Page:Popular Science Monthly Volume 12.djvu/580

562 the diaphragm near by, and thus the original talker gets his response. The action set up in one telephone is instantaneously repeated or copied by the other. And so by this marvelous chain of effects a person discoursing, apparently, to a piece of iron, may be intelligibly heard hundreds of miles away, and a conversation kept up between the distant parties as if they were in the same room. The "ear-shot" from the beginning of the world has been but a few hundred feet; by the invention of this little contrivance it is now extended to hundreds of miles.

So much for the theory of the telephone; let us now note how it may be practically constructed. To make one, procure a tube, bb (Fig. 5), of thin sheet-brass, one inch long, two inches in diameter,



and with a flange one-half inch wide. Then from a ferrotype-plate—the photographer's "tin-type"—cut a round plate, shown edgewise, at c c, to cover the tube bb over the flange. This is the vibrator or diaphragm. Next cut a wooden ring or "washer," d d, the width of the flange, and about one-eighth of an inch thick. Then make a spool, e e, one inch long, of thin sheet-brass again, with one flange wide enough to cover the wooden ring, the tube of the spool being made so as to fit tightly the magnet g, which is a strongly-magnetized steel rod, four inches long and three-eighths of an inch in diameter. The parts will then fit together, and may be screwed firmly through the flanges b b. The least polished side of the plate c c should face the magnet, and it is well to scrape the part opposite the end of the bar, so as to expose the iron. The spool is to be wound with about fifty yards of No. 36 or 38 silk-covered copper wire, the thickness of a bristle. The magnet is then shoved in, till it nearly touches the plate c c. After joining the ends of the spool-wire to the line-wires f f, that run to another instrument just like it, the telephone will be ready for use. It is important to concentrate the voice upon a narrow space at the middle of the plate, and for this purpose a movable wooden mouth-piece, a a, is used, with an opening at the bottom about the size of a dime. This mouth-piece should fit neatly, and reach to within about one-eighth of an inch of the diaphragm.