{
  "patent_number": "US 3735375",
  "country": "US",
  "title": "How Vacuum Tubes Detect Tiny Changes in High-Resistance Sensors",
  "original_title": "Circuit for detection of small resistance changes in ionization chamber devices",
  "summary": "A 1973 circuit design using a vacuum tube to detect microscopic resistance shifts in sensors like ionization chambers, commonly used in early smoke detectors.",
  "what_it_does": "This patent describes an electronic circuit designed to monitor high-resistance sensors, such as ionization chambers, by using a vacuum tube as a sensitive switch. The circuit biases the tube's control grid so that it is always conducting a small, steady amount of current, which allows it to detect even tiny fluctuations in the resistance of the connected sensor. When the sensor's resistance changes—perhaps due to smoke particles entering an ionization chamber—the circuit triggers a relay. It also incorporates positive feedback to ensure that once a threshold is reached, the switch activates decisively rather than flickering.",
  "what_it_does_not_cover": [
    "Does not cover modern solid-state sensors that use transistors or microcontrollers instead of vacuum tubes.",
    "Does not cover digital signal processing methods for detecting resistance changes.",
    "Does not cover circuits that do not maintain a constant flow of grid current during operation."
  ],
  "filed": "1969-12-04",
  "granted": "1973-05-22",
  "expires": "1990-05-22",
  "status": "expired",
  "holder": "Central Investment Corp",
  "holder_url": "https://patentbrief.org/company/central-investment-corp",
  "inventors": [
    {
      "name": "L Blackwell",
      "url": "https://patentbrief.org/inventor/l-blackwell"
    }
  ],
  "times_cited": 2,
  "tags": [
    "consumer_electronics",
    "mechanical"
  ],
  "abstract": "Electronic circuit for detecting small resistance changes in conductors of high resistance wherein the conductor is connected between grid and cathode of a tube connected as a triode and the grid bias is adjusted almost to the point necessary for tube current cut-off, such that some significant amount of grid current flows at all times. The high resistance conductor may be a sensing element such as an ion chamber. The level of grid current serves to identify an alarm, no-alarm or an outage condition with such a sensing device.",
  "url": "https://patentbrief.org/patent/us/3735375/smoke-detector-ionization",
  "markdown_url": "https://patentbrief.org/patent/us/3735375/smoke-detector-ionization/md",
  "google_patents_url": "https://patents.google.com/patent/US3735375",
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}