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 shell.

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 p