Rather than describe a machine which comparatively few of my readers would be able to build, I have explained below how it is possible to so alter an old telephone magneto that it may be made to serve as a small dynamo. Telephone magnetos, also sometimes called hand generators, are used in many telephone systems to supply the current which rings the telephone bell at the other end. The magneto is placed in a small box on the telephone, only the handle being exposed. In order to make a call the handle is given several brisk turns before raising the receiver. When the handle is turned the moving parts of the generator revolve and produce a current of electricity which goes forth over the line and rings the bell at the other end. [Illustration: Fig. 251—A Telephone Magneto.] Telephone magnetos are gradually being discarded in all the large telephone systems, a method known as "central energy," in which the current for ringing bells is supplied from the central office, taking their place. For that reason, there are a great many telephone magnetos to be found in second-hand shops and at electrical houses, where they can be purchased for a fractional part of the original cost. Fifty cents will buy a first-class second-hand telephone magneto. The author saw a pile of telephones as large as a haystack, each telephone containing a magneto, in the back yard of a second-hand shop, and the owner would have been glad to sell the complete instruments for fifty cents each. Before explaining how to reconstruct such a machine, it is best to impress upon the reader that a careful study of the principles of the dynamo is well worth the time spent. Almost any book on physics or electricity, or even the encyclopedia, will be found to contain a description of this wonderful machine that supplies the power for running the trolley cars, electric lights, etc., in fact all of the electricity in use to-day with the exception of that generated by batteries for telegraph and telephone lines. It will be remembered that if a bar magnet is suddenly plunged into a hollow coil of wire, a momentary electric current will be generated in the coil. The current is easily detected by means of an instrument called a galvanometer. The space in the vicinity of a magnet is filled with a peculiar invisible force called magnetism. The magnetism flows along a certain path, passing through the magnet itself and then spreading out in curved lines. If a sheet of paper is laid over a magnet and a few iron filings are sprinkled on the paper, they will follow the magnetic lines of force.