vibrations made by the diaphragm to which it is attached, as well as with the rapidity of their succession, it will readily be seen that the distance, and consequently the resistance to the passage of the current, between the lower end of the rod and the metallic plug, must vary in a similar manner, and this produces a series of corresponding variations in the strength of the battery

current.

The receiving apparatus consists simply of an electro-magnet, H, and armature, a diaphragm, D, and a mouthpiece, T. The soft iron armature which is attached to the diaphragm stands just in front of the electromagnet; consequently, when the latter acts, it does so in obedience to current pulsations, which have all the characteristics of the vibrating diaphragm, D,, and thus, through the additional intermediary of the soft iron, the vibrations produced by the voice in T,, are communicated to the diaphragm, T, of the receiving apparatus, and thus sounds of every character, including all the tones of the human voice, are reproduced with absolute fidelity and distinctness.

In the summer of 1876, Professor A. G. Bell, of the Boston University, exhibited at the Centennial Exhibition in Philadelphia, a telephonic apparatus by which articulate speech could be transmitted over an electric circuit, and reproduced at a distance with some degree of distinctness.

The accompanying diagram, figure 6, illustrates the principle and method of working of this apparatus. A represents the transmitting, and B the receiving, apparatus. When a person speaks into the tube, T, in the direction of the arrow, the acoustic vibrations of the air are communicated to a membrane tightly stretched across the end of the tube, upon which is cemented a light permanent bar magnet, n s. This is in close proximity to the poles of an electro-magnet,

|

M, in the circuit of the line, which is constantly charged by a current from the battery, E. The vibrations of the magnet n s induce magneto-electric pulsations in the coils of the electro-magnet, M, which traverse the circuit, and the magnitude of these pulsations is proportional to the rapidity and amplitude of the vibrations of the magnet. Consequently, this apparatus is capable of transmitting both the pitch and intensity of the tones which enter the tube, T. The receiving instrument consists simply of a tubular electro-magnet, R, formed of a single helix with an external soft iron case, into the top of which is loosely fitted the iron plate, r, which is thrown into vibrations by the action of the magnetizing helix. The sounds produced in this manner were quite weak and could only be transmitted a short distance; but the mere accomplishment of the feat of transmitting electric impulses over a metallic wire which should reproduce articulate speech, even in an imperfect manner, at the farther end, excited great interest in a scientific as well as a popular point of view throughout the civilized world.

During the ensuing autumn some important changes in the telephone were effected whereby its articulating properties were greatly improved. Professor A. E. Dolbear of Tufts College, observing that the actual function of the battery current with which the line was charged in Bell's method, had simply the effect of polarizing the soft iron cores of the transmitting and receiving instruments, or of converting them into permanent magnets, and that the mere passage of the constant voltaic current over the line had nothing to do with the result, conceived the idea of maintaining the cores in a permanently magnetic or polarized state by the inductive influence of a permanent magnet instead of by a voltaic current. Permanent magnets with small helices of insulated copper

FIG. 6. BELL'S SPEAKING TELEPHONE.

strument for both sending and receiving instead of employing instruments of different construction, as had been previously done. The principle and mode of operation of the improved apparatus is represented in figure 7.

It consists of an ordinary permanent bar magnet, N.S; a single helix, H, of insulated copper wire placed upon one end of the magnet, and a metallic diaphragm, D, consisting of a disk of thin sheet-iron two and a quarter inches in diameter and one fiftieth of an inch thick, forming an armature to the magnet, N S. The vibratory motions of the air produced by the voice or other cause are directed toward, and concentrated upon, the diaphragm, D, by means of a mouthpiece, T. It will thus be seen that when vibrations are communicated to the air in front of the mouthpiece the impact of the waves of air against the elastic diaphragm will cause a corresponding movement of the latter. This in turn, by reacting upon the magnet, disturbs the normal magnetic condition of the bar, and since any change of magnetism in this tends to generate electrical currents in the surrounding helix, the circuit in which the helix

may be placed will be traversed by a series of electrical pulsations or currents. Moreover, as these currents continue to be generated so long as the motion of the diaphragm continues, and as they increase and decrease in strength with the amplitude of its vibrations, thus varying with the variations of its amplitude, it is evident that they virtually possess all the physical characteristics of the agent acting upon the transmitting diaphragm. Consequently, by their electro-magnetic action upon the magnet of an apparatus, identical with the one above described and placed in the same circuit, they will cause its diaphragm to vibrate in exactly the same manner as that of the transmitting apparatus.

Mr. Thomas A. Edison, of Menlo Park, New Jersey, has invented a telephone, which, like that of Gray, shown in figure 6, is based upon the principle of varying the strength of a battery current in unison with the rise and fall of the vocal utterance. The problem of practically varying the resistance controlled by the diaphragm, so as to accomplish this result, was by no means an easy one. By constant experimenting, Mr. Edison at length made the discovery that, when properly prepared, carbon possessed the remarkable property of changing its resistance with pressure, and that the ratios of these changes, moreover, corresponded exactly with the pressure. Here then was the solution, for, by vibrating a diaphragm with varying degrees of pressure against a disk of carbon, which is made to form a portion of an electric circuit, the resistance of the disk would vary in precise accordance with the degree of pressure, and consequently a proportionate variation would be occasioned in the strength of the current. The latter would thus possess all the characteristics of the vocal waves, and by its reaction through the medium of an electromagnet, might then transfer them to a metallic diaphragm, causing the latter to vibrate, and thus reproduce audible speech. Figure 8 shows the telephone as now

for their action upon undulating electric currents produced by the vibratory motion of a transmitting diaphragm which increases and decreases the number and amplitude of the electric impulses transmitted over the wire without breaking the circuit.

use.

During the past year many ingenious per sons have turned their attention to the subject of speaking telephones, and by the introduction of various modifications have succeeded in greatly improving the invention, so as to make it available for practical Prominent among these is Mr. G. M. Phelps, an inventor of several valuable telegraph instruments, to whose ability in the scientific arrangement of details in the construction of the apparatus, the public is indebted for the most effective telephones yet introduced. The peculiar excellence of these instruments consists in their distinct articulation, combined with a loudness of utterance that is not met with in the numer ous other forms that have appeared up to the present time. Both of these qualities, manifestly so desirable, are, in these in

two platinum plates, D and G, which are connected in the battery circuit, as shown. by the lines. A small piece of rubber tubing, B, is attached to the center of the metallic diaphragm, and presses lightly against an ivory piece, C, which is placed directly over one of the platinum plates. Whenever, therefore, any motion is given to the diaphragm it is immediately followed by a corresponding pressure upon the carbon and by a change of resistance in the latter, as described above. It is obvious that any electro-magnet, properly fitted with an iron. diaphragm, will answer for a receiving instrument in connection with this apparatus.

Figure 9 shows a sending and receiving telephone and a box containing the battery.

Many other modifications of the speaking telephone have been devised, but they all possess certain common characteristics embodied in Mr. Gray's original discovery, and are essentially the same in principle although differing somewhat in matters of detail. All, for example, employ a diaphragm at the transmitting end capable of responding to the acoustic vibrations of the air; all employ a diaphragm at the receiving end, capable of being thrown into vibrations by the action of the magnetizing helix, corresponding to the vibrations of the transmitting diaphragm; and finally, all depend

hundred miles in length having been worked | by them with the most admirable results.

The form designed by Mr. Phelps, which is now being extensively introduced into practical use, consists of a polished ovalshaped case of hard rubber (figure 10), with magnet, diaphragm and coils inside. In connection with this there is also a small magneto-electrical machine, contained. in an oblong wooden box (figure 11), which is used for operating a call-bell when the attention of the correspondent at the distant station is required. currents, which are generated by this machine by turning a crank, are conveyed by the conducting wires through the helices of a polarized magnet, or relay, and cause a hammer attached to an armature lever to vibrate against a bell, thus producing a violent ringing, sufficiently loud to be heard at a considerable distance from the apparatus.

The

Probably the value of no invention was ever more promptly recognized by the public than the telephone. Already many thousands of them are in practical use in this country and abroad. It is employed as a means of communication between the counting-room and the factory, the merchant's residence and the office, the publishing house and the printing-office, and, in short, wherever oral communication is desired between persons separated by any distance beyond the ordinary reach of the human voice. In Germany it is being rapidly introduced into the various military establishments, and has also been adopted by the telegraph administration for connecting small villages and hamlets with the regular telegraph offices. It will undoubtedly afford the means of extending telegraphic facilities in this country to many thousands of places where the amount of business is insufficient to support a regular telegraph office, but where a line could be built and the telephone could be brought into use for the transmission of messages to the nearest telegraph office, at a trifling expense.

The speaking phonograph, also invented by Mr. Thomas A. Edison, is a purely mechanical invention, no electricity being used. It is, however, somewhat allied to the telephone in consequence of the fact that, like the latter, its action depends upon the vibratory motions of a metallic diaphragm capable of receiving from, and transmitting to, the air, sound vibrations.

In its simplest form, the speaking phonograph consists of a mounted diaphragm, so arranged as to operate a small steel stylus

tending each way beyond its ends for a distance about equal to its own length. A spiral groove is cut in the circumference of the cylinder, from one end to the other, each spiral of the groove being separated from its neighbor by about one-tenth of an inch. The shaft or axis is also cut by a screw thread corresponding to the spiral groove of the cylinder, and works in screw bearings, consequently when the cylinder is caused to revolve, by means of a crank that is fitted to the axis for this purpose, it receives a forward or backward movement of about onetenth of an inch for every turn of the same, the direction, of course, depending upon the way the crank is turned. The diaphragm, figure 13, is supported by an upright casting capable of adjustment, and so arranged that it may be removed altogether when necessary. When in use, however, it is clamped in a fixed position above or in front of the cylinder, thus bringing the stylus always opposite the groove as the cylinder is turned. A small, flat spring attached to the casting extends underneath the diaphragm as far as its center and carries the stylus, and between the diaphragm and spring a small piece of india rubber is placed to modify the action, it having been found that better results are obtained by this means than when the stylus is rigidly attached to the diaphragm itself.

The action of the apparatus will now be readily understood from what follows. The cylinder is first very smoothly covered with

tin-foil, and the diaphragm securely fastened in place by clamping its support to the base of the instrument. When this has been properly done, the stylus should lightly press against that part of the foil over the groove. The crank is now turned, while, at the same time, some one speaks into the mouth-piece of the instrument, which will cause the diaphragm to vibrate, and as the vibrations of the latter correspond with the movements of the air producing them, the soft and yielding foil will become marked along the line of the groove by a series of indentations of different depths, varying

sequently, rising and falling with the depressions in the foil, its motion is communicated to the diaphragm, and thence through the intervening air to the ear, where the sensation of sound is produced.

As the faithful reproduction of a sound is in reality nothing more than a reproduction of similar acoustic vibrations in a given time, it at once becomes evident that the cylinder should be made to revolve with absolute uniformity at all times, otherwise a difference more or less marked between the original sound and the reproduction will become manifest. To secure this uniformity

with the amplitude of the vibrations of the diaphragm; or, in other words, with the inflections or modulations of the speaker's voice. These inflections may therefore be looked upon as a sort of visible speech, which, in fact, they really are. If now the diaphragm is removed, by loosening the clamp, and the cylinder then turned back to the starting point, we have only to replace the diaphragm and turn in the same direction as at first, to hear repeated all that has been spoken into the mouth-piece of the apparatus; the stylus, by this means, being caused to traverse its former path, and con

of motion, and produce a practically working machine for automatically recording speeches, vocal and instrumental music, and perfectly reproducing the same, the inventor has devised an apparatus in which a plate replaces the cylinder. This plate, which is ten inches in diameter, has a volute spiral groove cut in its surface on both sides from its center to within one inch of its outer edge; an arm guided by the spiral upon the under side of the plate carries a diaphragm and mouthpiece at its extreme end. If the arm be placed near the center of the plate and the latter rotated, the motion will cause