GB331209A - Improvements in magnets - Google Patents

Abstract

331,209. Voigt, P. G. A. H. March 25, 1929. Magnets; electromagnets.-In a magnet of the type used in moving-coil loud-speakers, particularly a magnet as illustrated in Specifications 12141/11; 275,571 and 288,276, [both in Class 40 (iv), Telephones &c,], the central stem or peg is so shaped that the area of the path available for the flux in the iron is a minimum at the gap and then increases towards the base of the peg where it is a maximum, the area at a distance equal to the gap radius from the top of the peg being at least 50 per cent greater than the area at the gap. To this end the peg is made of stepped or smoothly-tapering form, the degree of taper decreasing towards the base of the peg. In order that the area of the path at the gap shall be a minimum, the axial length of the gap must be less than one-fourth, but not less than onefifteenth, of the inner diameter of the gap. The yoke is U-shaped and its cross-section is not less than that of the peg at corresponding points. The front plate is increased in thickness as the distance from the gap increases. In the form shown, the area of the flux path at the gap is #dL where d and L are the inner diameter and axial length of the gap respectively, and as this area must be less than the cross-sectional area of d the peg at any point, L must not exceed -. If 4 the iron is out away as at C, the gap length must be still smaller, so that #dL is less than the crosssection at C. The thickness of the top plate H increases away from the gap, the metal increasing in thickness in a direction away from the base. The top plate may be slotted at K to reduce leakage. The magnetizing coil E is preferably tapered internally to fit the peg.