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 disassembled 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 brass cup 1, which serves not only as a container for the magnet, but also as a seat for the diaphragm.  This receiver is therefore illustrative of the type mentioned above, wherein the relation between the diaphragm and the pole pieces is not dependent upon any connection through the shell.

[Illustration:  Fig. 52.  Automatic Electric Company Direct-Current Receiver]

[Illustration:  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 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.