[79] The copper electrodes are polarized by the formation of hydrogen on the cathode, but, in the course of a few seconds, the current becomes rather constant and is then read. The polarization may be considered as simply reducing the potential of the cell, and since, within the range of concentrations of acid used,—4-molar to 1/8-molar—the polarization current does not vary markedly, as compared with the potential of the storage cell, the total potential used through the series of dilutions may be considered sufficiently constant for the purposes of the experiment. Readings are made three or four seconds after each dilution, when the polarization has been fully established. Polarization may be entirely avoided by the use of a silver nitrate solution and silver electrodes or of a cupric salt solution and copper electrodes (Noyes and Blanchard). Hydrochloric acid is used here in order to carry the discussion in the text as far as possible with this typical ionogen. If one takes care to make readings as described, the result is quite satisfactory, as is shown by the comparison of the ratios of the readings with the ratios calculated from the known conductivities of the various dilutions (see table below). [80] Current = (Potential Difference) / Resistance, or Current = (Potential Difference) × Conductivity. For a ‹constant potential difference›, then, Current ~ Conductivity. [81] The ‹specific conductivity› of a solution (commonly designated by κ) is the conductivity of a cube of 1 cm. edge; the ‹molecular conductivity› is the conductivity of a mole of the electrolyte; the ‹equivalent conductivity› (designated by Λ) is the conductivity of a ‹gram-equivalent› of the electrolyte. Λ = κ × ‹v›, where ‹v› is the volume, expressed in cubic centimeters, containing the gram-equivalent. For instance, the resistance of 0.1 molar hydrochloric acid in a cube of 1 cm. edge is 28.5 ohms and its conductivity (κ) therefore 1 / 28.5 or 0.0351 reciprocal ohms. Since 10 liters or 10,000 c.c. of 0.1-molar hydrochloric acid is the volume (‹v›) containing one mole of the acid (the molar and the equivalent conductivities, for a monobasic acid being the same) Λ = 0.0351 × 10,000, or 351. [82] Kohlrausch and Holborn, p. 200. [83] Cf. Kahlenberg, ‹Transactions of the Faraday Society›, «1», 42 (1905). [84] Clausius, ‹Poggendorf's Ann.›, «101», 347 (1857) («Stud.»). His theory replaced the older one of Grotthuss. [85] ‹Phil. Mag.›, «5», 729 (1903), and‹ Transactions of the Faraday Society›, «1», 55, (1905). [86] ‹Vide›, Hudson, ‹J. Am. Chem. Soc.›, «31», 1136 (1909), for a recent summary of results. [87] ‹Lectures on Physical Chemistry›, «1», p. 131. [88] ‹Vide› A. A. Noyes and Blanchard, ‹J. Am. Chem. Soc.›, «22», 726 (1900). [89] The concentrations are figurative, but may be taken to represent actual concentrations, such as 0.015 molar, etc.