How Vacuum Tubes Detect Tiny Changes in High-Resistance Sensors
A 1973 circuit design using a vacuum tube to detect microscopic resistance shifts in sensors like ionization chambers, commonly used in early smoke detectors.
Original patent title: “Circuit for detection of small resistance changes in ionization chamber devices”
A 1973 circuit design using a vacuum tube to detect microscopic resistance shifts in sensors like ionization chambers, commonly used in early smoke detectors. Granted to Central Investment Corp in 1973 with 5 claims and 2 forward citations, and it is now in the public domain.
Coverage
What does this patent actually cover?
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.
The gap
What does this patent 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.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
Key facts
What made this novel
The circuit forces the vacuum tube to operate in a state where grid current is always flowing, effectively turning the tube into a high-impedance bridge that matches the resistance of the sensor itself.
The Patent Drawing
Schematic visualization of the patent's claim structure. Hand-drawn diagrams in progress for each landmark patent.
Where you've seen this
Real-world examples
Early electronic smoke detectors
Industrial ionization-based gas leak sensors
Why it matters
The bigger picture
This technology was foundational for the development of early electronic smoke and fire detection systems. By allowing a simple vacuum tube to act as a high-gain amplifier for extremely weak electrical signals, it enabled reliable, automated monitoring of environmental conditions before modern integrated circuits became affordable.
Filed
December 4, 1969
Granted
May 22, 1973
Market context
Who's building on this
Companies in this space
While the specific vacuum tube technology is largely obsolete, the principles of high-impedance sensing are now implemented by companies like Honeywell and Siemens using modern CMOS (Complementary Metal-Oxide-Semiconductor) sensors.
Market impact
This patent represents a period where industrial safety systems transitioned from mechanical or simple bimetallic switches to electronic monitoring. It helped define the architecture for ionization-based fire detection, which became a standard safety requirement in residential and commercial buildings for decades.
Claim 1 — Plain English
What this patent covers
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.
The clever bit
The circuit forces the vacuum tube to operate in a state where grid current is always flowing, effectively turning the tube into a high-impedance bridge that matches the resistance of the sensor itself.
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.
Patent timeline
Application submitted to the patent office
Application published, typically 18 months after filing
Patent officially issued
Patent enters public domain
This patent is in the public domain
See the Freedom to Build guide — what is free to use, what is not, and how to cite this patent.
PatentBrief Score
Impact Score
Limited data
Citation count
10/40
Early citations
Claim breadth
3/20
Moderate scope
Recency
0/20
Older than 20 years
Assignee scale
0/20
Independent or smaller assigneeassigneeThe entity that owns the patent — usually the inventor's employer or a company.Read more →
PatentBrief Impact Score — based on citation count, claim breadth, recency, and assignee scale. Not a legal assessment.
Heuristic Value Estimate
What this patent might be worth
$5K – $14K
Midpoint $9K · expired or expiring · industry ×1.5
Heuristic only — blends forward/backward citation counts, claim scope, time remaining, litigation history, and CPC-derived industry baseline. Real valuations need a professional appraisal.
Patent Claims
0 independent claims · 1 dependent
Claims are the legal boundaries of the patent. An independent claim stands alone. A dependent claim adds limitations to its parent, narrowing — but not broadening — the scope.
The original legal language
Original claims
5 claims as filed with the patent office.
Concepts involved
Citations
Patent lineage
Cite this patent
Blackwell, L. (1973). How Vacuum Tubes Detect Tiny Changes in High-Resistance Sensors (U.S. Patent No. 3,735,375). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/3735375/smoke-detector-ionization
Auto-generated from the patent record. Double-check author order and the issue date against the official USPTO document before submitting.
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Common Questions
Frequently Asked Questions
What does How Vacuum Tubes Detect Tiny Changes in High-Resistance Sensors cover?
A 1973 circuit design using a vacuum tube to detect microscopic resistance shifts in sensors like ionization chambers, commonly used in early smoke detectors.
Who owns patent US 3735375?
Central Investment Corp owns this patent, granted in 1973.
When does this patent expire?
This patent has expired and is now in the public domain — anyone can use the invention freely.
What is patent US 3735375 cited by?
This patent has been cited by 2 later patents that build on its ideas.
What problem does this patent solve?
This technology was foundational for the development of early electronic smoke and fire detection systems. By allowing a simple vacuum tube to act as a high-gain amplifier for extremely weak electrical signals, it enabled reliable, automated monitoring of environmental conditions before modern integrated circuits became affordable.
What does this patent NOT cover?
Does not cover modern solid-state sensors that use transistors or microcontrollers instead of vacuum tubes.
Patent monitoring