Popular Science Monthly/Volume 38/January 1891/The Storage of Electricity
|THE STORAGE OF ELECTRICITY.|
By SAMUEL SHELDON, Ph. D.,
PROFESSOR OF PHYSICS IN THE POLYTECHNIC INSTITUTE OF BROOKLYN.
THE problem how to save and store up the enormous amount of natural energy which is daily dissipated in producing natural phenomena has long occupied the attention of scientists. During the last fifteen years this attention has been especially directed toward electricity as an agent. This is, perhaps, because the majority of the really active investigators have been occupied in this department of science, or perhaps the popular superstitious credulity that electricity can be made to do anything, has, to a certain extent, taken possession of the scientific mind. At any rate, the result of experiments has been the development of the electrical storage batteries, or accumulators, as they are sometimes called.
The employment of these names for the apparatus is very unfortunate. They are the cause of the popular idea that electricity, which is considered as a subtle, indefinite, and intangible something, is stored up in them, as valuables are stored in a vault. The commercial current electricity can not, in large quantities, be stored and still preserve its character. It has but a flitting existence, and is no sooner produced than it dissipates itself and is converted into some other form of energy. It was because of this momentary existence that science had to wait so long for an accident to reveal to Galvani that such a thing could exist.
The energy which a current may at any instant be said to possess is immediately transformed into heat in the circuit, which will under certain conditions produce light; into chemical energy; into motion, which may or may not produce sound; or into magnetic and electrotonic conditions. The last may either be permanent or have the same evanescent existence as the original current.
When electricity is employed to charge a storage battery, only that part which is transformed into chemical energy is used. The rest is dissipated. The battery, then, instead of being a place where electricity is laid away, is a place where chemicals are left by the current, with the expectation that they will in turn produce a current when called upon. This may seem a fine distinction, but it is only apparently so. For instance, the current might be produced by a dynamo turned by Niagara water-power. The chemical left by it might be zinc deposited from a solution of zinc sulphate. This might be transported, preserved, bought and sold, and finally be employed by some physicist to produce another current. Were the electricity itself stored in its original form, then the imaginative reader can best tell what would become of it and how it must be handled.
To understand this transformation more clearly, and to obtain a clear idea of what goes on in a storage battery, one must first become acquainted with that part of electricity which treats of the phenomena resulting when a current of electricity passes through a liquid. This is called electrolysis, and the liquid through which a current can be made to pass is called an electrolyte.
If a current of electricity flows into a liquid solution of any metallic salt by means of a wire, and if, after traversing it, it flows out through another wire, then it will, by its passage, separate the salt into two parts and deposit the metal upon the latter wire.
If, for instance, the solution be one of silver cyanide, then silver will be deposited on the second wire. If a brass fork be connected with this wire and dipped in the solution, then it will receive a coating of silver by the process and will be silver-plated. Substitute a solution of nickel nitrate, and the article would become nickel-plated. By using copper sulphate we are enabled to cover the faces of types and cuts with a coating of copper, which increases their hardness and consequently their endurance.
This electrolytic action can be watched if a solution of tin chloride be used. Tin, instead of being deposited, like most other metals, in fine particles, comes out of the solution in quite large crystals. If the current of electricity be made to enter the solution through two wires, placed symmetrically on opposite sides of the wire through which it makes its exit, and the whole is performed in a vessel with glass sides, then, as the current passes, the crystals will appear, as if by magic, growing out around the central wire. This is but a modification of the "lead tree" which appears in many text-books on physics. The tin crystals, however, are much larger and more beautiful than those of lead.
The simplest storage battery, then, would seem to be one constructed of two copper plates suspended in a solution of some zinc salt. A current of electricity passed into this would deposit zinc upon one of the plates. After disconnecting the charging current, the battery of itself would give off a current until the zinc was redissolved. In fact, a modification of this form of storage battery has recently been placed upon the market. The question arises, however, whether it is cheaper to buy zinc sulphate and transform it by expensive horse-power into metallic zinc or to buy metallic zinc directly. Of course, in neither case is the zinc lost, for it can be recovered by chemical means from the solution. If solutions of zinc were abundant in nature and hence inexpensive, this style of storage battery would, undoubtedly, for economic reasons, prevail. Or, still further, if metallic zinc were inexpensive we would have no need of storage batteries at all, but could use primary batteries directly.It might be well, right here, to define a primary battery. If any two different metals be dipped in an acidulated liquid, and if their external extremities be connected by a wire, a current of electricity will flow through the wire. Such a combination is called a primary battery. Under the same conditions the amount of electricity obtained depends upon the character of. the metals. If nickel and iron were employed, a small amount of electricity would result. If, however, zinc be used in connection with either silver, gold, platinum, carbon, or copper, a large amount is obtained. The first three of the group are very expensive; hence, in most primary batteries, we find zinc combined with either carbon or copper, the differences between the various forms arising from difference in the liquids employed or in the shape of construction.
Furthermore, pieces of the same metal under different physical conditions, when combined with each other, will give a current. For instance, a piece of polished iron opposed to a rusty piece gives a current, and a plate of very rusty lead, if I may use the expression, combined with a piece of bright lead yields even more current than zinc and carbon. Unfortunately, lead does not rust sufficiently well to suit electricians, and other physical reasons prevent its being used in primary batteries.
It will thus be seen that a storage battery, when once charged, becomes nothing more or less than a primary battery. In the case before described, after charging, we have zinc and copper in a solution of zinc sulphate.
In describing the effects of electricity in passing through an electrolyte, we have assumed that the liquid contained a metal in solution. Suppose, now, that we take water, which has no metal in it, and subject it to the action of the current. The electricity can get no metal to deposit on the wire, where it passes out, and in consequence does the next "best thing and leaves one of the components of the water—viz., hydrogen gas. The other component, oxygen, collects around the entrance wire. The English physicist Grove showed that, if these two wires, around which the gases had collected, were connected together, a current of electricity would flow the same as if there were two metals instead of two gases. Now, water is cheap, and if there were not some serious technical difficulties as regards efficiency, Grove's battery would be universally employed.
It was reserved, however, for M. Gaston Planté to construct the first practical secondary battery. He considered the following points in its construction: Water is cheap; water, when subjected to the electric current, gives off oxygen and hydrogen; rusty lead, when combined with bright lead, has a high electromotive force; oxygen makes lead rusty and hydrogen makes it bright. His battery consisted, then, of two lead plates suspended in water, which contained a little sulphuric acid to assist in the conduction. When a current of electricity was passed through, hydrogen was thrown off at one plate, making it bright, and oxygen at the other plate, peroxidizing its surface. When the charging source was removed, the altered plates would send off a current, which was in a direction opposite to the one which had charged them, and this would keep up until the plates had assumed their original condition.
Planté's choice of materials was most wise, and all practical storage batteries of to-day are but modifications of his style.
In order that his battery might give a strong current, and one that would last a long time, it was found necessary that his two lead plates should be as near to each other, and that they should be as large as possible. He accomplished both of these ends with economy of space by winding large plates into a spiral form, they being separated from each other by strips of rubber. In charging this battery, care must be exercised that the current be not too strong; otherwise the gases would be sent off too rapidly for the lead to take them up, and they would then rise to the top of the liquid and escape into the air. The electrical energy which separated them would thus be lost. It accordingly takes a long time to charge a new Planté battery to its full capacity. After being subjected to the current for a day or two, if the plates be removed and examined, it will be found that the one which received the oxygen has changed its physical character: instead of having a smooth surface, it presents a spongy appearance, having little holes and cavities in it, and thus exposes a larger superficial area.
If the battery be now discharged, and be again subjected to the charging current, it will be found that a much stronger current may be used than at first, without any gas escaping. This is owing to the much larger surface exposed and to the spongy character of it.
This original charging of a new battery, to change the character of the lead surfaces, has been termed formation, and, inasmuch as only one plate is altered by a charge in one direction, a complete formation consists in a charging in two directions.
As the process of electrical formation is necessarily an expensive one, it was thought that the same end could be attained by mechanical means. Planté himself suspended the lead plates, for a few days, in strong nitric acid. The acid does not attack the lead, but seems to dissolve out small impurities, which are distributed throughout the metal, leaving it in a much more porous condition than after electrical formation.
Others cut the plates into fine fringes, thus exposing a large surface with a small weight of lead.
D'Arsonval, instead of using plates, employed lead shot, thinking to get the largest surface for the given weight. The particles could be effective, however, only under the condition that they were in good contact with the wires leading to the battery. After becoming oxidized, a large proportion of the shot did not satisfy this condition, and the method was abandoned.
Lead wire was then substituted for the shot, and was found very efficient. Lead wire, however, is very expensive; and, to obviate this, Simmen invented a very ingenious and economical process of manufacturing it. This consists in pouring molten lead into heated iron boxes, the bottoms of which are perforated with suitable-sized holes. The metal flows through these holes, and is suddenly cooled by dropping into cold water. The wire thus manufactured does not possess the same regular character as drawn wire, but is perfectly suited to the purpose for which it was intended. The wire, after removal from the water, is compressed into sheets, which, under the microscope, resemble, in texture, coarse felt. Simmen placed pieces of this felt in frames of cast lead, which acted as supports and improved the electrical contact.
Reynier sought to increase the exposed surface by taking thin lead foil and forming it into accordion-plaits. The compressed plaits were then attached to supporting frames. When Reynier's battery was charged, an unexpected phenomenon presented itself. The lead, in taking up the oxygen, had increased its weight. At the same time it had been transformed into peroxide of lead, which is less dense than pure lead—i. e., a pound of it would occupy more space than a pound of the metal. The plaits, therefore, required more room, and in expanding they buckled the frames holding them. To obviate this, Reynier then cut a longitudinal opening in the plaits after they had been placed in the frame, and when the battery was charged this opening was closed by the expansion.
In all the styles of lead batteries mentioned, the oxide of lead on one plate and the spongy metallic lead on the other were formed from the lead of the electrodes themselves. Camille Faure, however, lessened the loss of time in formation by using lead plates as a support, and covering them with a paste made of some powdered oxide of lead mixed with sulphuric acid. This paste he kept in place by covering with sheets of felt. When the charging current was connected, the oxide on one plate was changed to a higher oxide, and on the other plate transformed into metallic sponge. This idea of Faure was an excellent one, and is at the foundation of the construction of all the commercial lead accumulators. The percentage of energy recovered by discharge was greatly increased. His method of keeping the paste in place by felts was, however, soon abandoned, because fine lead needles soon filled up the interstices of the felt, and thus made a metallic connection between the electrodes. Holes were then punched in the lead plates and the paste pressed into them. A large number of the patents recently issued for accumulators refer to methods of making these holes and pressing in the paste, or to the shape of the holes themselves after they have been punched. The shapes vary from a slight depression on the surface to a hole completely
|Reynier's Modified Plaits (uncharged).||Reynier's Modified Accumulator (charged).|
through the plate, and even further, to a hollow plate, with small openings leading to the surface. A great deal depends upon this shape, for the paste changes its volume during the process of charging and discharging, the same as the metallic lead does, and it would tend to loosen itself from some shaped openings and fall to the bottom of the cell, while in others it would tend to tighten itself, and thus provide a better contact.
Although the electrical end is obtained by substituting paste for metallic lead, yet this does not prevent the charging current from attacking the lead frames which hold the paste. These in time are rendered porous, and after a while they break under their own weight To avoid this, alloys and many secret composition
|Plate with Holes of Larger External Diameter.||Plate with Holes of Larger External Diameter (filled with paste).|
metals have been substituted for the lead frames. Even then, the continual change in volume of the paste contained in them twists and warps them so that new plates have to be substituted after a while. No good battery has yet been constructed which can be said to have a reasonably long life.
From what has been said it will be seen that the electricity which is used for charging an accumulator is apparently used in
|Plate with Holes of Larger Internal Diameter (filled with paste).||Modern Accumulator.|
the production of oxygen and hydrogen gases. These are made to oxidize one plate and clean up the other. Now, an interesting question arises, whether it would not be more economical to employ gases, which can be more cheaply produced through chemical means. Difficulties, however, arise here, for the oxygen of electrolysis is generated in the form of nascent oxygen, which is far more active than ordinary oxygen. A molecule of the ordinary gas contains two elementary atoms, which work upon each other; with the electrolytic generation, however, a single atom is sent off, and this is chemically very active. It is sometimes called ozone; but chemists say that a molecule of ozone contains three atoms. Now, there is no known method of chemically manufacturing ozone in large quantities, and ordinary oxygen does not produce the required effect.
Again, Planté's supposition, that the charging current produced these two gases only, is incorrect. The sulphuric acid in the water, which he supposed only assisted in the conduction, really acts upon the lead in forming lead sulphate. This has its use in preventing the charged battery from running down when not in use, and from too rapidly expending itself when put to use.
A more perfect system of storage batteries is much to be desired. Already electricity is a staple article, and has a market price of so many cents per ampère-hour. But its sale is of necessity confined to limited areas. As soon as these can be extended, by means of storage, an improvement in our commercial welfare will become apparent, and the fear arising from the predicted loss of our coal-supplies, will not trouble the minds of our immediate posterity.