GB310839A - Improvements relating to automatic burglar alarms and to electrical instruments for use therefor - Google Patents

Abstract

310,839. Egai, A. May 1, 1928, [Convention date]. Theft and like alarms. - In a burglar alarm system, guard wires 26<1>, 26<2> are connected in series with differen tially acting coils 7, 8, of an ohmmeter, and a resistance is so adjusted that a contact carried by the ohmmeter needle 2 is normally midway between two fixed contacts 19<1>, 19<2>. If either of the guard wires 26<1>, 26<2> is tampered with so as to vary its resistance, the pointer 2 makes contact with one of the fixed contacts 19<1>, 19<2> and short-circuits a normally energized relay 21, causing the armature 22 to drop and close the circuit of alarm bells 25<1>, 25<2>. The alarm is set by means of a lock 30, the bolt 31 of which during its travel engages contacts 32', 32<2>, 32<3>, and at the ends of its travel operates through an insulating member a contact 33 connected to the battery 9. The contact 33 co-operates with a contact 34 which can also be connected, by operating a button 35 with a contact 36 connected to the positive pole of a battery 24<1>. By shooting the bolt 31 and depressing the button 35, the battery 24<1> is put in series with the battery 9 and with the relay coil 21, thus lifting the armature 22. When the motion of the bolt 31 is completed, the contacts 33, 34 are separated, disconnecting the battery 24<1>, but leaving the battery 9 connected to the relay coil 21, which is thereby sufficiently energized to retain the armature 22 in its raised position. A milliammeter 40 may be provided in the relay circuit.