Volume I Part 6 (1/2)
Materials. Of all the materials available for the variable-resistance element in telephone transmitters, carbon is by far the most suitable, and its use is well nigh universal. Sometimes one of the rarer metals, such as platinum or gold, is to be found in commercial transmitters as part of the resistance-varying device, but, even when this is so, it is always used in combination with carbon in some form or other. Most of the transmitters in use, however, depend solely upon carbon as the conductive material of the variable-resistance element.
Arrangement of Electrodes. Following the principles pointed out by Hughes, the transmitters of today always employ as their variable-resistance elements one or more loose contacts between one or more pairs of electrodes, which electrodes, as just stated, are usually of carbon. Always the arrangement is such that the sound waves will vary the intimacy of contact between the electrodes and, therefore, the resistance of the path through the electrodes.
A mult.i.tude of arrangements have been proposed and tried. Sometimes a single pair of electrodes has been employed having a single point of loose contact between them. These may be termed single-contact transmitters. Sometimes the variable-resistance element has included a greater number of electrodes arranged in multiple, or in series, or in series-multiple, and these have been termed multiple-electrode transmitters, signifying a plurality of electrodes. A later development, an outgrowth of the multiple-electrode transmitter, makes use of a pair of princ.i.p.al electrodes, between which is included a ma.s.s of finely divided carbon in the form of granules or small spheres or pellets. These, regardless of the exact form of the carbon particles, are called granular-carbon transmitters.
[Ill.u.s.tration: Fig. 38. Blake Transmitter]
Single Electrode. _Blake_. The most notable example of the single-contact transmitter is the once familiar Blake instrument. At one time this formed a part of the standard equipment of almost every telephone in the United States, and it was also largely used abroad.
Probably no transmitter has ever exceeded it in clearness of articulation, but it was decidedly deficient in power in comparison with the modern transmitters. In this instrument, which is shown in Fig. 38, the variable-resistance contact was that between a carbon and a platinum electrode. The diaphragm _1_ was of sheet iron mounted, as usual in later transmitters, in a soft rubber gasket _2_. The whole diaphragm was mounted in a cast-iron ring _3_, supported on the inside of the box containing the entire instrument. The front electrode _4_ was mounted on a light spring _5_, the upper end of which was supported by a movable bar or lever _6_, flexibly supported on a spring _7_ secured to the casting which supported the diaphragm. The tension of this spring _5_ was such as to cause the platinum point to press lightly away from the center of the diaphragm. The rear electrode was of carbon in the form of a small block _9_, secured in a heavy bra.s.s b.u.t.ton _10_. The entire rear electrode structure was supported on a heavier spring _11_ carried on the same lever as the spring _5_. The tension of this latter spring was such as to press against the front electrode and, by its greater strength, press this against the center of the diaphragm. The adjustment of the instrument was secured by means of the screw _12_, carried in a lug extending rearwardly from the diaphragm supporting casting, this screw, by its position, determining the strength with which the rear electrode pressed against the front electrode and that against the diaphragm.
This instrument was ordinarily mounted in a wooden box together with the induction coil, which is shown in the upper portion of the figure.
The Blake transmitter has pa.s.sed almost entirely out of use in this country, being superseded by the various forms of granular instruments, which, while much more powerful, are not perhaps capable of producing quite such clear and distinct articulation.
The great trouble with the single-contact transmitters, such as the Blake, was that it was impossible to pa.s.s enough current through the single point of contact to secure the desired power of transmission without overheating the contact. If too much current is sent through such transmitters, an undue amount of heat is generated at the point of contact and a vibration is set up which causes a peculiar humming or squealing sound which interferes with the transmission of other sounds.
Multiple Electrode. To remedy this difficulty the so-called multiple-electrode transmitter was brought out. This took a very great number of forms, of which the one shown in Fig. 39 is typical. The diaphragm shown at _1_, in this particular form, was made of thin pine wood. On the rear side of this, suspended from a rod _3_ carried in a bracket _4_, were a number of carbon rods or pendants _5_, loosely resting against a rod _2_, carried on a bracket _6_ also mounted on the rear of the diaphragm. The pivotal rod _3_ and the rod _2_, against which the pendants rested, were sometimes, like the pendant rods, made of carbon and sometimes of metal, such as bra.s.s. When the diaphragm vibrated, the intimacy of contact between the pendant rod _5_ and the rod _2_ was altered, and thus the resistance of the path through all of the pendant rods in multiple was changed.
[Ill.u.s.tration: Fig. 39. Multiple-Electrode Transmitter]
A mult.i.tude of forms of such transmitters came into use in the early eighties, and while they in some measure remedied the difficulty encountered with the Blake transmitter, _i.e._, of not being able to carry a sufficiently large current, they were all subject to the effects of extreme sensitiveness, and would rattle or break when called upon to transmit sounds of more than ordinary loudness.
Furthermore, the presence of such large ma.s.ses of material, which it was necessary to throw into vibration by the sound waves, was distinctly against this form of transmitter. The inertia of the moving parts was so great that clearness of articulation was interfered with.
Granular Carbon. The idea of employing a ma.s.s of granular carbon, supported between two electrodes, one of which vibrated with the sound waves and the other was stationary, was proposed by Henry Hunnings in the early eighties. While this idea forms the basis of all modern telephone transmitters, yet it did not prevent the almost universal adoption of the single-contact form of instrument during the next decade.
Western Electric Solid-Back Transmitter. In the early nineties, however, the granular-carbon transmitter came into its own with the advent and wide adoption of the transmitter designed by Anthony C.
White, known as the _White_, or _solid-back_, transmitter. This has for many years been the standard instrument of the Bell companies operating throughout the United States, and has found large use abroad. A horizontal cross-section of this instrument is shown in Fig.
40, and a rear view of the working parts in Fig. 41. The working parts are all mounted on the front casting _1_. This is supported in a cup _2_, in turn supported on the lug _3_, which is pivoted on the transmitter arm or other support. The front and rear electrodes of this instrument are formed of thin carbon disks shown in solid black.
The rear electrode, the larger one of these disks, is securely attached by solder to the face of a bra.s.s disk having a rearwardly projecting screw-threaded shank, which serves to hold it and the rear electrode in place in the bottom of a heavy bra.s.s cup _4_. The front electrode is mounted on the rear face of a stud. Clamped against the head of this stud, by a screw-threaded clamping ring _7_, is a mica washer, or disk _6_. The center portion of this mica washer is therefore rigid with respect to the front electrode and partakes of its movements. The outer edge of this mica washer is similarly clamped against the front edge of the cup _4_, a screw-threaded ring _9_ serving to hold the edge of the mica rigidly against the front of the cup. The outer edge of this washer is, therefore, rigid with respect to the rear electrode, which is fixed. Whatever relative movement there is between the two electrodes must, therefore, be permitted by the flexing of the mica washer. This mica washer not only serves to maintain the electrodes in their normal relative positions, but also serves to close the chamber which contains the electrodes, and, therefore, to prevent the granular carbon, with which the s.p.a.ce between the electrodes is filled, from falling out.
[Ill.u.s.tration: Fig. 40. White Solid-Back Transmitter]
The cup _4_, containing the electrode chamber, is rigidly fastened with respect to the body of the transmitter by a rearwardly projecting shank held in a bridge piece _8_ which is secured at its ends to the front block. The needed rigidity of the rear electrode is thus obtained and this is probably the reason for calling the instrument the _solid-back_. The front electrode, on the other hand, is fastened to the center of the diaphragm by means of a shank on the stud, which pa.s.ses through a hole in the diaphragm and is clamped thereto by two small nuts. Against the rear face of the diaphragm of this transmitter there rest two damping springs. These are not shown in Fig. 40 but are in Fig. 41. They are secured at one end to the rear f.l.a.n.g.e of the front casting _1_, and bear with their other or free ends against the rear face of the diaphragm. The damping springs are prevented from coming into actual contact with the diaphragm by small insulating pads. The purpose of the damping springs is to reduce the sensitiveness of the diaphragm to extraneous sounds. As a result, the White transmitter does not pick up all of the sounds in its vicinity as readily as do the more sensitive transmitters, and thus the transmission is not interfered with by extraneous noises. On the other hand, the provision of these heavy damping springs makes it necessary that this transmitter shall be spoken into directly by the user.
[Ill.u.s.tration: Fig. 41. White Solid-Back Transmitter]
The action of this transmitter is as follows: Sound waves are concentrated against the center of the diaphragm by the mouth-piece, which is of the familiar form. These waves impinge against the diaphragm, causing it to vibrate, and this, in turn, produces similar vibrations in the front electrode. The vibrations of the front electrode are permitted by the elasticity of the mica washer _6_. The rear electrode is, however, held stationary within the heavy chambered block _4_ and which in turn is held immovable by its rigid mounting.
As a result, the front electrode approaches and recedes from the rear electrode, thus compressing and decompressing the ma.s.s of granular carbon between them. As a result, the intimacy of contact between the electrode plates and the granules and also between the granules themselves is altered, and the resistance of the path from one electrode to the other through the ma.s.s of granules is varied.
New Western Electric Transmitter. The White transmitter was the prototype of a large number of others embodying the same features of having the rear electrode mounted in a stationary cup or chamber and the front electrode movable with the diaphragm, a washer of mica or other flexible insulating material serving to close the front of the electrode chamber and at the same time to permit the necessary vibration of the front electrode with the diaphragm.
[Ill.u.s.tration: Fig. 42. New Western Electric Transmitter]
One of these transmitters, embodying these same features but with modified details, is shown in Fig. 42, this being the new transmitter manufactured by the Western Electric Company. In this the bridge of the original White transmitter is dispensed with, the electrode chamber being supported by a pressed metal cup _1_, which supports the chamber as a whole. The electrode cup, instead of being made of a solid block as in the White instrument, is composed of two portions, a cylindrical or tubular portion _2_ and a back _3_. The cylindrical portion is externally screw-threaded so as to engage an internal screw thread in a f.l.a.n.g.ed opening in the center of the cup _1_. By this means the electrode chamber is held in place in the cup _1_, and by the same means the mica washer _4_ is clamped between the f.l.a.n.g.e in this opening and the tubular portion _2_ of the electrode chamber. The front electrode is carried, as in the White transmitter, on the mica washer and is rigidly attached to the center of the diaphragm so as to partake of the movement thereof. It will be seen, therefore, that this is essentially a White transmitter, but with a modified mounting for the electrode chamber.
A feature in this transmitter that is not found in the White transmitter is that both the front and the rear electrodes, in fact, the entire working portions of the transmitter, are insulated from the exposed metal parts of the instrument. This is accomplished by insulating the diaphragm and the supporting cup _1_ from the transmitter front. The terminal _5_ on the cup _1_ forms the electrical connection for the rear electrode, while the terminal _6_, which is mounted _on_ but insulated _from_ the cup _1_ and is connected with the front electrode by a thin flexible connecting strip, forms the electrical connection for the front electrode.
Kellogg Transmitter. The transmitter of the Kellogg Switchboard and Supply Company, originally developed by Mr. W.W. Dean and modified by his successors in the Kellogg Company, is shown in Fig. 43. In this, the electrode chamber, instead of being mounted in a stationary and rigid position, as in the case of the White instrument, is mounted on, and, in fact, forms a part of the diaphragm. The electrode which is a.s.sociated with the mica washer instead of moving with the diaphragm, as in the White instrument, is rigidly connected to a bridge so as to be as free as possible from all vibrations.