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.

CHAPTER VI
MAGNETS AND MAGNETISM

In many parts of the world there is to be found a kind of iron ore, some specimens of which have the peculiar power of attracting iron, and of turning to the north if suspended freely. This is called the lodestone, and it has been known from very remote times. The name Magnetism has been given to this strange property of the lodestone, but the origin of the name is not definitely known. There is an old story about a shepherd named Magnes, who lived in Phrygia in Asia Minor. One day, while tending his sheep on Mount Ida, he happened to touch a dark coloured rock with the iron end of his crook, and he was astonished and alarmed to find that the rock was apparently alive, for it gripped his crook so firmly that he could not pull it away. This rock is said to have been a mass of lodestone, and some people believe that the name magnet comes from the shepherd Magnes. Others think that the name is derived from Magnesia, in Asia Minor, where the lodestone was found in large quantities; while a third theory finds the origin in the Latin word magnus, heavy, on account of the heavy nature of the lodestone. The word lodestone itself comes from the Saxon laeden, meaning to lead.

It is fairly certain that the Chinese knew of the lodestone long before Greek and Roman times, and according to ancient Chinese records this knowledge extends as far back as 2600 B.C. Humboldt, in his Cosmos, states that a miniature figure of a man which always turned to the south was used by the Chinese to guide their caravans across the plains of Tartary as early as 1000 B.C. The ancient Greek and Roman writers frequently refer to the lodestone. Thales, of whom we spoke in [Chapter I]., believed that its mysterious power was due to the possession of a soul, and the Roman poet Claudian imagined that iron was a food for which the lodestone was hungry. Our limited space will not allow of an account of the many curious speculations to which the lodestone has given rise, but the following suggestion of one Famianus Strada, quoted from Houston’s Electricity in Every-Day Life, is really too good to be omitted.

“Let there be two needles provided of an equal Length and Bigness, being both of them touched by the same lodestone; let the Letters of the Alphabet be placed on the Circles on which they are moved, as the Points of the Compass under the needle of the Mariner’s Chart. Let the Friend that is to travel take one of these with him, first agreeing upon the Days and Hours wherein they should confer together; at which times, if one of them move the Needle, the other Needle, by Sympathy, will move unto the same letter in the other instantly, though they are never so far distant; and thus, by several Motions of the Needle to the Letters, they may easily make up any Words or Sense which they have a mind to express.” This is wireless telegraphy in good earnest!

The lodestone is a natural magnet. If we rub a piece of steel with a lodestone we find that it acquires the same properties as the latter, and in this way we are able to make any number of magnets, for the lodestone does not lose any of its own magnetism in the process. Such magnets are called artificial magnets. Iron is easier to magnetize than steel, but it soon loses its magnetism, whereas steel retains it; and the harder the steel the better it keeps its magnetism. Artificial magnets, therefore, are made of specially hardened steel. In this chapter we shall refer only to steel magnets, as they are much more convenient to use than the lodestone, but it should be remembered that both act in exactly the same way. We will suppose that we have a pair of bar magnets, and a horse-shoe magnet, as shown in [Fig. 13].