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.