containing shell was of hard rubber consisting of three pieces, the
barrel _4_, the ear-piece _5_, and the tail cap _6_. The barrel and
the ear piece engaged each other by means of a screw thread and served
to clamp the diaphragm between them. The compound bar magnet was held
in place within the shell by means of a screw _7_ passing through the
hard rubber tail cap _6_ and into the tail block _3_ of the magnet.
External binding posts mounted on the tail cap, 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