GB551729A - Improvements relating to electromagnetic telephone instruments - Google Patents

Abstract

551,729. Telephone instruments. SIEMENS BROS. & CO., Ltd., and SUTTON, G. W. Oct. 10, 1941, No. 13102. [Class 40 (iv)] In an electromagnetic telephone instrument having an armature of magnetic material which is supported at one place along its length and is free to vibrate at least at one of its ends, the armature has about it an electrical conductor closed upon itself and having a resistance such that it diminishes the rise of impedance with frequency of the instrument without producing substantial change in the amplitude-frequency response. The conductor may be a lapping of metal round the armature, such as a band or a winding of wire, or it may be formed by a deposit of a good conducting metal, such as copper or silver, on the armature. Fig. 2 shows an armature having slits 6 forming outer limbs 9 which support it so that it vibrates substantially about its centre line. A short-circuited copper band 5, which may be of copper strip 4 or 5 thousandths of inch thick, surrounds the armature close above the axis of vibration. It may be replaced or supplemented by a similar band 5a below the axis. In a modification, the armature is similarly slotted from both ends to form side supporting members which extend its whole length and are joined to it at the centre by narrow strips left between the inner adjacent ends of the slots. An armature supported at one end is also described. In all cases the supporting portions are gripped in U-section members four of which serve to space the pole pieces. Fig. 4 shows an instrument comprising pole-shoe stampings 11 with pole-pieces 12, 13 which are clamped to a magnet 14 and also to the members 4 carrying the armature 1 by screwing them to pillars 15.