single-pole receiver in two radical respects. In the first place, the
modern receiver is of the bi-polar type, consisting essentially of a
horseshoe magnet presenting both of its poles to the diaphragm. In the
second place, the modern practice is to either support all of the
working parts of the receiver, _i.e._, the magnet, the coils, and the
diaphragm, by an inner metallic frame entirely independent of the
shell; or, if the shell is used as a part of the structure, to rigidly
fasten the several parts close to the diaphragm rather than at the end
farthest removed from the diaphragm.

Western Electric Receiver. The standard bi-polar receiver of the
Western Electric Company, in use by practically all of the Bell
operating companies throughout this country and in large use abroad,
is shown in Fig. 50. In this the shell is of three pieces, consisting
of the barrel _1_, the ear cap _2_, and the tail cap _3_. The tail cap
and the barrel are permanently fastened together to form substantially
a single piece. Two permanently magnetized bar magnets _4-4_ are
employed, these being clamped together at their upper ends, as shown,
so as to include the soft iron block _5_ between them. The north pole
of one of these magnets is clamped to the south pole of the other, so
that in reality a horseshoe magnet is formed. At their lower ends,
these two permanent magnets are clamped against the soft iron pole
pieces _6-6_, a threaded block _7_ also being clamped rigidly between
these pole pieces at this point. On the ends of the pole pieces the
bobbins are wound. The whole magnet structure is secured within the
shell _1_ by means of a screw thread on the block _7_ which engages a
corresponding internal screw thread in the shell _1_. As a result of
this construction the whole magnet structure is bound rigidly to the
shell structure at a point close to the diaphragm, comparatively
speaking, and as a result of this close coupling, the relation between