Patent GB-892918-A (GB)
Title: An electromagnetic telephone receiver
IPC: H04M1/03
Filed: 1960-03-18
Abstract: 892,918. Telephone receivers. TESLA NARODNI PODNIK. March 18, 1960 [March 23, 1959], No. 9690/60. Class 40 (4). In a telephone receiver having two permanent magnets 1, 2 positioned in alignment and magnetized in the direction of their alignment, with one pole face of one magnet facing the pole face of opposite polarity of the other magnet, a clamp of magnetizable material, having two jars 3, 4 positioned in alignment with the magnets, holds them in position, one end of an armature 5 being held firmly between facing surfaces of adjacent ends of the jaws 3, 4, and the other end being firmly held either at its edges in position in an air gap between the said pole faces (Fig. 2), or by holding the other end between the other pair of opposing ends of the jaws 3, 4 (Fig. 8), the arrangement being such that substantially no flux passes through the armature 5 in its rest condition, and a depending part 5a being provided on the vibratable portion of the armature 5 for connection to the receiver diaphragm. The clamp also holds the bobbin 6 (Fig. 2), of coil 22 in position so that the armature 5 passes through its axial bore. In the first embodiment the two facing ends of the jaws 3, 4 which hold the armature are connected by rivets 10 (Fig. 2) or screws, and their other ends are bent towards each other to form stops for the magnets 1, 2. Spacing strips are inserted in notches near the edge of the armature 5 at its left-hand end (Fig. 2), to hold the borders of the armature central in the air gap while allowing free vibration of its middle part. The magnetic system is housed in a case 9 (Fig. 1), of plastic material to which it is fixed by a projection 11, on one of the jaws 4, which passes through an opening in a metallic washer 12 (pressed into the wall of the case) to which it is soldered. The magnetic system is located by a baffle 14 pressed against the jaw 3 when the lid 16 is closed, coaxial openings in the jaw 3 and baffle 14 permitting free passage of the pin 5a which connects to the diaphragm 13. A washer 23 of textile fabric or netting provides an acoustic impedance. Speech currents are connected to the coil 22 by means of a brass ring 17 to which one end of the coil is soldered, the other end being welded to one of the spacing strips, whereby the current passes through the jaw 4 into the washer 12. The armature 5 has a cut-out portion 20 (Fig. 9), to increase the reluctance of the armature at that part and so compel the bulk of the alternating flux to pass through the air gap. The armature 5 being of high permeability material the magnetic potential between the facing surfaces of jaws 3, 4 is very small and substantially no flux flows in the armature in its rest condition. In the second embodiment (Fig. 8), both ends of the armature 5 are fixed between facing surfaces of the jaws 3, 4 by solder 18, 19 or a suitable adhesive. The magnetic system is held in case 9 (Fig. 7) by a screw 21 engaging in jaw 4 and is prevented from rotating in the case by inner projections of the case between which it is inserted. An acoustic impedance and means for leading in to the coil are provided as in the first embodiment.