GB905576A - An electromagnetic device - Google Patents

Abstract

905,576. Transducers. KNOWLES, H. S. April 20, 1959 [April 22, 1958], No. 13401/59. Class 40 (4). In an electromagnetic transducer including two permanent magnets 33A, 33B, Fig. 3, secured on opposite sides of a coil 20, a balanced armature 27 works in a space between two polepieces 22, 23, Fig. 2, and is clamped at one end in a clamping gap formed by non-magnetic spacers 28W while its other end is capable of vibration in a working gap 29, a flux-conductive and bendable tab being provided to adjust the reluctance between the polepieces in the gap adjacent the clamped part of the armature. The tab may be a longitudinal extension 40 of the armature 27. Alternatively it may be a thin magnetic shim 51, Figs. 6, 6A, clamped with the armature 50 between non-magnetic spacers. In a modification the armature is T-shaped, having bendable arms integral with the leg portion, Figs. 7, 7A, not shown. The armature 60, Fig. 8, is flanked by transverse magnetic shims 61, 62 spaced from the polepieces by non- magnetic spacers 28W. Alternatively a flux conductive member 70, Fig. 9, is spot welded to pole piece 23 and adjustable relative to polepiece22. The magnets 33AX, 33BX, Fig. 5, may joint the upper polepiece 22 by a non- magnetic gap 65 to increase the reluctance into which the permanent magnets work, thereby producing little change in the total flux provided by the magnets when the reluctance is changed at the clamping gap. Permanent adjustments to the armature may also be made by knifing or bending or by producing certain strains in the armature. The device of Fig. 2 is a hearing-aid receiver, the armature 27 being connected by a link 32 to a diaphragm 31, a tube 19 being provided for insertion in the ear. In a microphone, Fig. 11, not shown, the cover plate 15 is partially covered by grill fabric.