diaphragm. The process by which direct current in winding produces an aiding field is called poling, because the polarity
of the direct current must be correct.
(3) Permanent magnets operate uniformly if they are not subjected to shocks or other abuse. Sudden and violent
jarring partially destroys their magnetism, and makes them less effective in telephone receivers. A weak magnet exerts a
weaker normal pull on the diaphragm, causing unequal displacement on each side. This results in distorted vibration and
Structure of Modern Receiver.
a. C, figure 18, shows the front view and a cross-sectional side view of a modern receiver unit, designed to be
mounted in several types of telephone instruments. The receiver winding is wound around two permalloy pole pieces,
each of which is welded to a cobalt-steel bar magnet. These magnets are made of a recently developed magnetic alloy
which has high permeability, giving a strong magnetic field. The magnets and pole pieces are fastened to a zinc-alloy
frame. The diaphragm is made of a special steel alloy. It is not clamped, but rests on a ring-shaped ridge; the pull of the
magnets holds it in place. It is protected in front by a silk screen, and its vibration is controlled by a silk acoustic
resistance disk attached at the rear. The entire unit is held together by a brass clamping ring. Two silver-plated contacts,
for electrical connections, are mounted on the back.