If there is no dissociation, _i.e._ if a = 0, then n' = n, and i = 1. If all the molecules are dissociated, a = 1, and i = k. _Faraday's Law._--Faraday found that the quantity of electricity required to liberate one gramme-molecule of any radical is 96.537 coulombs for each valency of the radical. _Electrochemical Equivalent._--The electrochemical equivalent of a radical is the weight liberated by one coulomb of electricity. It is equal to the molecular weight of the ion, divided by 96.537 times its valency. _Electrolytic Conductivity._--The conductivity of an electrolyte is the inverse of its resistance. C = 1/R. For a given difference of potential the conductivity of an electrolyte is proportional to the number of ions in unit volume, the electrical charge on each ion, and the velocity of the ions. _The specific conductivity_ [Delta] of an electrolyte is the conductivity of a cube of the solution, each face of which is one square centimetre in area. The _molecular conductivity_ of an electrolyte is the conductivity of a solution containing one gramme-molecule of the substance placed between two parallel conducting plates, one centimetre apart. The molecular conductivity is independent of the volume occupied by the gramme-molecule of the solute, depending only on the degree of dissociation. The molecular conductivity U is equal to the product of V, the volume of the molecule, by [Delta], its specific conductivity. U = V[Delta]. Whence [Delta] = U / V, _i.e._ the specific {29} conductivity equals the molecular conductivity divided by the volume. The conductivity of an electrolyte is proportional to the number of ions in a volume of the solution containing one gramme-molecule. Let M_{[infinity]} be the conductivity for complete dissociation and M_v the molecular conductivity at the volume V. Then M_v / M_{[infinity]} = n"k / nk = n" / n = a, the degree of dissociation. This is Ostwald's law, which says that the degree of dissociation is equal to the ratio of conductivity when the gramme-molecule occupies a volume V, to its conductivity when the solution is so dilute that dissociation is complete. Hence the degree of dissociation may also be determined by comparing the electrical conductivities of two solutions of different degrees of concentration. | -- -- -- | -- -- -- | | SO_4 SO_4 SO_4 | SO_4 SO_4 SO_4 | | | | | ++ ++ ++ | ++ ++ ++ | | Cu Cu Cu | Cu Cu Cu | | | | +----------------------------+--------------------------------+ FIG. 1.--Before the passage of the current.