THE AUTHOR"S TELEPHONE.
For several years past my regularly recurring duties have taken me over the various subjects treated of in this book, and each one has been extensively ill.u.s.trated in an experimental way, and a considerable number of new pieces of apparatus and new experiments to exhibit their phenomena have been devised by me.
Among these, I would mention the following:--
1. Measurement of the elongation of a magnetized bar.
2. A magneto-electric telegraph.
3. An electro-magnetic instrument for demonstrating the rotation of the earth.
4. The permanent magnetism of the magnetic phantom.
5. The convertibility of sound into electricity.
6. The induction of a vibrating magnet upon an electric circuit.
7. The origination of electric waves in a circuit by a sounding magnet.
8. The discovery of the action of the air in a sounding organ-pipe.
9. Two or three methods for studying the vibrations of membranes.
10. Lissajous forks for enlarged projections of sound vibrations.
As soon, therefore, as I gave attention to the subject of telephony, I was able, with a few preliminary experiments, to determine the proper conditions for the transmission of speech in an electric circuit; and, without the slightest knowledge of the mechanism which Prof. Bell had used, I devised the following arrangement for a speaking-telephone.
[Ill.u.s.tration: FIG. 14.--MY FIRST SPEAKING TELEPHONE.]
[Ill.u.s.tration: FIG. 14.--END VIEW.]
My first speaking-telephone, Fig. 14, consisted of a magnet made out of half-inch round steel bent into a U form, having the poles about two inches apart. Over these were slipped two bobbins taken from an old telegraph register, and were already fitted to a half-inch core. These bobbins, two inches and a half long, were wound with cotton-covered copper wire, No. 23, each bobbin containing about 150 feet. This magnet, with the bobbins slipped upon its poles, was made fast to a post two or three inches high. The steel was made as strongly magnetic as was possible, and would hold up three or four times its own weight. In front of the poles, a sheet of thin steel, one-fiftieth of an inch thick, was made fast to an upright board having a hole cut through it three and a half inches in diameter (Fig. 14, end view); the plate was screwed tightly to this board, so as to cover the hole; and the middle of the hole was at the same height as the two poles of the magnet. The wires from the two bobbins were connected, as if to make an electro-magnet; while the two free terminals were to be connected with the line-wires. Of course there were two of these instruments, both alike; and talking and singing were reproduced with these.
A very great number of experiments have been made to determine the best conditions for each of the essential parts,--the size and strength of the magnet, the size of the bobbins, as to length and fineness of wire, the best thickness for the plate for absorbing the vibrations, &c.; and it is really surprising, how little is the difference between very wide limits. The following directions will enable any one to construct a speaking-telephone with which good results may be obtained. The specifications will be for only one instrument; though of course two instruments made alike will be necessary for any purposes of speaking or other signals.
[Ill.u.s.tration: FIG. 15.]
[Ill.u.s.tration: FIG. 16.]
Procure three common horse-shoe magnets about six inches long, all of the same size; these retail in the market at about a dollar apiece. They should be strong enough to hold up several times their own weight each.
Next, have turned out of good hard wood,--such as maple or boxwood,--two spools not over half an inch long and an inch and a half broad, the sides cut square both inside and out, as shown at S, Fig. 15; a hole the third of an inch in diameter is to be made through the spool. Into this hole is to be fitted a short rod of soft iron, I, about an inch long, which should be a little rounded at the outer end. The bobbins may be wound with as much insulated copper wire as they will hold. The wire may be from the one-fortieth to the one-fiftieth of an inch in diameter, as is most convenient to obtain, the latter size being preferable. The resistance of such bobbins will probably be from two to three ohms each.
The soft-iron core I must project backwards far enough to be clamped between the two outer magnets 1 and 3, while the inner one, 2, is drawn back. When the bobbins are in their places, and are clamped between the upper and lower magnets, they will stand as shown in Fig. 16, where the view is from above; the magnets being b.u.t.toned down to the block they rest on (see Fig. 17), which at the same time holds the soft-iron rods with the bobbins upon them. The wires on these coils must be connected in the same way they would be in order to make opposite poles of their outer ends, if a current of electricity were to be sent through the coils. An upright board B (Fig. 17) six or seven inches square, having a round hole four inches in diameter cut out from the middle of it, must be fixed near the end of the base-board; and over this hole is to be screwed _tightly_ a piece of thin sheet iron or steel; it may be from the one-twentieth to the one-fiftieth of an inch in thickness. It does not seem to make much difference about the thickness of this plate. I have generally got the best results from a plate one-fiftieth of an inch thick. The upright board carrying this plate must be very rigid, otherwise the plate will be kept tight to the magnets all the time; and one of the conditions of success in working is, that this plate shall be as close as possible to the magnet-ends, but not to touch: therefore fix the board tight, and adjust the magnets by means of the b.u.t.ton shown on top of them in the perspective figure.
[Ill.u.s.tration: FIG. 17.]
The sounds to be transmitted, of whatever sort they may be, are to be made on the side P, Fig. 16; and likewise, when the instrument is used as a receiver, the ear is to be applied at the same place. A tube about two inches in diameter may be made fast to the front of the board, in a line with the centre of the plate; this will aid somewhat in hearing.
When two or three persons are to sing, it will be best to have each one supplied with a tube to sing through; one end of the tube to be placed close to the front of the plate. The sound of musical instruments, such as the flute and the cornet, will be reproduced much louder, if the front of such instrument be allowed to rest upon the rim of the hole in the board, just in front of the plate.
It is noticeable that low talking can be heard more distinctly than when a great effort is made; but the sounds though distinct are not strong at any time, and other sounds seriously interfere with hearing. It is probable that some way will hereafter be devised for increasing the usefulness of the invention by increasing the volume of sound. On account of the weakness of the sound it becomes necessary to provide a call to attract the attention of one in the room. This may be accomplished by having a small electric bell worked by a one or two cell battery. Another way which I have found to be quite as efficient is to have a rod of iron or steel about a foot long, and half an inch in diameter, bent into a U form. When this is held by the bend, and struck upon the floor or with a stick, it vibrates powerfully; and if one of its p.r.o.ngs be permitted to strike against the plate P, Fig. 16, the sound will be reproduced loud enough to hear over a large room. I have never failed to call with this when any one was in the same room with the telephone.
Wherever a telephone circuit has been made upon telegraph poles having other wires upon them, the inductive actions of the currents upon the other wires has been found to seriously interfere with the action of the telephones, inasmuch as the latter reproduce every other message. One skilled in reading by sound in the ordinary way can read through the telephone what message is travelling in a neighboring wire. Messages may be thus read upon wires as far distant as ten feet from the telephone circuit. It there fore seems to be essential that each telephone circuit should be isolated from every other one, else there can be no secrecy in messages.
A very interesting effect was noticed one night when there was a bright aurora display. There was a continuous current through the wires, accompanied with sounds which increased in intensity as the bright streamers pa.s.sed by. This will probably lead to some important results in science.
In all probability the telephone is as much in its infancy as was ordinary telegraphy in 1840. Since that time the sciences of electricity and magnetism have had the most of their growth, and telegraphy has kept pace with the advancing knowledge until its commercial importance is second to no other agency. Very many important principles that are invaluable in telegraphy to-day were wholly unknown in 1840; but it may here be noted that in the telephone, as it now is, there is not a single principle that was not well enough known in 1840. This will be apparent to one who follows out the phenomena from the sender to the receiver.
First, the sound in air causing a corresponding movement in a solid body, iron. This iron, acting inductively upon a magnet, originates magneto-electric currents in a wire helix about it; and these travel to another helix, and, re-acting upon the magnet in it, have electro-magnetic effects, and increase and decrease the strength of the magnet; and this variable magnetism affects the plate of iron in front of that magnet, and makes it to vibrate in a corresponding manner, and thus to restore to the air in one place the vibrations absorbed from the air in another place. To some it may seem strange that a simple thing as the telephone is, involving nothing but principles familiar enough to every one interested in physical science, should have waited nearly forty years to be invented. The reason is probably this: Men of science, as a rule, do not feel called upon to apply the principles which they may discover. They are content to be _discovering_, not _inventing_.
Now, the schools of the country ought to make the youth quite familiar with the general principles of physical science, that the inventive ones--and there are many such--may apply them intelligently. Mechanism is all that stands between us and aerial navigation; all that is necessary to reproduce human speech in writing; and all that is needed to realize completely the prophetic picture of the "Graphic," of the orator who shall at the same instant address an audience in every city in the world.