GB705330A - Improvements relating to electromagnetic telephone instruments - Google Patents

Abstract

705,330. Sound receiving and transmitting devices. SIEMENS BROS & CO., Ltd. Nov. 7, 1952 [Nov. 9, 1951], No. 26338/51. Class 40 (4) In a telephone instrument of the type employing an annular armature, the magnetic system is so disposed that there is no magnetic flux in the armature due to the permanent magnet. Pole shoes 2, 3 on each side of an annular permanent magnet 1 have pole pieces 4, 5, 6, 7 defining an air gap in which a rim-clamped annular armature 8 supporting an acoustic diaphragm 14 operates. The acoustic diaphragm which may have stiffening ribs 15 is cemented to the inner circumference of the armature and the outer edge of the armature is clamped by screws 16 between two resilient spacers 11 and 12. The screws 16 pass through cut away portions 22, Fig. 2, in the edge of the armature which may be so dimensioned as to give a desired compliance. The operating coils 9, 10 are located between pole-pieces 4, 5 and 6, 7 respectively, but in other embodiments, Figs. 3-5 (not shown), they are located either between a single polepiece and the spacer or in an annular recess in the permanent magnet itself. These further embodiments also disclose the employment of a permanent magnet on each side of the armature by means of which the rim is clamped. The reduction of compliance thereby introduced is compensated by reducing the thickness of the armature in the area between the magnet and the air gap or by drilling holes at intervals. The manner of connecting the coils to the insulated terminals 20 in the outer casing is also described.