You can freely build on How Vacuum Tubes Detect Tiny Changes in High-Resistance Sensors
This patent expired in 1990. Every claim — 0 independent, 1 dependent — is now unenforceable. Anyone can use, reproduce, manufacture, sell, or offer for sale this technology without a license.
Original assignee
Central Investment Corp
Patent granted
1973
Expired
1990
Forward citations
2
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.
What is now free to use
All 1 claims of US 3735375 are in the public domain. Specifically:
The 1 dependent claim add narrowing limitations and are also free.
What is NOT covered
Patent expiry frees this specific invention. Separately-patented improvements made after expiry may still be protected.
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.
Who is building on this today
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.
Products built on expired version of this technology
Early electronic smoke detectors
Industrial ionization-based gas leak sensors
How to cite this patent in your documentation
Central Investment Corp. US Patent 3735375. Circuit for detection of small resistance changes in ionization chamber devices. Granted 1973, expired 1990. Now in the public domain.
Note: This is a convenience citation. Consult a patent attorney for formal freedom-to-operate analysis.
PatentBrief is an educational resource and does not provide legal advice. Patent expiration information is derived from USPTO records and may not reflect continuation patents, divisional filings, or separately-patented improvements. For commercial use or production decisions, obtain a formal freedom-to-operate (FTO) opinion from a registered patent attorney.