p, as shown, were
connected by heavy leading-in wires to the terminals of the
electromagnet.

A casual consideration of the magnetic circuit of this instrument will
show that it was inefficient, since the return path for the lines of
force set up by the bar magnet was necessarily through a very long air
path. Notwithstanding this, these receivers were capable of giving
excellent articulation and were of marvelous delicacy of action. A
very grave fault was that the magnet was supported in the shell at the
end farthest removed from the diaphragm. As a result it was difficult
to maintain a permanent adjustment between the pole piece and the
diaphragm. One reason for this was that hard rubber and steel contract
and expand under changes of temperature at very different rates, and
therefore the distance between the pole piece and the diaphragm
changed with changes of temperature. Another grave defect, brought
about by this tying together of the permanent magnet and the shell
which supported the diaphragm at the end farthest from the diaphragm,
was that any mechanical shocks were thus given a good chance to alter
the adjustment.

[Illustration: Fig. 49. Single-Pole Receiver]

Modern Receivers. Receivers of today differ from this old
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 o