GB719179A - Improvements in and relating to electric signalling apparatus - Google Patents

Abstract

719,179. Detecting foreign bodies electrically. AKTIEBOLAGETTELEVAKT. Dec. 16, 1952, [Dec. 18, 1951.] No. 31893/52. Class 37. Apparatus for indicating a change in the condition at a location comprises two similar oscillator circuits tuned to a common frequency, and supplied with antennae so that a change in condition as produced e.g. by the presence of a body, loads one antenna to produce a difference in frequency between the two oscillators, which difference is detected and energizes an alarm circuit. As shown, two triodes 1, 2 with common cathode resistor, 17, have similar tuning circuits 3, 4 tuned to the same frequency, inductively coupled in opposition to a triode 6. Normally the beat frequency is zero, but with the presence of a body near one of the antennae A 1 , A 2 a beat frequency is produced which, after rectification at 6, is amplified to supply negative bias to a triode 9 thereby cutting its anode current to allow a relay Ri to energize and close an alarm circuit A via contacts 12. The circuit is reset by manually operated contacts 14 in parallel with normally closed contacts 13 of relay R 1 . The alarm beat frequency may be aurally detected by phones T in the anode circuit of triode 6. Failure of the supply voltage de-energizes a relay R 2 to break the alarm circuit A at contacts 15, and close a warning circuit F over contacts 16.