Chap. 10] ELECTRICAL METHODS 633 



conduction of current is accomplished by the "free" electrons in the outer- 

 most orbits of the atoms. In gases and liquids, current conduction is 

 associated with ions, that is, molecules combined with electrons or those 

 having a deficiency of electrons, or a positive charge. Because of this 

 association, matter is transferred, that is, deposited at one electrode and 

 dissolved at the other. Hence the name electrolytic (= solvent) phe- 

 nomena. Current conduction in electrolytes depends not only on the 

 mobility but also on the number of ions. The latter, in turn, depends on 

 the concentration and degree of dissociation, which increases with the 

 dielectric constant of the solvent. Hence, water with the highest dielectric 

 constant (about 80) is of great importance in enhancing conductivity. 



For D.C., conduction in both metallic and electrolytic conductors is 

 governed by Ohm's law. By it a conductor is defined as having a unit of 

 resistance if the potential difference 1 produces the current 1. Its re- 

 sistivity p is defined as resistance R referred to unit dimensions. If the 

 length of the conductor is I, and S its section, 



R = p.^. (10-4a) 



Although the resistance of a body of unit dimensions is numerically equal 

 to the resistance of a centimeter cubed, resisti\dty should not be expressed 

 in ohms per centimeter cubed. Since p = SR/l, and the square of dimen- 

 sions appears in the numerator and the first power in the denominator, the 

 dimension of resistivity is ohm-centimeter. In the practice of electrical 

 prospecting, the use of this unit often results in comparatively high figures, 

 and the ohm-meter (ohm-cm- 10"^) is frequently employed, in addition to 

 ohm-inch (ohm-cm 0.3937), ohm-foot (ohm-cm 0.0328) and kilohm-cm 

 (ohm-cm 10" ). In certain electrical problems it is necessary to convert 

 the practical into electromagnetic or electrostatic units: 1 ohm = 10^ 

 e.m.u. = 1.111 10"^ e.s.u. The reciprocal of resistivity is designated as 

 conductivity a: 



The unit is the reciprocal ohm-cm or mho-cm. Conductivity is the ratio 

 of current density and electric field strength E: 





(10-5) 



This relation follows directly from Ohm's law. Since V = IR and 

 R = pl/S, dV/dl = (potential gradient or field strength) = E = (//S)-l/cr, 

 with 7/S = current density i. 



