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586 RICHARDSON'S BLUEBOOK OF PROJECTION
monitors. The monitor in Figure 139 is a magnetic speaker, as shown by the crescent-shaped magnet placed just left of the winding. Magnetic speakers that are essentially only large telephone receivers equipped with long "lily" horns have been used in some theatres to increase the audibility of the vaudeville performance. The sneakers are scattered throughout the house, especially in the upper balcony, where the sound of vaudeville is most likely to need reinforcement. This method, however, is not typical of modern "reinforcement" practice, which is outlined briefly on Page 529.
The commonest type of magnetic speaker, and the type most often used for monitor work in the projection room, does not have a horn, but a "cone," which is a diaphragm from six to twelve inches across, usually made of paper. The magnetic fields of course cannot operate directly on a paper diaphragm. An armature is provided, consisting of a small bar of metal pivoted to respond to the fluctuating magnetic field, and connecting with the center of the diaphragm through a tiny lever. The elasticity of the paper cone pulls the armature away from the center of magnetic attraction, thus opposing the pull of the magnets. When the magnetic pull increases the center of the cone is drawn inward, and when the magnetic pull relaxes the elasticity of the cone causes its center to rebound outwards.
This arrangement needs no horn to provide a load for the diaphragm, since the area of the cone is large enough to act upon a sizeable body of air. At extreme volume magnetic speakers of this type will rattle, because the small metal armature will touch the pole-pieces if it moves far enough, but even a small speaker of this kind can produce sufficient volume for an average projection room before the point of rattling is reached.
(3) Magnetic speakers usually have two coils of wire, as most telephone receivers have (some have been made with three) but these coils are speech or voice coils. There is no field coil in a magnetic speaker, because its primary field is provided by a permanent magnet. The function