GB337836A - Improvements in or relating to sound reproducers or transmitters - Google Patents

Abstract

337,836. Matthew, E. M. July 1, 1929. Divided on 331,828. Magnetic instruments.- A moving coil sound transmitter or reproducer, such as a loud-speaker, is associated with switchgear and surge preventing devices which protect the moving coil and field exciting windings when these windings are being brought into or out of operation. The switchgear may either be mounted on the loud-speaker or employed as a remote control. On switching on the moving coil circuit a variable resistance of the order of one megohm is introduced into the circuit and is gradually cut out until it is inoperative when the circuit is completed. An anti-sparking or buffer resist. ance is also connected in parallel with the field exciting circuit immediately prior to and during connection of this circuit to the source of supply, the resistance being inoperative when the circuit has been completed. The switch is so arranged that the moving coil circuit is closed before the field exciting circuit, the latter being broken before the moving coil circuit. If desired, the. variable resistance may be completely traversed before the field buffer resistance is connected. Figs. 8 and 9 show a construction of switch in which the body B is provided with a recess D<1> for housing the moving coil variable resistance which contacts with a number of metallic segments S<1> - - S<5> adapted to be traversed by a spring loaded metallic plunger or flexible strip P connected to one metallic pivot G of the switch barrel E. The segment S<5> is connected to a terminal T<1> to which the moving coil M is connected whilst the pivot G is connected to a terminal T<2>. If desired, the segments may be dispensed with and the resistance itself traversed by the arm P. A metallic segment H connected to the metallic pivot K is fixed to or moulded with the barrel E and traverses over spring- loaded contacts L, N. The contact N is connected to a terminal T<4> which is connected to the field winding F and the pivot K is connected to a terminal T<5>. The buffer resistance is housed in a recess D<2> in the body B and has one of its ends connected to contact L and the other to a terminal T<6>. The pivot G may be located in the body B by a metallic bushing and the pivot K by a similar bushing in the switch cover (not shown), rotation of the barrel E being limited by pins fixed to one or both pivots and working in recesses in the body and cover.