147. Construction.- The RCA Photophone loudspeaker units are of the dynamic cone type, and are constructed to conform to the above general requirements. They consist essentially of two main parts; an iron housing, or "pot," which is magnetized by a field winding, and a movable cone, on which is mounted a "voice coil." The magnetic field is produced by a powerful electro-magnet made up of a large coil of wire wound on an iron core and enclosed in the iron housing in such a way that the housing itself is a part of the electro-magnetic circuit. The purpose of this is to bring the two magnetic poles very close to each other. A study of Figure 97 will show how this is accomplished. The left end of the core is one pole, while the other pole is the left end of the iron housing surrounding the end of the core. Between them is located the voice coil, a sectional view of which is shown. By bringing the poles together in this manner the lines of force from one pole to the other, when the field coil is energized, are concentrated in a very small area.

The cone is about eight inches in diameter at its outside edge, or mouth, and is made of heavy paper impregnated with a moisture resisting compound. It is corrugated in concentric rings to improve the quality of reproduction. At the top, or apex, of the cone is placed a cylindrical fibre ring about an inch in diameter and onequarter inch wide, cemented to the paper of the cone. On this is wound the "voice coil," a small coil of fine wire which is the end of the circuit carrying the amplifier output. This ring, with the coil on it, is centered in a position in the air gap between the two poles of the field. It is held in place by a screw passing through the center of a piece of "aeroplane cloth" stretched over the end of the fibre ring and at that end cemented to the cone. The monitor speaker, however, uses a thin piece of perforated fibre instead of aeroplane cloth. The outer edge of the cone is cemented to a very light, thin, flexible ring of leather, which in turn is secured to an iron ring flange rigidly supported on the field housing. When supported in this way the cone is free to move in one plane, along its axis, due to the flexibility of the aeroplane cloth and leather ring supporting the cone. This means that the voice coil may move in and out along the air gap.

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