Volume I Part 7 (2/2)
A number of other manufacturers have gone even a step further than this in securing permanency of adjustment between the receiver diaphragm and pole pieces. They have done this by not depending at all on the hard rubber sh.e.l.l as a part of the structure, but by enclosing the magnet coil in a cup of metal upon which the diaphragm is mounted, so that the permanency of relation between the diaphragm and the pole pieces is dependent only upon the metallic structure and not at all upon the less durable sh.e.l.l.
Direct-Current Receiver. Until about the middle of the year 1909, it was the universal practice to employ permanent magnets for giving the initial polarization to the magnet cores of telephone receivers. This is still done, and necessarily so, in receivers employed in connection with magneto telephones. In common-battery systems, however, where the direct transmitter current is fed from the central office to the local stations, it has been found that this current which must flow at any rate through the line may be made to serve the additional purpose of energizing the receiver magnets so as to give them the necessary initial polarity. A type of receiver has come into wide use as a result, which is commonly called the _direct-current receiver_, deriving its name from the fact that it employs the direct current that is flowing in the common-battery line to magnetize the receiver cores. The Automatic Electric Company, of Chicago, was probably the first company to adopt this form of receiver as its standard type.
Their receiver is shown in cross-section in Fig. 52, and a photograph of the same instrument partially disa.s.sembled is given in Fig. 53. The most noticeable thing about the construction of this receiver is the absence of permanent magnets. The entire working parts are contained within the bra.s.s cup _1_, which serves not only as a container for the magnet, but also as a seat for the diaphragm. This receiver is therefore ill.u.s.trative of the type mentioned above, wherein the relation between the diaphragm and the pole pieces is not dependent upon any connection through the sh.e.l.l.
[Ill.u.s.tration: Fig. 52. Automatic Electric Company Direct-Current Receiver]
[Ill.u.s.tration: Fig. 53. Automatic Electric Company Direct-Current Receiver]
The coil of this instrument consists of a single cylindrical spool _2_, mounted on a cylindrical core. This bobbin lies within a soft iron-punching _3_, the form of which is most clearly shown in Fig. 53, and this punching affords a return path to the diaphragm for the lines of force set up in the magnet core. Obviously a magnetizing current pa.s.sing through the winding of the coil will cause the end of the core toward the diaphragm to be polarized, say positively, while the end of the enclosing sh.e.l.l will be polarized in the other polarity, negatively. Both poles of the magnet are therefore presented to the diaphragm and the only air gap in the magnetic circuit is that between the diaphragm and these poles. The magnetic circuit is therefore one of great efficiency, since it consists almost entirely of iron, the only air gap being that across which the attraction of the diaphragm is to take place.
The action of this receiver will be understood when it is stated that in common-battery practice, as will be shown in later chapters, a steady current flows over the line for energizing the transmitter. On this current is superposed the incoming voice currents from a distant station. The steady current flowing in the line will, in the case of this receiver, pa.s.s through the magnet winding and establish a normal magnetic field in the same way as if a permanent magnet were employed.
The superposed incoming voice currents will then be able to vary this magnetic field in exactly the same way as in the ordinary receiver.
An astonis.h.i.+ng feature of this recent development of the so-called direct-current receiver is that it did not come into use until after about twenty years of common-battery practice. There is nothing new in the principles involved, as all of them were already understood and some of them were employed by Bell in his original telephone; in fact, the idea had been advanced time and again, and thrown aside as not being worth consideration. This is an ill.u.s.tration of a frequent occurrence in the development of almost any rapidly growing art. Ideas that are discarded as worthless in the early stages of the art are finally picked up and made use of. The reason for this is that in some cases the ideas come in advance of the art, or they are proposed before the art is ready to use them. In other cases the idea as originally proposed lacked some small but essential detail, or, as is more often the case, the experimenter in the early days did not have sufficient skill or knowledge to make it fit the requirements as he saw them.
Monarch Receiver. The receiver of the Automatic Electric Company just discussed employs but a single electromagnet by which the initial magnetization of the cores and also the variable magnetization necessary for speech reproduction is secured. The problem of the direct-current receiver has been attacked in another way by Ernest E.
Yaxley, of the Monarch Telephone Manufacturing Company, with the result shown in Fig. 54. The construction in this case is not unlike that of an ordinary permanent-magnet receiver, except that in the place of the permanent magnets two soft iron cores _1-1_ are employed.
On these are wound two long bobbins of insulated wire so that the direct current flowing over the telephone line will pa.s.s through these and magnetize the cores to the same degree and for the same purpose as in the case of permanent magnets. These soft iron magnet cores _1-1_ continue to a point near the coil chamber, where they join the two soft iron pole pieces _2-2_, upon which the ordinary voice-current coils are wound. The two long coils _4-4_, which may be termed the direct-current coils, are of somewhat lower resistance than the two voice-current coils _3-3_. They are, however, by virtue of their greater number of turns and the greater amount of iron that is included in their cores, of much higher impedance than the voice-current coils _3-3_. These two sets of coils _4-4_ and _3-3_ are connected in multiple. As a result of their lower ohmic resistance the coils _4-4_ will take a greater amount of the steady current which comes over the line, and therefore the greater proportion of the steady current will be employed in magnetizing the bar magnets. On account of their higher impedance to alternating currents, however, nearly all of the voice currents which are superposed on the steady currents, flowing in the line will pa.s.s through the voice-current coils _3-3_, and, being near the diaphragm, these currents will so vary the steady magnetism in the cores _2-2_ as to produce the necessary vibration of the diaphragm.
[Ill.u.s.tration: Fig. 54. Monarch Direct-Current Receiver]
This receiver, like the one of the Automatic Electric Company, does not rely on the sh.e.l.l in any respect to maintain the permanency of relation between the pole pieces and the diaphragm. The cup _5_, which is of pressed bra.s.s, contains the voice-current coils and also acts as a seat for the diaphragm. The entire working parts of this receiver may be removed by merely uns.c.r.e.w.i.n.g the ear piece from the hard rubber sh.e.l.l, thus permitting the whole works to be withdrawn in an obvious manner.
Dean Receiver. Of such decided novelty as to be almost revolutionary in character is the receiver recently put on the market by the Dean Electric Company and shown in Fig. 55. This receiver is of the direct-current type and employs but a single cylindrical bobbin of wire. The core of this bobbin and the return path for the magnetic lines of force set up in it are composed of soft iron punchings of substantially =E= shape. These punchings are laid together so as to form a laminated soft-iron field, the limbs of which are about square in cross-section. The coil is wound on the center portion of this _E_ as a core, the core being, as stated, approximately square in cross-section. The general form of magnetic circuit in this instrument is therefore similar to that of the Automatic Electric Company's receiver, shown in Figs. 52 and 53, but the core is laminated instead of being solid as in that instrument.
[Ill.u.s.tration: Fig. 55. Dean Steel Sh.e.l.l Receiver]
The most unusual feature of this Dean receiver is that the use of hard rubber or composition does not enter into the formation of the sh.e.l.l, but instead a sh.e.l.l composed entirely of steel stampings has been subst.i.tuted therefor. The main portion of this sh.e.l.l is the barrel _1_. Great skill has evidently been exercised in the forming of this by the cold-drawn process, it presenting neither seams nor welds. The ear piece _2_ is also formed of steel of about the same gauge as the barrel _1_. Instead of screw-threading the steel parts, so that they would directly engage each other, the ingenious device has been employed of swaging a bra.s.s ring _3_ in the barrel portion and a similar bra.s.s ring _4_ in the ear cap portion, these two being slotted and keyed, as shown at _8_, so as to prevent their turning in their respective seats. The ring _3_ is provided with an external screw thread and the ring _4_ with an internal screw thread, so that the receiver cap is screwed on to the barrel in the same way as in the ordinary rubber sh.e.l.l. By the employment of these bra.s.s screw-threaded rings, the rusting together of the parts so that they could not be separated when required--a difficulty heretofore encountered in steel construction of similar parts--has been remedied.
[Ill.u.s.tration: Fig. 56. Working Parts of Dean Receiver]
The entire working parts of this receiver are contained within the cup _5_, the edge of which is f.l.a.n.g.ed outwardly to afford a seat for the diaphragm. The diaphragm is locked in place on the sh.e.l.l by a screw-threaded ring _6_, as is clearly indicated. A ring _7_ of insulating material is seated within the enlarged portion of the barrel _1_, and against this the f.l.a.n.g.e of the cup _5_ rests and is held in place by the cap _2_ when it is screwed home. The working parts of this receiver partially disa.s.sembled are shown in Fig. 56, which gives a clear idea of some of the features not clearly ill.u.s.trated in Fig. 55.
It cannot be denied that one of the princ.i.p.al items of maintenance of subscribers' station equipment has been due to the breakage of receiver sh.e.l.ls. The users frequently allow their receiver to fall and strike heavily against the wall or floor, thus not only subjecting the cords to great strain, but sometimes cracking or entirely breaking the receiver sh.e.l.l. The innovation thus proposed by the Dean Company of making the entire receiver sh.e.l.l of steel is of great interest. The sh.e.l.l, as will be seen, is entirely insulated from the circuit of the receiver so that no contact exists by which a user could receive a shock. The sh.e.l.l is enameled inside and out with a heavy black insulating enamel baked on, and said to be of great durability. How this enamel will wear remains to be seen. The insulation of the interior portions of the receiver is further guarded by providing a lining of fiber within the sh.e.l.l at all points where it seems possible that a cross could occur between some of the working parts and the metal of the sh.e.l.l. This type of receiver has not been on the market long enough to draw definite conclusions, based on experience in use, as to what its permanent performance will be.
Thus far in this chapter only those receivers which are commonly called _hand receivers_ have been discussed. These are the receivers that are ordinarily employed by the general public.
[Ill.u.s.tration: Fig. 57. Operator's Receiver]
Operator's Receiver. At the central office in telephone exchanges the operators are provided with receivers in order that they may communicate with the subscribers or with other operators. In order that they may have both of their hands free to set up and take down the connections and to perform all of the switching operations required, a special form of receiver is employed for this purpose, which is worn as a part of a head-gear and is commonly termed a _head receiver_. These are necessarily of very light construction, in order not to be burdensome to the operators, and obviously they must be efficient. They are ordinarily held in place at the ear by a metallic head band fitting over the head of the operator.
[Ill.u.s.tration: GRANT AVENUE OFFICE OF HOME TELEPHONE COMPANY, SAN FRANCISCO, CAL. A Type of Central-Office Buildings in Down-Town Districts of Large Cities.]
Such a receiver is shown in cross-section in Fig. 57, and completely a.s.sembled with its head band in Fig. 58. Referring to Fig. 57 the sh.e.l.l _1_ of the receiver is of aluminum and the magnets are formed of steel rings _2_, cross-magnetized so as to present a north pole on one side of the ring and a south pole on the other. The two L-shaped pole pieces _3_ are secured by screws to the poles of these ring magnets, and these pole pieces carry the magnet coils, as is clearly indicated.
These poles are presented to a soft iron diaphragm in exactly the same way as in the larger hand receivers, the diaphragm being clamped in place by a hard rubber ear piece, as shown. The head bands are frequently of steel covered with leather. They have a.s.sumed numerous forms, but the general form shown in Fig. 58 is the one commonly adopted.
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