The visible electrolysis of water begins at an E.M.F. of about 1.7 V. Below this there is no disengagement of bubbles. If the E.M.F. be increased at the terminals of the voltameter, the current (and consequently the production of gas) will become proportional to the excess of the value over 1.7 V; but, at the same time, the current will heat the circuit—that is to say, will produce a superfluous work, and there will be waste. At 1.7 V the rendering is at its maximum, but the useful effect is nil. In order to make an advantageous use of the instruments, it is necessary to admit a certain loss of energy, so much the less, moreover, in proportion as the voltameters cost less; and as the saving is to be effected in the current, rather than in the apparatus, we may admit the use of three volts as a good proportion—that is to say, a loss of about half the disposable energy. Under such conditions, a voltameter having an internal resistance of 1 ohm produces 0.65 liter of hydrogen per hour, while it will disengage 6.500 liters if its resistance be but 0.0001 of an ohm. It is true that, in this case, the current would be in the neighborhood of 15,000 amperes. Laboratory voltameters frequently have a resistance of a hundred ohms; it would require a million in derivation to produce the same effect. The specific resistance of the solutions that can be employed in the production of gases by electrolysis is, in round numbers, twenty thousand times greater than that of mercury. In order to obtain a resistance of 0.0001 of an ohm, it is necessary to sensibly satisfy the equation
20,000 l/s = 1/10,000
l expressing the thickness of the voltameter expressed in meters, and s being the section in square millimeters. For example: For l = 1/10, s = 20,000,000, say 20 square meters. It will be seen from this example what should be the proportions of apparatus designed for a production on a large scale.
The new principles that permit of the construction of such voltameters are as follows: (1) the substitution of an alkaline for the acid solution, thus affording a possibility of employing iron electrodes; (2) the introduction of a porous partition between the electrodes, for the purpose of separating the gases.
Electrolytic Liquid.—Commandant Renard's experiments were made with 15 per cent, solution of caustic soda and water containing 27 per cent. of acid. These are the proportions that give the maximum of conductivity. Experiments made with a voltameter having platinum electrodes separated by an interval of 3 or 4 centimeters showed that for a determinate E.M.F. the alkaline solution allows of the passage of a slighter intenser current than the acidulated water, that is to say, it is less resistant and more advantageous from the standpoint of the consumption of energy.
Porous Partition.—Let us suppose that the two parts of the trough are separated by a partition containing small channels at right angles with its direction. It is these channels alone that must conduct the electricity. Their conductivity (inverse of resistance) is proportional to their total section, and inversely proportional to their common length, whatever be their individual section. It is, therefore, advantageous to employ partitions that contain as many openings as possible.
The separating effect of these partitions for the gas is wholly due to capillary phenomena. We know, in fact, that water tends to expel gas from a narrow tube with a pressure inversely proportional to the tube's radius. In order to traverse the tube, the gaseous mass will have to exert a counter-pressure greater than this capillary pressure. As long as the pressure of one part and another of the wet wall differs to a degree less than the capillary pressure of the largest channel, the gases disengaged in the two parts of the trough will remain entirely separate. In order that the mixing may not take place through the partition above the level of the liquid (dry partition), the latter will have to be impenetrable in every part that emerges. The study of the partitions should be directed to their separating effect on the gases, and to their electric resistance. In order to study the first of these properties, the porous partition, fixed by a hermetical joint to a glass tube, is immersed in the water (Fig. 2). An increasing pressure is exerted from the interior until the passage of bubbles is observed. The pressure read at this moment on the manometer indicates (transformed above the electrolytic solution) the changes of level that the bath may undergo. The different porcelains and earths behave, from this point of view, in a very unequal manner. For example, an earthen vessel from the Pillivayt establishment supports some decimeters of water, while the porcelain of Boulanger, at Choisy-le-Roi, allows of the passage of the gas only at pressures greater than one atmosphere, which is much more than is necessary. Wire gauze, canvas, and asbestos cloth resist a few centimeters of water. It might be feared, however, that the gases, violently projected against these partitions, would not pass, owing to the velocity acquired. Upon this point experiment is very reassuring. After filling with water a canvas bag fixed to the extremity of a rubber tube, it is possible to produce in the interior a tumultuous disengagement of gas without any bubbles passing through.
FIG. 2.—ARRANGEMENT FOR THE STUDY OF CAPILLARY REACTION IN POROUS VESSELS.
From an electrical point of view, partitions are of very unequal quality. Various partitions having been placed between electrodes spaced three centimeters apart, currents were obtained which indicated that, with the best of porcelains, the rendering of the apparatus is diminished by one-half. Asbestos cloth introduces but an insignificant resistance.