To the boy interested in science, a wide field is open. There is no better education for any boy than to begin at the bottom of the ladder and climb the rungs of scientific knowledge, step by step. It equips him with information which may prove of inestimable worth in an opportune moment. There is an apt illustration in the boy who watched his mother empty a jug of molasses into a bowl and replace the cork. His mother told him not to disturb the jug, as it was empty. He persisted, however, and turned the jug upside down. No more molasses came, but _out crawled a fly_. New developments in science will never cease. Invention will follow invention. The unexpected is often a valuable clue. The Edisons and Teslas of to-day have not discovered everything. _There is a fly in the molasses_, to be had by persistence. Inspiration is but a starting-point. Success means work, days, nights, weeks, and years. There can be no boy who will follow exactly any directions given to him, or do exactly as he is told, of his own free will. He will "bolt" at the first opportunity. If forced or obliged to do as he is directed, his action will be accompanied by many a "why?" Therefore in presenting the following chapters I have not only told how to _make_ the various motors, telegraphs, telephones, radio receivers, etc. but have also explained the principles of electricity upon which they depend for their operation, and how the same thing is accomplished in the every-day world. In giving directions or offering cautions, I have usually stated the reason for so doing, in the hope that this information may be a stimulant to the imagination of the young experimenter and a useful guide in enabling him to proceed along some of the strange roads on which he will surely go. ALFRED P. MORGAN UPPER MONTCLAIR, N. J.