In the case of gases, their neutral molecules do not conduct current, and electrical conductivity depends exclusively on the ionization of the gas, the latter being caused by external influences. In contrast, in the case of solutions, the formation of ions occurs through the dissociation of the electrolyte, and the degree of dissociation is uniquely determined by the properties of the solution itself (the nature of the solvent, the composition and concentration of the dissolved electrolyte). The electrical conductivity of an electrolyte solution depends on the concentration of ions in the solution and their mobility. The concentration of ions, in turn, is determined by the total concentration of the dissolved electrolyte and the degree of its dissociation. As is known, the degree of electrolytic dissociation increases as the concentration decreases, approaching in the limit to unity (i.e., to complete dissociation). By dividing the specific electrical conductivity by the molar concentration, it can always be related to the same (specifically to gram-molecular) amount of electrolyte; this quantity is called molar electrical conductivity. As the concentration decreases, molar electrical conductivity increases, gradually approaching a certain constant value—the so-called limiting molar electrical conductivity. This latter quantity, being independent both of the concentration of the electrolyte (molar concentration) and of the degree of its dissociation (complete dissociation), must obviously be determined entirely by the mobility of the electrolyte ions. Indeed, Kohlrausch showed that the limiting molar electrical conductivity is an additive property of the electrolyte, composed of the electrolytic mobility of its anion and cation. According to the classical concepts of Arrhenius and Kohlrausch, the mobility of each ion represents a characteristic quantity for it, independent of concentration (at constant temperature). The decrease in molar electrical conductivity with increasing concentration must depend exclusively on the decrease in the degree of dissociation, whereby the latter can be determined exactly by measuring electrical conductivity. In contrast, the modern theory of activity has shown that the mobility of an ion depends on the interionic electrostatic fields, whereby it should decrease as the total ionic concentration increases (see Electrolytic Dissociation, Solutions). As a result, molar electrical conductivity should decrease with increasing concentration even in cases where the degree of dissociation of the electrolyte does not undergo significant changes (strong electrolytes). As for the effect of temperature, it is due to the fact that with increasing temperature, the viscosity of the solution decreases, and accordingly both the speed of ion movement and the electrical conductivity of the solution increase. Lit.: Kohlrausch H. and Holborn L., Das Leitvermögen der Elektrolyte, Lpz., 1916. See also lit. to art. Electrolytic Dissociation. D. Rubinstein.