Volume I Part 7 (1/2)
Acousticon Transmitter. In Fig. 46 is shown a transmitter adapted for such use. This has been termed by its makers the _acousticon transmitter_. Like all the transmitters previously discussed, this is of the variable-resistance type, but it differs from them all in that it has no damping springs; in that carbon b.a.l.l.s are subst.i.tuted for carbon granules; and in that the diaphragm itself serves as the front electrode.
This transmitter consists of a cup _1_, into which is set a cylindrical block _2_, in one face of which are a number of hemispherical recesses. The diaphragm _3_ is made of thin carbon and is so placed in the transmitter as to cover the openings of the recesses in the carbon block, and lie close enough to the carbon block, without engaging it, to prevent the carbon particles from falling out. The diaphragm thus serves as the front electrode and the carbon block as the rear electrode. The recesses in the carbon block are about two-thirds filled with small carbon b.a.l.l.s, which are about the size of fine sand. The front piece _4_ of the transmitter is of sheet metal and serves to hold the diaphragm in place. To admit the sound waves it is provided with a circular opening opposite to and about the size of the rear electrode block. On this front piece are mounted the two terminals of the transmitter, connected respectively to the two electrodes, terminal _5_ being insulated from the front piece and connected by a thin metal strip with the diaphragm, while terminal _6_ is mounted directly on the front piece and connected through the cup _1_ with the carbon block _2_, or back electrode of the transmitter.
[Ill.u.s.tration: Fig 46. Acousticon Transmitter]
When this transmitter is used in connection with outfits for the deaf, it is placed in a hard rubber containing case, consisting of a hollow cylindrical piece _7_, which has fastened to it a cover _8_. This cover has a circular row of openings or holes near its outer edge, as shown at _9_, through which the sound waves may pa.s.s to the chamber within, and thence find their way through the round hole in the center of the front plate _4_ to the diaphragm _3_. It is probable also that the front face of the cover _8_ of the outer case vibrates, and in this way also causes sound waves to impinge against the diaphragm.
This arrangement provides a large receiving surface for the sound waves, but, owing to the fact that the openings in the containing case are not opposite the opening in the transmitter proper, the sound waves do not impinge directly against the diaphragm. This peculiar arrangement is probably the result of an endeavor to prevent the transmitter from being too strongly actuated by violent sounds close to it. Instruments of this kind are very sensitive and under proper conditions are readily responsive to words spoken in an ordinary tone ten feet away.
[Ill.u.s.tration: Fig. 47. Switchboard Transmitter]
Switchboard Transmitter. Another special adaptation of the telephone transmitter is that for use of telephone operators at central-office switchboards. The requirements in this case are such that the operator must always be able to speak into the transmitter while seated before the switchboard, and yet allow both of her hands to be free for use.
This was formerly accomplished by suspending an ordinary granular-carbon transmitter in front of the operator, but a later development has resulted in the adoption of the so-called breast transmitter, shown in Fig. 47. This is merely an ordinary granular-carbon transmitter mounted on a plate which is strapped on the breast of the operator, the transmitter being provided with a long curved mouthpiece which projects in such a manner as to lie just in front of the operator's lips. This device has the advantage of automatically following the operator in her movements. The breast transmitter shown in Fig. 47, is that of the Dean Electric Company.
[Ill.u.s.tration: Fig. 48. Transmitter Symbols]
Conventional Diagram. There are several common ways of ill.u.s.trating transmitters in diagrams of circuits in which they are employed. The three most common ways are shown in Fig. 48. The one at the left is supposed to be a side view of an ordinary instrument, the one in the center a front view, and the one at the right to be merely a suggestive arrangement of the diaphragm and the rear electrode. The one at the right is best and perhaps most common; the center one is the poorest and least used.
CHAPTER VI
RECEIVERS
The telephone receiver is the device which translates the energy of the voice currents into the energy of corresponding sound waves. All telephone receivers today are of the electromagnetic type, the voice currents causing a varying magnetic pull on an armature or diaphragm, which in turn produces the sound waves corresponding to the undulations of the voice currents.
Early Receivers. The early forms of telephone receivers were of the _single-pole_ type; that is, the type wherein but one pole of the electromagnet was presented to the diaphragm. The single-pole receiver that formed the companion piece to the old Blake transmitter and that was the standard of the Bell companies for many years, is shown in Fig. 49. While this has almost completely pa.s.sed out of use, it may be profitably studied in order that a comparison may be made between certain features of its construction and those of the later forms of receivers.
The coil of this receiver was wound on a round iron core _2_, flattened at one end to afford means for attaching the permanent magnet. The permanent magnet was of laminated construction, consisting of four hard steel bars _1_, extending nearly the entire length of the receiver sh.e.l.l. These steel bars were all magnetized separately and placed with like poles together so as to form a single bar magnet.
They were laid together in pairs so as to include between the pairs the flattened end of the pole piece _2_ at one end and the flattened portion of the tail piece _3_ at the other end. This whole magnet structure, including the core, the tail piece, and the permanently magnetized steel bars, was clamped together by screws as shown. The containing sh.e.l.l was of hard rubber consisting of three pieces, the barrel _4_, the ear-piece _5_, and the tail cap _6_. The barrel and the ear piece engaged each other by means of a screw thread and served to clamp the diaphragm between them. The compound bar magnet was held in place within the sh.e.l.l by means of a screw _7_ pa.s.sing through the hard rubber tail cap _6_ and into the tail block _3_ of the magnet.
External binding posts mounted on the tail cap, as shown, were connected by heavy leading-in wires to the terminals of the electromagnet.
A casual consideration of the magnetic circuit of this instrument will show that it was inefficient, since the return path for the lines of force set up by the bar magnet was necessarily through a very long air path. Notwithstanding this, these receivers were capable of giving excellent articulation and were of marvelous delicacy of action. A very grave fault was that the magnet was supported in the sh.e.l.l at the end farthest removed from the diaphragm. As a result it was difficult to maintain a permanent adjustment between the pole piece and the diaphragm. One reason for this was that hard rubber and steel contract and expand under changes of temperature at very different rates, and therefore the distance between the pole piece and the diaphragm changed with changes of temperature. Another grave defect, brought about by this tying together of the permanent magnet and the sh.e.l.l which supported the diaphragm at the end farthest from the diaphragm, was that any mechanical shocks were thus given a good chance to alter the adjustment.
[Ill.u.s.tration: Fig. 49. Single-Pole Receiver]
Modern Receivers. Receivers of today differ from this old single-pole receiver in two radical respects. In the first place, the modern receiver is of the bi-polar type, consisting essentially of a horseshoe magnet presenting both of its poles to the diaphragm. In the second place, the modern practice is to either support all of the working parts of the receiver, _i.e._, the magnet, the coils, and the diaphragm, by an inner metallic frame entirely independent of the sh.e.l.l; or, if the sh.e.l.l is used as a part of the structure, to rigidly fasten the several parts close to the diaphragm rather than at the end farthest removed from the diaphragm.
Western Electric Receiver. The standard bi-polar receiver of the Western Electric Company, in use by practically all of the Bell operating companies throughout this country and in large use abroad, is shown in Fig. 50. In this the sh.e.l.l is of three pieces, consisting of the barrel _1_, the ear cap _2_, and the tail cap _3_. The tail cap and the barrel are permanently fastened together to form substantially a single piece. Two permanently magnetized bar magnets _4-4_ are employed, these being clamped together at their upper ends, as shown, so as to include the soft iron block _5_ between them. The north pole of one of these magnets is clamped to the south pole of the other, so that in reality a horseshoe magnet is formed. At their lower ends, these two permanent magnets are clamped against the soft iron pole pieces _6-6_, a threaded block _7_ also being clamped rigidly between these pole pieces at this point. On the ends of the pole pieces the bobbins are wound. The whole magnet structure is secured within the sh.e.l.l _1_ by means of a screw thread on the block _7_ which engages a corresponding internal screw thread in the sh.e.l.l _1_. As a result of this construction the whole magnet structure is bound rigidly to the sh.e.l.l structure at a point close to the diaphragm, comparatively speaking, and as a result of this close coupling, the relation between the diaphragm and the pole piece is very much more rigid and substantial than in the case where the magnet structure and the sh.e.l.l were secured together at the end farthest removed from the diaphragm.
[Ill.u.s.tration: Fig. 50. Western Electric Receiver]
Although this receiver shown in Fig. 50 is the standard in use by the Bell companies throughout this country, its numbers running well into the millions, it cannot be said to be a strictly modern receiver, because of at least one rather antiquated feature. The binding posts, by which the circuit conductors are led to the coils of this instrument, are mounted on the outside of the receiver sh.e.l.l, as indicated, and are thus subject to danger of mechanical injury and they are also exposed to the touch of the user, so that he may, in case of the wires being charged to an abnormal potential, receive a shock. Probably a more serious feature than either one of these is that the terminals of the flexible cords which attach to these binding posts are attached outside of the receiver sh.e.l.l, and are therefore exposed to the wear and tear of use, rather than being protected as they should be within the sh.e.l.l. Notwithstanding this undesirable feature, this receiver is a very efficient one and is excellently constructed.
[Ill.u.s.tration: Fig. 51. Kellogg Receiver]
Kellogg Receiver. In Fig. 51 is shown a bi-polar receiver with internal or concealed binding posts. This particular receiver is typical of a large number of similar kinds and is manufactured by the Kellogg Switchboard and Supply Company. Two straight permanently magnetized bar magnets _1-1_ are clamped together at their opposite ends so as to form a horseshoe magnet. At the end opposite the diaphragm these bars clamp between them a cylindrical piece of iron _2_, so as to complete the magnetic circuit at the end. At the end nearest the diaphragm they clamp between them the ends of the soft iron pole pieces _3-3_, and also a block of composite metal _4_ having a large circular f.l.a.n.g.e _4'_ which serves as a means for supporting the magnet structure within the sh.e.l.l. The screws by means of which the disk _4'_ is clamped to the shouldered seat in the sh.e.l.l do not enter the sh.e.l.l directly, but rather enter screw-threaded bra.s.s blocks which are moulded into the structure of the sh.e.l.l. It is seen from this construction that the diaphragm and the pole pieces and the magnet structure itself are all rigidly secured together through the medium of the sh.e.l.l at a point as close as possible to the diaphragm.
Between the magnets _1-1_ there is clamped an insulating block _5_, to which are fastened the terminal plates _6_, one on each side of the receiver. These terminal plates are thoroughly insulated from the magnets themselves and from all other metallic parts by means of sheets of fiber, as indicated by the heavy black lines. On these plates _6_ are carried the binding posts for the receiver cord terminals. A long tongue extends from each of the plates _6_ through a hole in the disk _4'_, into the coil chamber of the receiver, at which point the terminal of the magnet winding is secured to it. This tongue is insulated from the disk _4'_, where it pa.s.ses through it, by means of insulating bus.h.i.+ng, as shown. The other terminal of the magnet coils is brought out to the other plate _6_ by means of a similar tongue on the other side.
In order that the receiver terminals proper may not be subjected to any strain in case the receiver is dropped and its weight caught on the receiver cord, a strain loop is formed as a continuation of the braided covering of the receiver cord, and this is tied to the permanent magnet structure, as shown. By making this strain loop short, it is obvious that whatever pull the cord receives will not be taken by the cord conductors leading to the binding posts or by the binding posts or the cord terminals themselves.