The coil machine is not only useful in a medical point of view, but when properly arranged offers a good reception to a run-away bellringer, and is an excellent preventive against illicit attempts at cheap rides by small boys. [Illustration: Fig. 233. Boy, _evidently shocked_, behind doctor's carriage provided with a small coil machine.] [Page 241] CHAPTER XIX. MAGNETO-ELECTRICITY. [Illustration: Fig. 234. Clarke's magneto-electrical machine.] The correlation of the physical forces, heat, light, electricity, magnetism, and motion, is one of the most interesting subjects for study that can be suggested to the lover of science. The examination of the precise meaning of the term correlation, so ably considered by Professor Grove, indicates a necessary mutual or reciprocal dependence of one force on the other. Thus, electricity will produce heat, and _vice versâ_; motion, such as friction, produces electricity, and the latter, by its attraction and repulsion, establishes itself as a source of motion. Electricity produces light, also magnetism, and contrariwise light is said to possess [Page 242] the power of magnetizing steel, whilst magnetism again produces light and electricity. Such are the intimate connexions that exist between these imponderable agents, and we may trace cause and effect and its reversal amongst these forces, until the mind is lost in the examination of the bewildering mazes, and is content to return to the beaten track and work out experimentally the practical truths. We have had occasion to notice in another part of this playbook the fact that a current of electricity causes the evolution of magnetism in its passage through various conducting media, and the truth has been specially illustrated by the various experiments in the chapter devoted to electro-magnetism. In commencing this portion of electrical science, we have no new terms to coin for the title of the discourse, as we merely reverse the other when we examine the nature and peculiarities of MAGNETO-ELECTRICITY. The source of the power must necessarily be a bar or horse-shoe shaped piece of steel permanently endowed with magnetism. If the former is thrust into a cylinder of wood or pasteboard, around which coils of covered copper wire have been carefully wound, so that the extremities communicate with a galvanometer, an immediate deflection of the needle occurs, which, however, quickly returns to its first position, but is again deflected in the opposite direction on the withdrawal of the steel magnet from the coil of copper wire. (Fig. 235.) [Illustration: Fig. 235. A B. Coil of copper wire. C. Permanent bar magnet placed inside the coil, when the galvanometer needle, D, is deflected.]