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.