_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.
[Illustration: 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.
To this inconvenience of porous vessels is added their fragility,
their high price, and the impossibility of obtaining them of the
dimensions that large apparatus would call for. The selection of
asbestos cloth is therefore clearly indicated; but, as it does not
entirely separate the gases, except at a pressure that does not exceed
a few centimeters of water, it was always necessary to bring back the
variation of the level to these narrow limits by a special
arrangement. We cannot, in fact, expect that the entire piping shall
be always in such conditions that no difference in pressure can occur.
The levels are brought back to equality within the effective limits by
interposing between the voltameter and the piping an apparatus called
a compensator, which consists of two vessels that communicate in the
interior part through a large tube. The gases enter each vessel
through a pipe that debouches beneath the level of the water. If a
momentary stoppage occurs in one of the conduits, the water changes
level in the compensator, but the pressure remains constant at the
orifice of the tubes. The compensator is, as may be seen, nothing more
than a double Mariotte flask. When it is desired to obtain pure gases,
there is introduced into the compensator a solution of tartaric acid,
which retains the traces of alkalies carried along by the current of
gas. The alkaline solution, moreover, destroys the ozone at the moment
of its formation.
It will be seen that laboratory studies have furnished all the
elements of a problem which is now capable of entering the domain of
practice. The cheapness of the raw materials permits of constructing
apparatus whose dimensions will no longer be limited except by reasons
of another nature. The electrodes may be placed in proximity at will,
owing to the use of the porous partition. It may be seen, then, that
the apparatus will have a considerable useful effect without its being
necessary to waste the electric energy beyond measure.
_Industrial Apparatus._--We have shown how the very concise researches
of Commandant Renard have fixed the best conditions for the
construction of an industrial voltameter. It remains for us to
describe this voltameter itself, and to show the rendering of it.
[Illustration: FIG. 3.--PLANT FOR THE INDUSTRIAL ELECTROLYSIS OF
WATER.]
The industrial voltameter consists of a large iron cylinder. A battery
of such voltameters is shown to the left of Fig. 3, and one of the
apparatus, isolated, is represented in Fig. 4. The interior electrode
is placed in an asbestos cloth bag, which is closed below and tied at
its upper part. It is provided with apertures which permit of the
ascent of the gases in the interior of the cylinder. The apparatus is
hermetically sealed at the top, the two electrodes being naturally
insulated with rubber. Above the level of the liquid the interior
electrode is continuous and forms a channel for the gas. The hydrogen
and oxygen, escaping through the upper orifices, flow to the
compensator. The apparatus is provided with an emptying cock or a cock
for filling with distilled water, coming from a reservoir situated
above the apparatus.
[Illustration: FIG. 4.--DETAILS OF AN INDUSTRIAL VOLTAMETER.]
The constants of the voltameter established by Commandant Renard are
as follows:
Height of external electrode 3.405 m.
" internal " 3.290 "
Diameter of external " 0.300 "
" internal " 0.174 "
The iron plate employed is 2 millimeters in thickness. The electric
resistance is about 0.0075 ohm. The apparatus gives 365 amperes under
2.7 volts, and consequently nearly 1 kilowatt. Its production in
hydrogen is 158 liters per hour.
It is clear that, in an industrial exploitation, a dynamo working
under 3 volts is never employed. In order to properly utilize the
power of the dynamo, several voltameters will be put in series--a
dozen, for example, if the generating machine is in proximity to the
apparatus, or a larger number if the voltameters are actuated by a
dynamo situated at a distance, say in the vicinity of a waterfall.
Fig. 3 will give an idea of a plant for the electrolysis of water.
It remains for us to say a few words as to the net cost of the
hydrogen and oxygen gases produced by the process that we have just
described. We may estimate the value of a voltameter at a hundred
francs. If the apparatus operates without appreciable wear, the
amortizement should be calculated at a very low figure, say 10 per
cent., which is large. In continuous operation it would produce more
than 1,500 cubic meters of gas a year, say a little less than one
centime per cubic meter. The caustic soda is constantly recuperated
and is never destroyed. The sole product that disappears is the
distilled water. Now one cubic meter of water produces more than 2,000
cubic meters of gas. The expense in water, then, does not amount to a
centime per cubic meter. The great factor of the expense resides in
the electric energy. The cost of surveillance will be minimum and the
general expenses _ad libitum_.
Let us take the case in which the energy has to be borrowed from a
steam engine. Supposing very small losses in the dynamo and piping, we
may count upon a production of one cubic meter of hydrogen and 500
cubic decimeters of oxygen for 10 horse-power taken upon the main
shaft, say an expenditure of 10 kilogrammes of coal or of about 25
centimes--a little more in Paris, and less in coal districts. If,
consequently, we fix the price of the cubic meter of gas at 50
centimes, we shall preserve a sufficient margin. In localities where a
natural motive power is at our disposal, this estimate will have to be
greatly reduced. We may, therefore, expect to see hydrogen and oxygen
take an important place in ordinary usages. From the standpoint alone
of preservation of fuel, that is to say, of potential energy upon the
earth, this new conquest of electricity is very pleasing. Waterfalls
furnish utilizable energy in every locality, and, in the future, will
perhaps console our great-grandchildren for the unsparing waste that
we are making of coal.--_La Nature._
* * * * *
[Continued from SUPPLEMENT, No. 818, page 13066.]