GB248412A - Improvements in electromagnetic acoustic devices - Google Patents

Abstract

248,412. Western Electric Co., Ltd., (Western Electric Co., Inc.). Sept. 9, 1924. Electromagnets.-Relates to telephone receivers and like instruments in which the steady magnetic pull on the armature is wholly or partly balanced in all positions of the armature. According to one feature of the invention, the balancing is effected solely by the inherent characteristics of springs or other resilient means, and according to another feature, the balancing is effected in such a manner as to give a resultant restoring force on the armature proportional to its displacement from its mean position. The armature system may have a resultant elasticity which is a function of the frequency over a wide range of frequencies. In the construction shown in Figs. 1 and 2, the armature 23 connected to the diaphragm 33 is supported between the bifurcated pole-pieces 18, 19 of a permanent or electromagnet 11 by means of short springs 30, 31 fixed at both ends to an E-shaped support 22 of nonmagnetic material. The line coils 20, 21 are arranged as shown and the armature 23 may also be supported by a pin 34 at its centre. The springs 30 may be so designed as to vary the restoring force at exactly the same rate as the pull due to the steady magnetic field, in which case the armature would remain in any position to which it was moved. In order to prevent freezing of the armature to the pole-pieces, the springs may provide a restoring force slightly greater than the force due to the magnetic field by an amount proportional to the displacement. A force giving a low and constant elasticity over a wide range of frequencies of such a value as to give the vibrating system a natural period of about 50 to 200 cycles per second is suitable. In another construction the armature 64, Figs. 7 and 8, connected to a diaphragm 68, passes through line coils 61 arranged between pairs of polar extensions 60 of pole-pieces 58, 59 of the magnet 57. The armature is supported by torsion members 63 screwed to the pole-pieces and is connected to a member 69 the ends of which are supported between elastic cushions 70 built up of sheets of thin foil or paper. The cushions are clamped between parts 71 - - 74 which are bevelled so as to provide a larger space for the inner ends of the sheets. The presence of air between the sheets modifies the elastic properties of the system in such a way that the elasticity varies with the frequency. In a modification, the armature is connected to a plunger arranged between thin discs of foil or paper in a cylindrical chamber. Heavier discs may be arranged between the thin discs to increase the mass of the cushioning system. In the construction shown in Fig. 5, in which the resultant restoring force is made proportional to the displacement of the armature 42, the latter is supported by a convex bearing surface 41 against which it is held by a resilient member 43 having a knife edgo engaging a, groove in the armature. As the armature vibrates, the instantaneous centre varies and consequently the resultant force.