Special attention is called to the dictionary of electrical terms given in the Appendix. The young student should never pass over a word or a term that he does not thoroughly understand. Always look it up at once and _every time_ it occurs, until you are sure that its meaning is fixed in your mind. This is an education in itself, at least so far as the theoretical knowledge of our subject is concerned. As a final word, I should like every boy interested in electricity to hear what Thomas A. Edison once said to me when I was a boy working in his laboratories. I often recall it when things do not go just right at first. I asked the great inventor one day if invention was not made up largely of inspiration. He looked at me quizzically for a moment, and then replied: “My boy, I have little use for a man who works on inspiration. Invention is two parts inspiration and ninety-eight per cent. perspiration.” You will never get what you are after unless you work hard for it. You must stick to it until you produce results. If the history of the world’s most valuable inventions could be fully known, the fact would be clearly established that the vital spark of inspiration is but the starting-point. Then follow the days, weeks, and sometimes years of industrious toil, failures, and disappointments, until finally the desired end is attained. One must work for success; there is no other means of winning it. As the table of contents shows, Part I. of this book explains principles and the simpler forms of electrical appliances. From this we advance to Part II., which deals with more complex forms of electrical work, most of which, however, are within the reach of intelligent boys who have followed the chapters carefully from the first. In a final chapter we have simple explanations of the great commercial uses of electricity, which we see all about us, although very few of us have a clear idea as to their operation. Chapter II CELLS AND BATTERIES Simple Cells In order to generate electricity it is necessary to employ cells, batteries, or dynamos. Since the construction and operation of a dynamo is somewhat intricate, it will be better to start with the simpler methods of electric generation, and so work up to the more complicated forms. For small apparatus, such as electric bells and light magnets and motors, the zinc-carbon-sal-ammoniac cell will answer very well; but for larger machinery, where more current is required, the bluestone and the bi-chromate batteries will be found necessary. [Illustration: FIG. 1 FIG. 2 FIG. 3 FIG. 5 SIMPLE BATTERY ELEMENTS]