GB351560A - Improvements in or relating to magnets for sound reproducing or recording instruments of the moving coil type - Google Patents

Abstract

351,560. Casting. SWIFT LEVICK & SONS, Ltd., HORSBURGH, G. D. L., and LEVICK, H. S., Clarence Steel Works, and DARWINS, Ltd., Fitzwilliam Works, Templeborough, both in Sheffield. March 28, 1930, No. 10007. [Class 83 (i).] Casting processes ; moulder's tools.-A permanent or electromagnet of the kind comprising a gap formed between concentrically arranged polepieces for sound reproducing or transmitting instruments of the moving-coil type has its central pole or a polepiece thereon initially made movable and correctly positioned relatively to the other pole by means which hold it so positioned while it is permanently secured in the correct position. The invention is particularly suited for use in connection with a permanent magnet of the kind described in Specification 340,207, [Group XXXV]. In the construction, Fig. 1, a magnet A is formed with concentrically arranged poles B, C, the centre pole being provided with a separate polepiece B<1>. This polepiece is formed with a reduced concentric extension b<1> adapted to enter a hole D in the pole B. When the parts are in this condition a tube E of subssantially the same dimentions as the required gap is inserted between the outer pole C and the polepiece on the pole B<1> so as to position correctly the polepiece on the pole B. Molten substance, such as an alloy of lead and antimony, is then poured into the hole D, and when this substance G has just solidified it may be subjected to pressure as by means of a ram F. Finally, the tube E is withdrawn by screwing a bolt through the hole e<1> so that its end presses against the end of the polepiece. The surface of the extension bl may be roughened or grooved and its diameter may be enlarged as at b<2>, Fig. 3.