Electricity

A current of electricity has the power of decomposing certain liquids. If we pass a current through water, the water is split up into its two constituent gases, hydrogen and oxygen, and this may be shown by the apparatus seen in Fig. 12. It consists of a glass vessel with two strips of platinum to which the current is led. The vessel contains water to which has been added a little sulphuric acid to increase its conducting power, and over the strips are inverted two test-tubes filled with the acidulated water. The platinum strips, which are called electrodes, are connected to a battery of Daniell cells. When the current passes, the water is decomposed, and oxygen collects at the electrode connected to the positive terminal of the battery, and hydrogen at the other electrode. The two gases rise up into the test-tubes and displace the water in them, and the whole process is called the electrolysis of water. If now we disconnect the battery and join the two electrodes by a wire, we find that a current flows from the apparatus as from a voltaic cell, but in the opposite direction from the original battery current.

It will be remembered that one of the troubles with a simple voltaic cell was polarization, caused by the accumulation of hydrogen; and that this weakened the current by setting up an opposing electro-motive force tending to produce another current in the opposite direction. In the present case a similar opposing or back electro-motive force is produced, and as soon as the battery current is stopped and the electrodes are connected, we get a current in the reverse direction, and this current continues to flow until the two gases have recombined, and the electrodes have regained their original condition. Consequently we can see that in order to electrolyze water, our battery must have an electro-motive force greater than that set up in opposition to it, and at least two Daniell cells are required.

This apparatus thus may be made to serve to some extent as an accumulator or storage cell, and it also serves to show that an accumulator does not store up or accumulate electricity. In a voltaic cell we have chemical energy converted into electrical energy, and here we have first electrical energy converted into chemical energy, and then the chemical energy converted back again into electrical energy. This is a rough-and-ready way of putting the matter, but it is good enough for practical purposes, and at any rate it makes it quite clear that what an accumulator really stores up is not electricity, but energy, which is given out in the form of electricity.

The apparatus just described is of little use as a source of current, and the first really practical accumulator was made in 1878 by Gaston Planté. The electrodes were two strips of sheet lead placed one upon the other, but separated by some insulating material, and made into a roll. This roll was placed in dilute sulphuric acid, and one strip or plate connected to the positive, and the other to the negative terminal of the source of current. The current was passed for a certain length of time, and then the accumulator partly discharged; after which current was passed again, but in the reverse direction, followed by another period of discharge. This process, which is called forming, was continued for several days, and its effect was to change one plate into a spongy condition, and to form a coating of peroxide of lead on the other. When the plates were properly formed the accumulator was ready to be fully charged and put into use. The effect of charging was to rob one plate of its oxygen, and to transfer this oxygen to the other plate, which thus received an overcharge of the gas. During the discharge of the accumulator the excess of oxygen went back to the place from which it had been taken, and the current continued until the surfaces of both plates were reduced to a chemically inactive state. The accumulator could be charged and discharged over and over again as long as the plates remained in good order.

In 1881, Faure hit upon the idea of coating the plates with a paste of red-lead, and this greatly shortened the time of forming. At first it was found difficult to make the paste stick to the plates, but this trouble was got rid of by making the plates in the form of grids, and pressing the paste into the perforations. Many further improvements have been made from time to time, but instead of tracing these we will go on at once to the description of a present-day accumulator. There are now many excellent accumulators made, but we have not space to consider more than one, and we will select that known as the “Chloride” accumulator.

The positive plate of this accumulator is of the Planté type, but it is not simply a casting of pure lead, but is made by a building-up process which allows of the use of a lead-antimony mixture for the grids. This gives greater strength, and the grids themselves are unaffected by the chemical changes which take place during the charging and discharging of the cell. The active material, that is the material which undergoes chemical change, is pure lead tape coiled up into rosettes, which are so designed that the acid can circulate through the plates. These rosettes are driven into the perforations of the grid by a hydraulic press, and during the process of forming they expand and thus become very firmly fixed. The negative plate has a frame made in two parts, which are riveted together after the insertion of the active material, which is thus contained in a number of small cages. The plate is covered outside with a finely perforated sheet of lead, which prevents the active material from falling out. It is of the utmost importance that the positive and negative plates should be kept apart when in the cell, and in the Chloride accumulator this is ensured by the use of a patent separator made of a thin sheet of wood the size of the plates. Before being used the wood undergoes a special treatment to remove all substances which might be harmful, and it then remains unchanged either in appearance or composition. Other insulating substances, such as glass rods or ebonite forks, can be used as separators, but it is claimed that the wood separator is not only more satisfactory, but that in some unexplained way it actually helps to keep up the capacity of the cell. The plates are placed in glass, or lead-lined wood or metal boxes, and are suspended from above the dilute sulphuric acid with which the cells are filled. A space is left below the plates for the sediment which accumulates during the working of the cell.

In all but the smallest cells several pairs of plates are used, all the positive plates being connected together and all the negative plates. This gives the same effect as two very large plates, on the principle of connecting in parallel, spoken of in Chapter IV. A single cell, of whatever size, gives current at about two volts, and to get higher voltages many cells are connected in series, as with primary cells. The capacity is generally measured in ampere-hours. For instance, an accumulator that will give a current of eight amperes for one hour, or of four amperes for two hours, or one ampere for eight hours, is said to have a capacity of eight ampere-hours.

Accumulators are usually charged from a dynamo or from the public mains, and the electro-motive force of the charging current must be not less than 2½ volts for each cell, in order to overcome the back electro-motive force of the cells themselves. It is possible to charge accumulators from primary cells, but except on a very small scale the process is comparatively expensive. Non-polarizing cells, such as the Daniell, must be used for this purpose.

The practical applications of accumulators are almost innumerable, and year by year they increase. As the most important of these are connected with the use of electricity for power and light, it will be more convenient to speak of them in the chapters dealing with this subject. Minor uses of accumulators will be referred to briefly from time to time in other chapters.