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 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 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.