« НазадПродовжити »
any one set. By properly combining dry cells in this manner batteries of any desired current strength may be obtained.
The terms “volt” and “ampere” are merely units by which current strength is gauged. The "volt” is the unit of pressure or potential which exists between the terminals of a circuit. The “ampere” measures current quantity or flow, and is independent of the pressure.
One may have a current of high amperage at low potential or one having great pressure and but little amperage or current strength. Voltage is necessary to overcome resistance, while the amperage available determines the heating value of the current. As the resistance to current flow increases, the voltage must be augmented proportionately to overcome it. A current having the strength of one ampere with a pressure of one volt is said to have a value of one watt, which is the unit by which the capacity of generators and the amount of current consumption is gauged.
One of the disadvantages of primary cells, as those types which utilize zinc as an active element are called, is that the chemical action produces deterioration and waste of material by oxidization. Dry cells are usually proportioned so that the electrolyte and depolarizing materials become weaker as the zinc is used, and when a dry cell is exhausted it is not profitable to attempt to recharge it because new ones can be obtained at a lower cost than the expense of renewing the worn elements would be.
Principles of Storage Battery Construction.—Some voltaic couples are reversible, i. e., they may be recharged when they have become exhausted by passing a current of electricity through them in a direction opposite to that in which the current flows on discharge. Such batteries are known as “accumulators” or “storage batteries.” A storage battery belies its name, as it does not really store current, and its action is somewhat similar to that of the simpler chemical cell previously described. In its simplest form a storage cell would consist of two elements and an electrolyte, as outlined at Fig. 1, D. The storage battery differs from the primary cell in that the elements are composed of the same metal before charging takes place, usually lead, instead of being zinc or carbon. One of the plates is termed the "positive," and may be distinguished from the other because it is brown or chocolate in color after charging, while the negative plate is usually a light gray or leaden color. The active material of a charged storage battery is not metallic lead, but oxides of that material.
The simple form shown at Fig. 1, C, consists of two plates of lead, which are rolled together, separated by insulating bands of rubber at the top and bottom to keep them from touching. This roll is immersed in an electrolyte composed of a weak solution of sulphuric acid in water. Before such a cell can be used it must be charged, which consists of passing a current of electricity through it until the lead plates have changed their nature. After the charging process is complete the lead plates have become so changed in nature that they may be considered as different substances, and a chemical action results between the oxidized plate and the electrolyte and produces current, just as in the simple cell shown at Fig. 1, A. When the cell is exhausted the plates return to their discharged condition and are practically the same, and as there is but little difference in electrical condition existing between them, they do not deliver any current until electricity has been passed through the cell so as to change the sulphate on the positive lead, plates to peroxide of lead.
The changes that occur in the plates of a storage battery when a current is passed through them and the electrolyte in which they are placed results in altering their nature to such an extent that they act just the same as two elements of more widely differing nature, such as zinc and copper or zinc and carbon might. Primary cell action depends on the wasting of one of the elements, and the only way that more current can be obtained when the cell is exhausted is by replenishing the electrolyte and also putting in a new plate of metal in place of that eaten away by the acid.
Discovery of Reversible Chemical Action.— The basic principle on which storage battery action is based has been known for over a century, as a French scientist, Gautherot, while experimenting with the electrolysis or decomposition of water in 1801 by passing an electric current through it discovered that the silver or platinum wires employed as electrodes for this purpose would send a current of electricity back through a circuit when the battery that furnished the decomposing current was removed from them. The electrical flow from the wires was in a reverse direction to that passed through by the main battery, and was naturally very weak. Further studies by other scientists, notably De la Rue, Ritter and Grove, led to the development of a gas battery which was really a variation of the apparatus used in the electrolysis of water. Iri 1834 Farraday made experiments with lead electrodes that resulted in the development of storage battery forms from which those used to-day are patterned. Following the experiments of Sinsteden, who used plates of nickel, silver and lead in a voltampmeter in 1854 and obtained reverse currents from that instrument of sufficient power to raise a wire to red heat after it had been used in measuring a source of electricity, Gaston Planté, who was familiar with these experiments, did the experimental work that formed the basis for the practical storage battery of to-day.
The early Planté battery consisted of two sheets of lead, which were separated from each other by canvas and immersed in a sulphuric acid and water electrolyte. After sending a current of electricity through from primary cells of the Bunsen type, a much more powerful current was obtained from the lead plates than either Farraday, with his lead peroxide element, or Sinsteden, with his combination of metals, had secured, and as a result he is generally credited as being the inventor of the storage battery. After considerable experimenting Planté found that it was possible to increase the current output of a secondary battery by repeated charges and discharges. It was learned that the capacity augmented with use, and that the lead plate surfaces were changed into lead sulphate and lead oxide, and that the coatings penetrated deeper into the plate with each added charge and discharge.
Inasmuch as the knowledge was available to men of science even at that early day (1860) that the action of electric current on lead plates would cause a chemical change that would in turn result in a reverse current flow, the only drawback to making the storage battery of practical value was the lack of economical means for charging the plates. The dynamo had not been perfected to the point that it reached a few years later, so the only
current means available for "forming" or preparing the lead plates was by the action of expensive primary batteries. The first chemical changes that took place were of very limited depth, and even after long service the chemical action of changed material could hardly be measured. It was only by frequent and repeated charges and discharges, extending over months of time, that it was possible to obtain cells with sufficient capacity to be of practical value. Even though laboring under these disadvantages, Planté was able to make numerous cells of this kind and perform laboratory experiments which created great interest among the scientists of that time.
The chemical change which results in the production of electricity in lead plate batteries is rather complicated, and its exact nature is not definitely known even to-day. It was advanced by Planté that the charging action, or rather chemical change when current was passed through the cell from an outside source, resulted in the formation of lead peroxide (Pb0,) on the positive plate and metallic lead on the negative. Both of these were said to be converted into lead oxide (Pb0) when the current was drawn from the battery. Later investigations showed that the formation of lead sulphate also was of enough consequence to be taken into consideration. This reaction on discharge is probably about as follows:
Charged Condition Becomes Discharged Condition.
+ Therefore, during charging, the plates must be brought to their original state and the sulphate driven out of the plates into the electrolyte, as expressed chemically in the following:
When Discharged. After Charging.
Negative Plate, PbSO4 + 2H=Pb + H2SO4. Considering the equations previously outlined, it will be evident that the active material on both plates of a storage battery is changed into lead sulphate when the battery discharges. There