II. TELEPHONIC INSTRUMENTS
One of the best-known forms of the Reis telephone is shown in fig. 1. The transmitter consists of a box A, provided with a mouthpiece M. In the top of the box a round hole is cut and across it a membrane S of hogs bladder is stretched. A thin strip of platinum p fixed to the box at one side of the hole and extending to the centre of the membrane, supports at that point one foot a light metal tripod egf. One of the feet, e or f, rests in a cup containing mercury, which is in metallic connexion with the terminal b, while the end of the strip p is similarly in connexion with the terminal a. The receiver consists of a electromagnet made up of a magnetizing coil H, with a stout knitting needle for a core. When in use these two instruments are joined in circuit with a battery B, so that under ordinary circumstances a continuous current is flowing through the line. Suppose a sound is then produced in front of the mouthpiece M, the successive variations it the pressure of the air are communicated to the inside of the box, and cause the membrane to vibrate in unison with the sound. Reiss theory of the action of the instrument was that at each outward impulse of the membrane the point g would be thrown out of contact with the plate underneath it and would thus break the circuit. There would consequently result as many breaks in the circuit as there were vibrations in the sound, and, in conformity with Pages discovery, the electromagnetic receiver would give out a rapid successive of beats, which would together a continuous and sound of the same pitch as that to which the transmitter was subjected.
Fig. 2 shows the first telephone made by Bell for transmitting speech. It consisted of a wooden frame F, to one side of which a tube T was fixed ; over the end of the tube of membrane M was stretched taut by a stretching ring R. To the opposite side of the frame and with its axis in line with that of the tube T was fixe an electromagnet H, and between the membrane M and the end of the electromagnet a hinged armature A was arranged in such a way that its motions would be controlled by the membrane. The instrument was joined in circuit with a battery and another similar instrument placed at a distance. A continuous current was made to flow through the circuit, which kept the electromagnet magnetized. Bell reasoned thus : when words are spoken in front armature A, and the vibration of the armature in front of the electro-magnet will induce variations in the line current ; their magnitude will be proportional to the amplitude, and their frequenly to the frequency, of the vibrations of the armature ; in fact, the difference between the actual and the average current in the circuit will be at each instant proportional to the rate of motion of the armature. It follows from this that armature and membrane of the distant instrument should have induced in them a motion precisely similar to that of the membrane of the transmitter. This telephone was made in July 1875, but was put aside after trial as unsatisfactory on account of the feebleness of the sounds it produced ; since then, however, a successful telephone has been made on precisely the same plan as that here indicated.
The next form tried is shown in fig. 3. It is very similar except in constructive details to first ; the hinged armature, however, is omitted, its place being taken by a small iron disk A fixed to the centre of the diaphragm D. The electromagnet H is, as before, placed so as to have the centre of the soft iron core C opposite to the centre of the disk, and the theory according to which it was expected to act is the same. The results obtained with this instrument were much more satisfactory it was with one precisely like that shown in the figure that the remarkable results of the Philadelphia exhibition in 1876 were obtained. A perspective and a sectional view of the receiving instrument used along with that shown in fig. 3 are illustrated in figs. 4 and 5 It consisted of an iron cylindrical box B, through the axis of which a rod of soft iron C was passed to form the core of an electromagnet, having the magnetizing helix H. wound on the upper half of its length. Across the top of the box a thin disk D of soft iron was fixed, the core C being just clear of the disk when the strongest current is flowing through the helix. In the perspective view the disk is removed, showing the end of the core. These instruments are interesting, not because they may be considered the first really successful speaking telephone, but cause they are of the same form as those brought to Great Britain in 1876 by Sir W. Thomson, and exhibited before the British Association at Glasgow in that year.
Fig. 6 shows one of the earliest forms brought into commercial use. On each pole of a somewhat large horse-shoe permanent magnet M a short coil E with a soft iron core was fixed. This is one of the early forms of permanent magnet telephones, of which there were at that time several, including a hand telephone very similar to that shown in fig. 7. In another form. Introduced about the end of 177, the small magnetizing coils and soft iron cores were fixed on the side and opposite the poles of the horse-shoe magnet, and the diaphragm was placed with its plane parallel to that of the magnet. The diaphragm in these telephones was of thin sheet iron and a little over 4 inches in diameter.
The form of telephone now almost universally in use is shown in fig. 7. It was introduced in December 1877 and consists of compound permanent magnet M, fitted into the centre of a tube of vulcanite or " hard rubber" and carrying at one end a short electromagnet, the coil of which through its terminals t, t is included in the circuit when the instrument is in use. In front of the electromagnet, with its plance normal to the axis of the magnet, if fixed a thin soft iron disk about 1 _ inches in diameter, which has its cover cut to a convenient shape to form a mouthpiece. This telephone acts well either a transmitter or as a transmitter or as receiver ; but the former purpose it is now seldom used on account of the great advances which have been make in "microphone" transmitters.
It has been stated that Bell and Elisha Gray almost simultaneously suggested the use of a column of liquid to vary the resistance in the circuit. The form of instrument proposed by the former and said to have been exhibited at the Philadelphia exhibition is shown in fig. 8. It consists of a speaking tube or mouthpiece M, across the lower end of which a membrane D is stretched. To the centre of the membrane a light rod R, made of metal or of carbon, is fixed with its length at right angles to the plane of the membrane. Under the lower end of R a small metallic vessel C is supported on a threaded rod, working in a nut fixed to the sole F, so that its height may be readily adjusted. Suppose C to be filled with water or any other conducting liquid, and the rod R to be metal. C is raised until the liquid just touches the point of the rod, when advantage is taken of the change of contact resistance with the greater or less immersion of R during the vibration of D. Good results were obtained with mercury as the liquid and with a rod of carbon.
The arrangement proposed by Elisha Gray is almost identical in E. form with Bells. The only difference seems to be that Gray intended the rod R (fig. 9) to reach near to the bottom of the vessel B or to the end of another rod, prolongation of b, projecting up from the bottom. The variation of the current was produced by the variation of the distance between the ends of the rod caused by the vibrations of the diaphragm. This plan was not tried until after the success of Bells experiments was known, and when it was tried the results did not prove encouraging. Indeed the variations of the resistance which ca be produced in this way must be excessively small, unless the liquid has a very high specific resistance, the distance between the ends is very small, and the sides of the rods are prevented by an insulting covering from interfering with the results. Neither of these transmitters has any great merit as such but they show that both Bell and Gray clearly recognized the principle on which successful transmission of the different forms of sounds, including speech, could be accomplished.
The first successful microphone transmitter was Edisons. An early form of it (fig. 10) somewhat resembles Bells hand telephone in external form. A cell of insulating material has at its bottom a flat-headed platinum screw G; on the top of G is a layer of carbon powder C, on the top of that a thin platinum disk D, and above that, forming the cover of the cell, a disk of ivory B, held in position by a ring E. Resting on the centre of this disk is a small piece of rubber tubing, which is lightly pressed by the diaphragm A, and this, as in the hand telephone, is held in position by the mouthpiece M. The varying pressure on A, when a sound is produced near it, causes corresponding variations in the pressure on the carbon powder, and this produces similar variations in its electrical resistance. Thus, when the instrument is included in an electric circuit through which is flowing, undulations in the pressure on the diaphragm produce corresponding undulations in the current.
Perhaps the best known forms of the microphone are those introduced by Prof. Hughes. One of the commonest is shown in fig. 11. Consists of two rectangular pieces of wood, B and D, fixed together with their planes at right angles together with their planes at right angles to each other. D forms the base, and to B two small blocks of carbon C, C are attached. Between these a light rod A of the same material is supported on small cups formed in C, C. To the blocks two electrodes e, e are connected for the purpose of inserting the instrument in an electric circuit. The material which Hughes found most suitable for the carbon blocks and rod was wood charcoal metallized by heating it to redness and plunging it while hot in to mercury. If this microphone is joined in circuit with a telephone and a small battery, say one or two small Deniell cells, the vibration produced by a fly walking on the base D can be distinctly heard in the telephone. The same apparatus will also act as a microphone transmitter, but the sounds are apt to be harsh. A better form for this purpose is shown in fig. 12. In this a light pencil of carbon M is pivoted at h and has one end resting on two blocks of carbon c, c, the lower one being fixed to the base. The pressure of M on the carbon block is regulated by a spring s. This arrangement is enclosed in a box of thin wood, against which the sound is directed. It is capable of acting well as a microphone receiver. The lower block c is then attached to the centre of a vertical diaphragm and against it the sounds are directed.
The Blake transmitter, which is perhaps most widely used of all, is a simple modification of the Hughes instrument last described. It consists (fig. 13) of a frame F, to which is attached a diaphragm D of thin sheet iron ; in front of this is a cover M, M provided with a suitable cavity for directing the sound-waves against the diaphragm. The microphonic arrangement consists of a spring S, about the hundredth or an inch thick and the eighth of an inch broad, fixed at one end to a lever L, and carrying at its free extremity a brass block W. In one side of W a small disk C of gas carbon is inserted, resting on the hemispherical end of a small platinum pin K, about the twentieth of an inch in diameter, held in position by a thin spring A. The pressure of the carbon on the platinum point can be adjusted by the screw N, which turns the lever about the flexible joint G. The electrical connexions of the instrument as arranged for actual use are illustrated in the figure. The current goes through S, W, C, K, A, and the primary circuit of the induction coil I to the battery B, and thence to S again. This forms a local circuit at the transmitting station. The line of circuit passes through the secondary of the induction coil I to the line, from that to the telephone T at the receiving station, and then either to earth or back to the induction coil b a return line of wire.
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