Patent GB-814518-A (GB)
Title: Improvements in and relating to electromagnetic bells
Inventor: MELTON RONALD HENRY
IPC: G10K1/26
Filed: 1957-09-09
Abstract: 814,518. Electric bells. ERICSSON TELEPHONES Ltd. Sept. 9, 1957, No. 28414/57. Class 13. In a flame-proof electromagnetic bell the casing 1, of non-magnetic material such as cast gun metal, encloses two sealable chambers 9 and 18 separated by a dividing wall 8, the first chamber 9 having connecting wires led into it through a flameproof cable sealing gland 15, such as is described in Specification 774,662, [Group XXXVI], and connected via terminals 32 sealed through the dividing wall 8 to the windings 31 of an electromagnet housed in the second chamber 18. The pole pieces 21, 25 of this magnet are sealed through an exterior wall of the chamber 18, and an armature 37 is mounted externally of the casing 1. Movement of this armature, which occurs when alternating current is fed through the windings 31, causes the strikers 43 to strike the gong 48. As shown, the casing 1 consists of a base portion 2 and a lid 3 hinged together by means of lugs 4, 5. The chamber 9 in the base portion 2 contains brass terminals 13, mounted on a plate 12 of insulating material, on which the incoming conductors are terminated. Three feet 16 are provided with holes to enable the base portion 2 to be fastened to a wall or the like by screws. The chamber 9 is sealed by the plate 8 which is clamped between the plane faces 6, 7 of the base portion 2 and the lid 3 by bolts 50. Mid-way along a side wall 19 of the chamber 18 is a tapped hole 20 into which a pole piece 21 is screwed from the inside, this pole piece 21 having a recess 22 in its lower face (Fig. 4) into which the centre limb 59 of an E- shaped electromagnet fits. The centre limb 59 is a permanent magnet whereas the outer limbs 23 are not. On each side of the hole 20 there is a hole 24 in which the lower part of a pole piece 25 forms a sliding fit, the lower end of each pole piece 25 screwing into a tapped hole 26 in the end of one of the outer limbs 23 of the electromagnet. The pole pieces 25 are locked in position by screwing the horizontal portion of an L-shaped bracket 29 to the lid 3, the vertical portion of the bracket 29 having projections 30 which co-operate with notches 28 on the flanges 27 of these pole pieces. The windings 31 on the outer limbs 23 are connected to the terminals 13 by way of two screws 32 which pass through insulating collets in two holes in the plate 8 and are provided with wiring tags such as 54 extending into the two chambers 9 and 18. As shown in Fig. 5 an armature 37 is mounted so as to be free to rock about a pivot pin 58 passing through a hole 57 (Fig. 4) in the centre pole piece 21. The upper face of this pole piece has secured thereto a plate 41 having upstanding ears 42 which carry strikers 43. When the armature rocks in response to alternating current applied to the windings 31 the balls at the ends of the strikers 43 alternately strike the gong 48 (Fig. 1). The chamber 18 is sealed by the plate 8 which is fastened to the plane face 7 thereof by countersunk screws passing through the plate 8 into tapped holes 60 in the lid 3. The triangular heads of the bolts 50 which fit within raised perimeters integral with the lid 3 prevent unauthorized access to chambers 9 and 18. Flameproofing requirements.-The plane faces 6, 7 and the co-operating plane faces of the plate 8 are suitably worked, as by a grinding process, to provide surfaces which, when fastened together as described above, form a flameproof seal between the chambers 9 and 18 and the surrounding atmosphere. The surfaces of the pole pieces 25 and the co-operating surfaces of the wall 19 are similarly worked, and the threaded surfaces of the pole piece 21 and wall 19 are sealed as by soft solder 39 (Fig. 4). Modification.-The pole pieces may, if desired, be integral with the limbs of the electromagnet, and each may be provided with an integral flange abutting the inner surface of the wall, another flange being swaged down against the outer surface of the wall. Specification 537,568 also is referred to.