Fig. 52. Automatic Electric Company Direct-Current Receiver
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Fig. 53. Automatic Electric Company Direct-Current Receiver
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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 passing 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 shell 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, pass 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 astonishing 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 illustration 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 pass 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 pass 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.