Patent GB-394533-A (GB)
Title: Improvements in or relating to magnet systems of electromagnetic devices for converting electrical oscillations into mechanical vibrations or inversely
IPC: H04R11/00
Filed: 1932-10-07
Abstract: 394,533. Current and pressure meters. NAAMLOOZE VENNOOTSCHAP PHILIPS' GLOEILAMPENFABRIEKEN, 29, Emmasingel, Eindhoven, Holland. Oct. 7, 1932, No. 27972. Convention date, Jan. 23. [Class 37.] Relates to magnet systems of electromagnetic devices for converting electrical oscillations into mechanical vibrations or inversely, such as loud-speakers, microphones, gramophone pick-ups, measuring instruments or relays, having two or more pairs of polepieces and with an armature arranged in the air gaps formed therebetween and consists in adapting the restoring force exerted on the armature to the magnetic force in such manner that the curve of the restoring force when graphically plotted as a function of the position of the armature in the air-gap, over part of the latter coincides with or is at a small distance parallel or substantially parallel to the descending arms of the hysteresis curve obtained by plotting the magnetic force exerted on the armature against displacements of the armature in the air gap. The restoring force is preferably provided by springs of suitable dimensions. The resultant directive force, the curve of which also takes the form of a hysteresis loop, is therefore slight and nearly constant over a large part of the air gap, thus favouring the faithful reproduction of low frequencies. The range over which the directive force is small and substantially constant may be increased by forming the armature of a material which is not magnetically saturated until the deflections of the armature from the geometrical mid-position are larger than half the air gap and/or by suitably dimensioning the armature. Thus, the armature may be formed of a silicon-iron or nickeliron alloy and preferably one containing about 50 per cent of nickel and about 50 per cent of iron, this alloy having small hysteresis losses and an electric resistance at least three times as large as that of iron or nickel alone. The polepieces are also preferably formed of this material. An air gap of at least 0.3 mm. may thus be used particularly if the magnet employed is dimensioned as set out in Specification 382,514, [Group XXXIX], so that its volume is smaller than four times the volume of the air gap multiplied by the square of the field strength and divided by (BH) max. of the steel used, all magnitudes being expressed in C.G.S. units. The figures show the invention applied to the driving unit of a loudspeaker comprising a bar magnet 15 of U-shapedcross-section to which is clamped by a bolt 16 soft-iron pieces 17, 18 supporting four polepieces 19 .. 22 forming two pairs of air gaps 23, 24 in which the ends of an armature 25 are located. Rods 26, 27 on the ends of the armature are bridged by a rod 30 carrying the diaphragm actuating rod 30<1>, the rods 26, 27 being mounted on flat steel springs 28, 29 secured by plates 31 and bolts 32 to brass blocks 33 clamped to the pieces 17, 18 by the bolts 34, 35. The portions of the springs 28, 29 located in the recesses 37 of the blocks 33 are so dimensioned that the effect according to the invention is produced. As it is found in mass production that magnets of the same kind of steel and of the same dimensions and springs of determined dimensions always differ, auxiliary means are provided in the form of an adjustable magnetic shunt to ensure that the springs and magnetic field are exactly adapted to one another when the system is mass produced. In Figs. 4 and 5, this shunt takes the form of a soft-iron pin 38 located for the greater part in the space between the magnet limbs and for the remainder in a bore 39 in the supporting piece 18 in which it can be secured by the screw 40. Fig. 6 shows a modification in which the magnet consists of two bars 41, 42 having soft-iron supporting pieces 44, 45 clamped thereto by a bolt 43, the shunt being in the form of a soft-iron plate 46 adjustably mounted by screw 48 on the piece 45 so as to slide along the bottom of bar 42.