How Early Telephone Systems Managed Busy Lines Automatically
A 1925 patent describing a mechanism to automatically signal that a telephone line is occupied, preventing callers from interrupting an active conversation.
Original patent title: “Make-busy scheme for telephone systems”
A 1925 patent describing a mechanism to automatically signal that a telephone line is occupied, preventing callers from interrupting an active conversation. Granted to Automatic Electric Co in 1925.
Key facts
Coverage
What does this patent actually cover?
The patent describes a make-busy circuit designed for automatic telephone exchanges. When a specific telephone line is currently in use, this circuit applies a potential—a specific electrical voltage—to the test terminal of that line. This signal effectively marks the line as busy, so that if another caller attempts to connect to it, the automatic switching equipment detects the signal and prevents the connection from being completed.
The gap
What does this patent NOT cover?
- Does not cover digital or packet-switched communication networks.
- Does not cover call-waiting features that allow a second caller to interrupt an active line.
- Does not cover software-based line management systems.
- Does not cover wireless or cellular telephony.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
What made this novel
The invention uses the electrical state of the test terminal itself to signal busy status, allowing the mechanical switch to 'read' the line's availability before attempting a connection.
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 20th-century automatic telephone exchanges
Strowger automatic switching systems
Why it matters
The bigger picture
This technology was essential for the transition from manual operator-connected calls to automated switching systems. By preventing multiple users from connecting to the same line simultaneously, it provided the reliability required for large-scale telephone networks to function without human intervention.
Filed
August 28, 1922
Granted
May 12, 1925
Market context
Who's building on this
Companies in this space
The Automatic Electric Company was a pioneer in the Strowger switch technology, which laid the foundation for modern automated telephony. While this specific mechanical patent is obsolete, the logic of signaling line status remains a fundamental concept in network traffic management.
Market impact
This patent helped standardize the reliability of the early automatic telephone network. It enabled the rapid expansion of telephone infrastructure by reducing the need for human operators to monitor line availability, which was a critical bottleneck for network growth in the 1920s.
Claim 1 — Plain English
What this patent covers
The patent describes a make-busy circuit designed for automatic telephone exchanges. When a specific telephone line is currently in use, this circuit applies a potential—a specific electrical voltage—to the test terminal of that line. This signal effectively marks the line as busy, so that if another caller attempts to connect to it, the automatic switching equipment detects the signal and prevents the connection from being completed.
The clever bit
The invention uses the electrical state of the test terminal itself to signal busy status, allowing the mechanical switch to 'read' the line's availability before attempting a connection.
What it does not cover
- Does not cover digital or packet-switched communication networks.
- Does not cover call-waiting features that allow a second caller to interrupt an active line.
- Does not cover software-based line management systems.
- Does not cover wireless or cellular telephony.
Patent timeline
Application submitted to the patent office
Application published, typically 18 months after filing
Patent officially issued
PatentBrief Score
Impact Score
Limited data
Citation count
0/40
No citations yet
Claim breadth
0/20
Narrow claimsclaimsThe numbered statements at the end of a patent that legally define what the inventor owns.Read more →
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
$2K – $7K
Midpoint $4K · expired or expiring · industry ×1.4
Heuristic only — blends forward/backward citation counts, claim scope, time remaining, litigation history, and CPC-derived industry baseline. Real valuations need a professional appraisal.
Concepts involved
Cite this patent
Willis, B. D. (1925). How Early Telephone Systems Managed Busy Lines Automatically (U.S. Patent No. 1,537,326). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/1537326/insulin-extraction-and-purification
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 Early Telephone Systems Managed Busy Lines Automatically cover?
A 1925 patent describing a mechanism to automatically signal that a telephone line is occupied, preventing callers from interrupting an active conversation.
Who owns patent US 1537326?
Automatic Electric Co owns this patent, granted in 1925.
When does this patent expire?
This patent has expired and is now in the public domain — anyone can use the invention freely.
What problem does this patent solve?
This technology was essential for the transition from manual operator-connected calls to automated switching systems. By preventing multiple users from connecting to the same line simultaneously, it provided the reliability required for large-scale telephone networks to function without human intervention.
What does this patent NOT cover?
Does not cover digital or packet-switched communication networks.
Patent monitoring