562 



THE POPULAR SCIENCE MONTHLY. 



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 per- 

 son 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, 



Fig. 5. 



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 b b over the flange. This is the vibrator or 

 diaphragm. Next cut a wooden ring or "washer," dd, 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 cc. After joining the ends 

 of the spool-wire to the line-wires ff, 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. 



