He reasoned in this way: From the nature of the magnet it follows that when waves of electricity are passed through the wire coil around the magnet, the strength of the magnet must vary with the force of the electric impulses. Its pull on the plate of iron near it must vary in the same manner. The varying pull on the plate must make it move, and this movement must set the air against the plate in motion in sound-waves corresponding exactly with the motion setting up the electric waves in the first place; in other words, the sound-motion in one telephone must be exactly reproduced as sound-waves in a similar instrument joined to it by wire. Experiment proved the reasoning correct; and thus the speaking-telephone was invented. But it took a long time to find the simplest and best way to make it. At last, however, Mr. Bell's telephone was perfected in the form illustrated below. Fig. 1 shows the inner structure of the instrument. A is the spool carrying the coil of wire; B, the magnet; C, the diaphragm; E, the case; F, F, the wires leading from the coil, and connecting at the end of the handle with the ground and line wires. Fig. 2 shows how a telephone looks on the outside. [Illustration: BELL'S TELEPHONE. Fig. 1 and Fig. 2] So much for description. You will understand it better, perhaps, if you experiment a little. You can easily make a pair for yourself, rude and imperfect, it is true, but good enough for all the tests you may want to apply. For each you will want: (1) a straight magnet; (2) a coil of silk-covered copper wire; (3) a thin plate of soft iron; (4) a box to hold the first three articles. You will also want as much wire as you can afford, to connect the instruments, and two short pieces of wire to connect your telephones with the ground. (Two wires between the instruments would make the ground-wires unnecessary, but this would use up too much wire.) The magnet and the coil you will have to buy from some dealer in electrical apparatus. They need not cost much. A small cigar-box will answer for the case. [Illustration: Fig. 3. A "CIGAR-BOX" TELEPHONE.] In one end of the box cut a round hole, say, three inches across. Against this hole fasten a disk of thin sheet-iron for the vibrator or "diaphragm." For a mouth-piece use a small can, such as ground spices come in, or even a round paper box. Now, on the inside of the box, place the magnet, the end carrying the coil almost touching the middle of the diaphragm, and fix it firmly. Then, to the ends of the copper wire of the main coil fasten two wires,--one for the line, the other for the "ground-wire." This done, you will have an instrument (or rather two of them) very much like Fig. 3. A is the mouth-piece; B, the diaphragm; C, the coil; D, the magnet; E, E, the wires. The receiving and sending instruments are precisely alike, each answers for both purposes; but there must be two, since one must always be hearing while the other is speaking.