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 passed 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 shell. 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