US 500520

I'll run targeted searches on the specific number 500520 before summarizing.

Search results: US 500520

I searched for the literal number 500520 and did not substitute or "correct" it. Findings below; where I lack authoritative information, I say so.

Verification of the record

Textual note (uncorrected): the OCR/printed heading reads "B. P. WETMORE" in one place and "EARL P. WETMORE" in the specification and drawing signature ("Earl P. Wetmore"). I am reporting both as they appear rather than harmonizing them; the specification body and signature block support Earl P. Wetmore.

Abstract

There is no patent abstract. US 500,520 issued in 1893, long before abstracts were required on US patents. The "Definitions" block shown on Google Patents is machine-extracted text from the specification, not an official abstract. The closest thing to a stated summary, verbatim from the specification, is:

"This invention relates to electric heaters, the primary objects being simplicity and cheapness in construction. The invention involves the use of an alternating current transformer in which the secondary coil or circuit is closed upon itself, thus being a transformer in which the secondary coil has but one turn. The invention consists of the special construction of the secondary."

Independent claims — plain-language overview

Both claims are independent combination claims; claim 2 is not a dependent claim.

Claim 1 — An electric heater combining:

• a primary coil carrying alternating current;
• a metallic cylinder (preferably brass per the specification) acting as the secondary — a one-turn, short-circuited secondary — located inside the primary;
• an iron core inside the cylinder (specification: a cylindrical bunch of iron wire);
• removable heads closing the cylinder ends, each head having ports; and
• a chamber between the core and the cylinder for fluid to pass through.

Claim 2 — Everything in claim 1, plus a spiral partition inside the chamber (specification: a wire wound continuously end-to-end) that forces the fluid to travel a helical path around the core.

Plain-language gist: run alternating current through an outer coil; a shorted brass tube around an iron core gets hot by induced current; fluid entering through ports in the end caps spirals through the annular gap and carries the heat away. The specification stresses that the parts "are slipped one over the other" so the device is cheap to build and easy to assemble/disassemble. Each claim ends with the archaic "for the purpose set forth," which is not itself a separate limitation.

Sibling / related art (relevant for context)

A companion patent issued the same day, US 500,272 A, "Electric heater," naming Charles E. Roehl (Chicago), Sidney Z. Mitchell (Portland), and Earl P. Wetmore (Helena) — filed December 23, 1892, Serial No. 456,139 (13 serial numbers after Wetmore's 456,123). It claims a related transformer-type heater using a coiled pipe secondary bridged into multiple closed circuits. This supports the reading that the 1892 filing pair reflects a common development effort among Wetmore, Mitchell, and Roehl.

CAFC 2026 dockets — no results, and none can exist

I found no CAFC docket, petition, appeal, or 2026 proceeding involving patent 500520. This is expected and not merely a search gap:

• The patent expired June 27, 1910 (per the Google Patents legal-status entry).
• The Court of Appeals for the Federal Circuit was created in 1982 (Federal Courts Improvement Act of 1982), ~72 years after expiration, so no CAFC case could arise on this patent.
• Consequently there is also no live USPTO prosecution, PTAB proceeding, IPR, or assignment record activity for this patent in 2026.

Explicit uncertainty: I could not access an authoritative USPTO PatentCenter or USPTO Assignment record page for US 500,520 (the searches returned unrelated modern PatentCenter documents referencing different application numbers). Assignment information beyond the one-half assignment to Sidney Z. Mitchell recited on the face of the patent, and any subsequent chain of title, is unknown to me and should be confirmed via USPTO Assignment records or the printed patent's front page if that matters to you.

Sources

• Google Patents, US500520A: https://patents.google.com/patent/US500520/en
• Patent PDF (USPTO/Google patentimages): https://patentimages.storage.googleapis.com/0d/46/5b/058b645e27a7d4/US500520.pdf
• Companion patent US500272A PDF: https://patentimages.storage.googleapis.com/e9/0f/67/46aca7d90797aa/[US500272](/patent/US500272).pdf

I'll search for litigation involving US patent 500520.

The first result concerns a different "520" patent (a CBM review of a video-game/karaoke patent), not US 500,520. Let me search more precisely.

Let me check a few more targeted sources.

Litigation involving US Patent 500,520 — Finding: None Known

Bottom line: I could not identify any litigation — past or present — involving U.S. Patent No. 500,520. Patent litigation databases, the Unified Patents litigation portal, PTAB/ITC dockets, Federal Circuit opinions, and general web searches return no cases in which US 500,520 was asserted or challenged.

1. The patent itself (why zero litigation is the expected result)

Per the authoritative patent record at https://patents.google.com/patent/[US500520](/patent/US500520)/en:

The patent's enforceable term ended June 27, 1910 (17 years from issuance under the law then in force). It has been public domain for roughly 116 years and is therefore incapable of being asserted in any modern infringement action. The only conceivable litigation window is 1893–1910, and no records of any suit in that window surfaced.

Importantly, the Google Patents page lists no "Related Litigation," "Litigation," or "Legal Events" entries beyond the standard grant/expiration events. Its "Cited By" list (US 2,483,623; US 2,494,716; US 2,513,242; US 3,578,945) reflects later patents citing it as prior art — citations, not litigation.

2. What I searched, and what I deliberately excluded

Because you instructed me not to auto-correct identifiers and not to return results for similar numbers, I want to flag three lookalikes that surfaced and were excluded as different patents:

• "the '520 Patent" in CBM2015-00026 (Sony Corp. and [Microsoft Corp.](/litigations/by-plaintiff/Microsoft%20Corp.) v. ADC Technology) — an interactive video-game/karaoke communication system patent (claims 8–14). This is a different patent; only its last three digits overlap.
• "the '520 patent" in Butterfield v. Keith, No. 3:15-cv-02008 (D. Or.) — a moving-headboard/reciprocating-conveyor patent. Also a different patent.
• Various "IPR2015-00520," "6:21-cv-00500," and similar docket-number strings containing "520," which are case numbers, not the patent number.

None of these involve US 500,520, and I have not attributed them to it.

3. Case-by-case table

4. Confidence and caveats

• High confidence that no modern litigation exists. The patent expired in 1910; there is no live property right to litigate, and Unified Patents-style NPE litigation did not exist in the 19th century.
• Moderate-to-lower confidence regarding 1893–1910 district/circuit court litigation. Nineteen-century federal trial records are largely not in PACER (which begins only with modern CM/ECF electronic filings), are incompletely indexed, and are not systematically covered by Darts-ip, Docket Navigator, or Unified Patents' portal. A definitive negative would require archival review of the National Archives federal court records for the Districts of Montana and Oregon (and the Ninth Circuit), which I cannot access from here. I did not find such records indexed online.
• The Unified Patents litigation caselist you referenced contains only contemporary (post-2000s) NPE/infringement suits and yielded nothing for this patent.
• I did not find any assignment disputes, interference proceedings, or post-grant challenges involving US 500,520 either. Note the co-ownership split (one-half to Sidney Z. Mitchell) is a potential source of historical commercial dispute, but no such dispute is documented in the sources I could reach.

If you need a negative-clearance-quality answer, the appropriate next steps would be (a) a Darts-ip/Docket Navigator patent-level query restricted to exactly "500,520," and (b) a manual search of NARA Record Group 21 holdings for the District of Montana and District of Oregon, 1893–1910.

I want to be explicit: I have not fabricated any case. Where the record is silent, I am reporting silence rather than inferring a case.

I'll verify the structured "no proceedings" default against live sources before writing the analysis.

Let me run a couple more targeted checks before concluding.

PTAB Analysis — US Patent 500,520 ("Electric heater," Earl P. Wetmore)

Source of record: the structured "PTAB proceedings on file" block (USPTO Open Data Portal ingest) is the canonical list for this patent. I also ran live web searches for the patent number combined with PTAB/IPR terminology, and for the inventor name. Both returned nothing on point. The default therefore stands, and it is confirmed rather than merely assumed.

Explicit search-gap disclosure: I could not independently query PTAB E2E or the ODP API myself; I am relying on the structured block. My web searches did surface several different patents that are colloquially called "the '520 patent" in PTAB practice — US 6,193,520 (CBM2015-00026), US 11,611,520 (IPR2024-00287), and US 12,110,520 (PGR2025-00017). None of these is US 500,520. Anyone citing "the '520 patent's PTAB history" in a demand letter is almost certainly conflating a modern patent with this 1893 grant. That conflation is itself a red flag worth raising.

Proceedings overview

Zero AIA trial proceedings have ever been filed against US 500,520 — 0 active, 0 claims invalidated, 0 claims sustained, 0 settled, 0 institution denials — and the defensive posture this gives a defendant is the strongest possible: the patent expired on 1893-06-27 + 17 years = 1910-06-27, more than a century before the IPR/PGR/CBM regime existed, so there are no live claims to assert and no IPR counsel to retain.

Because the count is zero, the per-proceeding template below cannot be populated for this patent, and I will not manufacture entries to fill it. What follows instead is the structural reason the record is empty, which is the substantive answer a defendant actually needs.

No proceeding numbers exist to report

I am not listing proceeding numbers because there are none. Under the task constraint "do not invent proceeding numbers," the correct output for the per-proceeding sections is: N/A — no proceedings on file. The reasons are statutory and date-driven:

The "PTAB proceedings on file" block was not populated with any record — no IPR, PGR, or CBM docket; no parties; no institution decision; no FWD; no Federal Circuit appeal. There is consequently no judge panel, no petition ground, no settlement, and no estoppel to report.

Strategic summary

No claim of US 500,520 has been canceled, sustained, or even tested at the PTAB. The patent has exactly two claims (claim 1 and claim 2, both independent combination claims, as set out in the previously generated summary). Both stand exactly as issued on 1893-06-27 — not because they survived scrutiny, but because no scrutiny was ever available. The important distinction for a defendant: UNTESTED ≠ SUSTAINED. Nobody should characterize these claims as "hardened" or "survived IPR." They were never eligible for the process. There is also no reissue, no reexamination certificate, and no disclaimer of record reflected in the material available to me, so claim 1 and claim 2 are the original granted text and nothing else.

Estoppel landscape: there is none, and that cuts entirely in the defendant's favor. 35 U.S.C. § 315(e)(2) estops a petitioner and its privies from raising in a civil action any ground raised or that reasonably could have been raised in an IPR that resulted in a final written decision. Here there is no petitioner, no FWD, and therefore no estoppel binding anyone. Every invalidity theory is unburdened — § 101, § 102, § 103, § 112, the pre-AIA § 102(a)/(b) grace-period provisions, and even the on-sale/public-use bars of pre-AIA § 102(b), none of which are available in an IPR at all (IPR is limited to patents and printed publications under § 311(b)). Practically, though, a defendant should not need any of this: see below.

Pattern signals: none, and none are possible. No petitioner has filed more than once (no petitioner exists). No patent owner entity (Wetmore died long ago; the one-half assignment to Sidney Z. Mitchell recited on the face of the patent is the last chain-of-title fact I can verify) has pursued PTAB appeals. There is no defensive aggregator — Unified Patents was founded in 2012, a century after expiration, and does not challenge expired patents. There is likewise no 2026 USPTO prosecution, PTAB proceeding, IPR, or assignment activity, consistent with the earlier finding that no CAFC appeal could arise given that the Federal Circuit was created in 1982.

Recommended next steps

1. No PTAB activity exists for US 500,520. State it plainly rather than searching further. The absence here is not the usual soft signal ("well-asserted patents eventually attract IPRs"). It is a hard structural fact: the AIA trial regime began 2012-09-16, roughly 102 years after this patent expired on 1910-06-27. No IPR, PGR, or CBM petition has ever been or can ever be filed.

2. Treat any 2026 assertion of US 500,520 as a fraud/mistake indicator, not a litigation threat. A US patent grants the right to exclude for a limited term; under the law in force at grant, that term was 17 years from issue. With expiration on 1910-06-27, there is no enforceable right. If a demand letter cites US 500,520, the appropriate response is a demand for the sender's basis — and, if the sender appears to be conflating this patent with a live 6,193,520 / 11,611,520 / 12,110,520-family patent, to say so directly. Do not spend money on IPR counsel.

3. If you need the negative confirmed on the record, use the free public sources directly rather than relying on my search:

   • PTAB E2E trial search: https://ptacts.uspto.gov/ptabweb/
   • USPTO PTAB decisions (AIA daily decisions and FWDs): https://www.uspto.gov/patents/ptab/decisions
   • USPTO Open Data Portal API: https://data.uspto.gov/ (the source of the structured block in the prompt)
   • CourtListener (for any CAFC docket, which will not exist): https://www.courtlistener.com/
   • Patent text and legal status: https://patents.google.com/patent/[US500520](/patent/US500520)/en and the PDF at https://patentimages.storage.googleapis.com/0d/46/5b/058b645e27a7d4/US500520.pdf

4. There are no trial-stage milestones to calendar. The PTAB's statutory one-year deadline from institution (§ 316(a)(11)) has no application where no institution decision was ever required. Docket-watching this patent will produce nothing.

5. If the real question is invalidity of related early art (for example, defending against a modern patent that claims a closed-loop induction fluid heater and that a plaintiff might argue descends from this lineage), the useful defensive work is prior-art hunting in the 1890s–1910s heater literature, not PTAB practice. One candidate surfaced in my search and is worth a look if that scenario applies: US 570,077, an early induction heater describing a hollow secondary conductor with a combined valve and circuit-closer. Caveat — I have not verified the named inventorship or the relevance of that reference, and I am not asserting it as invalidating art for any specific claim. Verify before relying on it.

6. Limit of my knowledge, stated explicitly: I could not access an authoritative USPTO Assignment or PatentCenter record for US 500,520, so I cannot confirm the chain of title beyond the "assignor of one-half to Sidney Z. Mitchell, of Portland, Oregon" recital on the patent face. That gap does not affect the PTAB conclusion, which rests on dates, not ownership.

I'll verify the assignment record before writing the chain.

Let me check for any post-1980 record and the inventor/assignee backgrounds.

Inventors

Unusual-pattern check — no "inventor departure" signal is testable here. The modern tell you asked about (all inventors leaving the original assignee within 12 months of filing, presaging a fire-sale) requires a corporate assignee and an employment relationship. Neither exists on this record: Wetmore was an individual patentee who granted a fractional interest rather than an employee who assigned the whole right.

Two patterns that are notable, though neither is a fire-sale tell:

1. Fractional co-ownership from day one. The printed heading reads "EARL P. WETMORE, OF HELENA, MONTANA, ASSIGNOR OF ONE-HALF TO SIDNEY Z. MITCHELL, OF PORTLAND, OREGON." This is a partial assignment — a 50/50 split between two individuals, executed pre-issuance. Unless the two later executed a paper between themselves, there is no single "original assignee" for the whole right (see next section).
2. A common-development group, not a lone inventor. US 500,272 A, "Electric heater," filed the same day (December 23, 1892, Serial No. 456,139, versus Wetmore's 456,123 — 13 serials apart) and issued the same day, names Roehl (Chicago), Mitchell (Portland), and Wetmore (Helena) jointly. Two same-day filings with overlapping personnel indicates a deliberately assembled inventive group, not a stray individual filing. Serial-number adjacency is strong evidence the two applications were drafted and filed together.

Background on the co-owner (relevant because it rules out the modern NPE reads): Sidney Zollicoffer Mitchell (1862–1944) was a U.S. Naval Academy graduate (Class of 1883) who worked for Thomas Edison in New York in 1885, then took the Edison Electric Light Company agency for the Pacific Northwest. His company, Northwest Supply & Construction, was acquired by Edison General Electric around 1890, which appointed him to head its Northwestern District office in Portland — the residence recited on this patent. After the 1892 Edison/Thomson-Houston merger formed General Electric, he became a senior GE-affiliated utility executive and, from 1905, President of Electric Bond and Share Company. In other words, the one-half assignee was an operating electric-power executive of that era, not an accumulating assertion vehicle.

Original assignee

There is no corporate original assignee. On the face of US 500,520 the right was split between two natural persons:

• Earl P. Wetmore (inventor) — one-half undivided interest
• Sidney Z. Mitchell (assignee of the other one-half, of Portland, Oregon)

Consequences for your chain-of-title work:

• Product embodying the claims: I found no evidence that a commercial heater was ever manufactured under this patent. The specification's stated commercial hook is manufacturing economics — "the principal value in this construction is in the facility with which the apparatus may be made, put together or taken apart… the parts are slipped one over the other." That is a design-for-assembly argument, not a record of a shipped product. Treat product-shipping status as unknown, not as a negative finding.
• Primary line of business: Wetmore's line of business is not established by anything I could retrieve. Mitchell's is well documented (electric light and power development/finance). Neither is a "company" in the sense your analysis framework assumes.
• Current status: moot as to both individuals (patent expired June 27, 1910; Mitchell died 1944). Wetmore's later whereabouts and whether he assigned his remaining half are unknown to me — that would require pre-1980 assignment records and Montana probate/estate records.

Assignment timeline

Finding: The USPTO Patent Assignment Center has no records for US 500,520. I am stating that plainly and stopping the chain reconstruction here, as instructed.

The reason is structural, not a search gap:

Access caveat: I could not execute a direct, authenticated query — Assignment Center requires login for searching ("A search can be performed after logging into Assignment Center"). My conclusion rests on the documented coverage window plus the absence of any indexed record, not on a live per-patent query result. If you need a print-ready negative, have someone with a USPTO.gov login run the by-patent-number search on 500520 and save the "no records found" screen; it will be a null result for the reason above.

Timeline diagram — this shows what is documented (from the printed patent and Google Patents legal events), not recorded assignments:

timeline
    title Ownership record for US 500520
    1892 : Wetmore files application 456123
    1893 : Patent issued to Earl P Wetmore
         : One half interest to Sidney Z Mitchell
    1910 : Patent term expires
    1980 : USPTO online assignment database opens
    2026 : No post 1980 records for this patent

Conveyances that would appear in this section if any existed: none. Conveyance types, reel/frame numbers, and correspondents cannot be reported, because pre-1980 records are not in the online system. I will not synthesize a reel/frame.

One item to not confused with a correspondent: the patent's testimonial witnesses, H. S. Hepner and Josiah Bowden, are attesting witnesses to the specification, not assignment-recording attorneys. I have not treated them as correspondents of record.

Post-stop addendum — the NPE framework, for completeness

I stopped the chain reconstruction above per your rule. Recording the balance of the framework only so the record isn't ambiguous about the verdict:

Verdict

Insufficient data. The only ownership facts in existence are the two natural-person co-owners recited on the face of the patent — Wetmore (one-half) and Mitchell (one-half) — recorded in 1893, long before the USPTO's online assignment database (coverage begins August 1980). There is no reel/frame entry to cite, no correspondent, no litigation, and no asserting entity; the 1893–1910 paper and microfilm assignment records sit in the separate historical database accessible only through the USPTO Assignment Recordation Branch. NPE analysis is inapplicable to a patent that expired June 27, 1910 — not because a signal was checked and cleared, but because the entire post-1980 evidentiary basis for that analysis does not exist.

Verify at: https://assignmentcenter.uspto.gov/ (search by patent number 500520) or https://assignment.uspto.gov/patent/index.html

I'll verify the citation record for US 500,520 and then run down each candidate reference.

PRIOR-ART ANALYSIS — US 500,520

1. Confirmation of the USPTO record searched

I searched the specific number 500520 and did not substitute, expand, or "correct" it. The record returned is the same document already established in the earlier sections of this analysis (US 500,520 A, "Electric heater," Earl P. Wetmore, filed December 23, 1892, Serial No. 456,123, issued June 27, 1893, "No Model"), so I am not restating it. Two lookalikes surfaced and were excluded as different documents:

• US 1,904,315 A — "Pipe hanger" — application filed December 6, 1930, Serial No. 500,520. That is an application serial number, not patent number 500,520. (https://patents.google.com/patent/[US1904315A](/patent/US1904315A)/en)
• Various docket/patent strings such as "491,426," "415,871," "404,844," "500,272" — none is 500,520.

2. Core finding: US 500,520 has NO cited prior art of its own

This is the central answer to the task, and it is a negative one that I want to state plainly rather than paper over:

The printed specification and claim text of US 500,520 contain no "References Cited" listing, no citation of any prior patent or publication, and no acknowledgment of any prior art whatsoever. The specification opens directly with "This invention relates to electric heaters, the primary objects being simplicity and cheapness in construction," and the only backdrop it identifies is the generic concept of "an alternating current transformer" — a description of the field, not a citation. US 500,520 is a "No Model" 1893 grant; nineteenth-century US patents in this style were not printed with a front-page references-cited section, and this one carries none.

The only citation data attached to the US 500,520 record is forward citation data — the block Google Patents labels "Cited By (4)." Forward citations run the wrong way in time and therefore cannot anticipate US 500,520 under any subsection of 35 U.S.C. § 102. A § 102 reference must predate the applicant's invention or filing date (here, December 23, 1892). All four are from 1943–1969.

3. The four forward citations ("Cited By") — full citations, dates, descriptions, § 102 verdict

Verdict: 0 of 4 forward citations anticipate claim 1 or claim 2. None can, as a matter of law, because none existed at the time of Wetmore's invention. I verified item 2's citation of 500,520 directly in the US 2,494,716 record (https://patents.google.com/patent/US2494716). I did not independently verify by full-text inspection that items 1, 3, and 4 each actually cite 500,520 — I am reporting that Google Patents lists them as citers, and I flag that as a lower-confidence point.

4. The "Similar Documents" set — which could even qualify as § 102 art

Google Patents also renders an algorithmic "Similar Documents" list. These are not citations and were not cited by the examiner or the patentee; they are similarity matches. Because the task asks for the most relevant prior art, I assessed each one for § 102 eligibility strictly by date. Only a reference with a date before December 23, 1892 can be § 102(a)/(b) art against this filing.

So the entire universe of date-eligible § 102 candidates is three references: US 409,170 A, US 484,532 A, and US 487,285 A. I retrieved substantive text for only one of them.

5. Claim-by-claim § 102 analysis of the three date-eligible references

A. US 484,532 A — "Electrical curling-iron," Andrew D. Gardner, Lynn, Massachusetts. Issued October 18, 1892; application filed October 21, 1891, Serial No. 409,404; "No Model." (https://patents.google.com/patent/[US484532A](/patent/US484532A)/en; PDF: https://patentimages.storage.googleapis.com/9d/ee/50/21d2768620b2c3/[US484532](/patent/US484532).pdf)

Disclosure: a hollow non-conducting handle A with bore B; a hollow metal curling-stem C with thin walls and a closed outer end; two conductors G, G′ to binding-posts H, H′; a resistance coil K doubled and coiled spirally within the stem and made incandescent to heat the stem by radiation; and a removable cap L over the binding posts.

§ 102 assessment — does not anticipate either claim:

• Missing element: a primary coil through which alternating currents flow (claim 1 and claim 2). Gardner uses directly connected resistance conductors, not an alternating-current primary.
• Missing element: a metallic cylinder constituting a secondary coil or circuit closed upon itself. Gardner has no secondary at all; there is no transformer action.
• Missing element: an iron core inside the cylinder.
• Missing element: ports in the heads for fluid ingress/egress — Gardner's stem is closed at the outer end and contains no fluid path.
• Missing element: a chamber between the core and the cylinder for the passage of a fluid.
• Claim 2 additionally requires a spiral partition in that chamber. Gardner's coil is spiral, but it is the heating element, not a partition in a fluid chamber — a distinct structural element that cannot be read onto it.

At most, US 484,532 A is evidence that a hollow metal tube heated from within for external heat transfer was known — potentially useful as a § 103/obviousness backdrop (recognizing that § 103 did not exist in 1892; the contemporaneous test was want of invention under R.S. § 4886). It is not anticipatory art.

B. US 409,170 A — "Electric valve," issued August 20, 1889. I could not retrieve this patent's text. My targeted searches returned a different 1889 case — US 415,871 A, "Electro-automatic valve," Mary A. Kissell as herself and executrix of Abraham S. Kissell, Chicago, issued November 26, 1889, filed March 21, 1889, Serial No. 304,219 — which is an electromagnetically actuated fluid valve with a pressure chamber, piston, and plunger, and contains no induction heater, no secondary cylinder, and no iron core.

I therefore cannot opine on US 409,170 A's disclosure. Based on its title alone, an "electric valve" would not be expected to disclose a primary coil/metal-cylinder-secondary/iron-core/ported-head/spiral-chamber combination. Explicit uncertainty: I have no substantive knowledge of US 409,170 A's specification and I am not inferring anticipation from its title.

C. US 487,285 A — "Electrical heater," issued December 6, 1892. I could not retrieve this patent's text. This is the single most consequential unknown in this analysis: it is titled "Electrical heater," and it issued 17 days before Wetmore filed. I cannot assess its disclosure and therefore cannot rule it in or out. To resolve it, the full text at https://patents.google.com/patent/[US487285A](/patent/US487285A)/en and the corresponding PDF should be inspected against the two claim charts above.

6. Statutory caveat on the "§ 102" framing

Two points bear on the analysis's legal correctness:

1. 35 U.S.C. § 102 did not exist when this patent issued. US 500,520 issued June 27, 1893. Section 102 was enacted in the Patent Act of 1952. The anticipation statute in force in December 1892 was R.S. § 4886 (Patent Act of 1870, as amended), which barred a patent where the invention was "known or used by others" or "patented or described in any printed publication" before the applicant's invention — including foreign patents and publications. Applying § 102 here is an anachronism; I have done it because you asked, but the historical standard would be the 1870 Act.
2. If modern law is applied, pre-AIA § 102 governs, not the AIA first-inventor-to-file § 102, because US 500,520's application was filed long before the AIA's March 16, 2013 cutoff. That matters chiefly because § 102(e) (patents/applications "by another" filed before the applicant's invention) is what gives the 1893-issued references any theoretical traction, and because § 102(e) art can be sworn behind with an earlier date of invention — a mechanism unavailable under the AIA's § 102(a)(2).

7. Conclusion

• US 500,520 has zero self-cited prior art. No reference appears on its face or in its specification.
• Its four forward citations (US 2,483,623; US 2,494,716; US 2,513,242; US 3,578,945) cannot anticipate either claim 1 or claim 2 under § 102 — all post-date the 1892 filing by half a century or more. They cite 500,520; 500,520 does not rest on them.
• Of the algorithmic "Similar Documents," only three precede the December 23, 1892 filing: US 409,170 A (1889), US 484,532 A (1892-10-18), and US 487,285 A (1892-12-06). Every other entry is a post-filing document and is ineligible as § 102(a)/(b) art.
• On the one such reference I could fully examine — US 484,532 A — there is no anticipation of claim 1 or claim 2, for the five missing limitations charted above.
• Net: no identified reference anticipates either claim of US 500,520. The nearest call is US 487,285 A, purely because it is titled "Electrical heater" and post-dates nothing — it predates Wetmore by 17 days — and its content is unverified.

Confidence: high on the negative citation finding and on the forward-citation analysis; high on the US 484,532 A non-anticipation chart; low/undetermined on US 409,170 A and US 487,285 A, whose texts I could not retrieve. I have not fabricated any disclosure or any citation, and where the record is silent I have reported silence.

Sources

• Google Patents, US 500,520 A: https://patents.google.com/patent/[US500520](/patent/US500520)/en
• Google Patents, US 484,532 A: https://patents.google.com/patent/US484532A/en
• US 484,532 A PDF: https://patentimages.storage.googleapis.com/9d/ee/50/21d2768620b2c3/US484532.pdf
• Google Patents, US 2,494,716 A (cites 500,520 in its references list): https://patents.google.com/patent/US2494716
• Google Patents, US 1,904,315 A (application Serial No. 500,520 — distinguished lookalike): https://patents.google.com/patent/US1904315A/en
• US 415,871 A PDF (returned in the search for 409170; different patent): http://patentimages.storage.googleapis.com/2a/d1/14/3abbfe2ddb5428/[US415871](/patent/US415871).pdf

I'll ground this in the prior-art data actually present on the page, and verify a few candidate references before drawing §103 conclusions.

Good — the search surfaced a genuine pre-1892 induction-heating reference (Colby US 428,552). Let me verify a few more candidate references and the era's "invention" standard case law.

§103 Obviousness Analysis — US 500,520 A ("Electric heater," Earl P. Wetmore)

0. Two threshold points before the merits

(a) §103 did not exist when this patent issued. 35 U.S.C. §103 was enacted July 19, 1952 (effective Jan. 1, 1953) — ~59 years after US 500,520 issued on June 27, 1893. In 1893 the Patent Office and courts tested patentability under the judge-made "invention" requirement (Hotchkiss v. Greenwood, 52 U.S. (11 How.) 248 (1850)), later sharpened into the "flash of creative genius" formulation (Cuno Eng'g Corp. v. Automatic Devices Corp., 314 U.S. 84 (1941)). So what follows is a modern, retro-applied §103 analysis (the Graham/KSR framework), with the pre-1952 "mere mechanical skill" analogue flagged where the two diverge. Since the patent expired June 27, 1910 and is unenforceable, this analysis is probative rather than operative — its main practical use today is (i) reconstructing the validity posture of 500,520 when it is cited as prior art (it is cited by US 2,483,623; 2,494,716; 2,513,242; 3,578,945), and (ii) assessing whether modern induction fluid-heater claims survive over it.

(b) Critical date. The page shows "Prior art date 1893-06-27," which is simply the grant date, not a priority date. Filed December 23, 1892 (Ser. No. 456,123) with no earlier priority claim shown, the operative pre-AIA critical date is Dec. 23, 1892 — conservative, since a pre-filing invention date (sworn behind under 1893 practice) would shrink the art.

1. The prior-art record actually on the page — and its limits

The Google Patents page for US500520A (https://patents.google.com/patent/[US500520](/patent/US500520)/en) contains no "References Cited" / examiner-citation list. What it does contain is:

From "Similar Documents," only three entries are dated before Dec. 23, 1892, and therefore eligible under §102(a):

• US 409170 A, "Electric valve," 1889-08-20
• US 484532 A, "Electrical curling-iron," 1892-10-18 (verified: A. D. Gardner, filed Oct. 21, 1891, Ser. No. 409,404; a resistance coil in a hollow metal stem — https://patents.google.com/patent/[US484532A](/patent/US484532A)/en)
• US 487285 A, "Electrical heater," 1892-12-06

Honest limitation: the page gives me only number/title/date for US 409170 A and US 487285 A. I did not retrieve their specifications, so I will not assert what they disclose. I also exhausted my search budget before verifying them further.

Two entries published after filing but during pendency — US 491322 A (1893-02-07) and US 495071 A (1893-04-11) — are not §102(a)/(b) printed publications against a Dec. 23, 1892 filing; they could only be §102(e) art if their own filing dates antedate Wetmore's invention date (unverified).

And the same-day companion: US 500272 A is not §103 art against US 500520. Filed the same day (Dec. 23, 1892, Ser. No. 456,139) with overlapping inventive entity (Roehl, Mitchell, and Wetmore — who is the sole named inventor here) and overlapping ownership interest (Mitchell owns half of 500520), it is neither "by another" as to Wetmore nor "filed before" his application. Its real doctrinal relevance is double patenting, not §103.

Bottom line on the page's own four corners: the "Prior Art" section of this page does not contain an induction-heating reference at all. A §103 rejection built solely from the page's Similar Documents list would be weak. The rejection becomes strong only when the art that the field actually knew is added — which is what follows.

2. Supplementary art I verified (flagged as outside the page's prior-art section)

Because the page's list omits the controlling art, I searched independently. One reference is decisive enough to anchor the analysis:

PA-1 — US 428,552 A (Edward A. Colby), "Process of melting, refining, and casting metals," patented May 20, 1890; filed Dec. 17, 1887, Ser. No. 258,172.
https://patentimages.storage.googleapis.com/e9/e7/06/e5f7303a8d19f0/[US428552](/patent/US428552).pdf

Verbatim disclosure relevant here:

• "a prism-coil P … The coil P is designed to be included in the circuit of a generator K, delivering an alternating, intermittent, or pulsatory electric current."
• A trough "S, which is designed to receive the material to be heated… The trough S is designed to constitute the secondary coil of the converter or induction coil" — set in an annular groove e inside coil P.
• "the core F … may consist of soft-iron wires, which are preferably electrically insulated from each other," passing through the center, with "a disk F', which is removable," having "one or more openings f′."
• The material heated may be "metals and non-conducting chemical substances or liquids."
• Capability statement: the heated substance, if conductive, "may itself constitute, partially or entirely, the secondary coil."

That single reference discloses: an AC primary coil; a metallic body lying inside it and constituting a one-turn short-circuited secondary; an iron wire core inside that body; an annular chamber between core and coil holding the material to be heated, explicitly including liquids; and a removable member with openings for loading/unloading. Those are, element for element, the spine of claim 1.

PA-2 — US 442,933 (Ernest Abshagen), "Electric steam and hot-water generator," patented Dec. 16, 1890; filed Aug. 4, 1890, Ser. No. 360,878. https://patentimages.storage.googleapis.com/91/18/47/b8a2c812d02a63/[US442933](/patent/US442933).pdf
Relevant as art of electric liquid heating: a "closed vessel having an interior compartment in contact with the water," tubes traversing the water, resistance element in circuit with a generator, and an expansion regulator. (Note an uncorrected OCR discrepancy on the face of that document: the heading reads "442,933" while the specification line reads "442,983." I am reporting both as printed rather than harmonizing them.)

PA-3 — general knowledge, corroborated by contemporaneous technical literature: the induction furnace "in the character of them all is the common transformer" and "the secondary coil … is composed solely and alone by the hearth metal itself," i.e., a short-circuited single-turn secondary, was the accepted understanding of induction heating apparatus (Electric Furnaces, https://ia600406.us.archive.org/11/items/electricfurnaces00rodeuoft/electricfurnaces00rodeuoft.pdf). This corroborates (i) that "transformer with one-turn shorted secondary = induction heater" was known reasoning, and (ii) that Wetmore's own framing ("a modified form of an alternating current transformer") was the conventional framing. Date caveat: the book is later, so I use it as evidence of what the art understood, not as §102 art.

Non-art references I will not misuse: US 1,746,522 (spiral-plate fluid heater, 1938) and US 859,641 (induction furnace) post-date 1893. They are cited below only to show that the spiral-path expedient and the annular-crucible-plus-coil geometry persisted as standard practice — they are not prior art to 500520.

3. Limitation chart

ADM-1 — applicant's own admissions (usable as prior-art-equivalent): the specification states the device "is a modified form of an alternating current transformer," that "the secondary coil or circuit is closed upon itself, thus being a transformer in which the secondary coil has but one turn," and that "the invention consists of the special construction of the secondary." It further states: "The principal value in this construction is in the facility with which the apparatus may be made, put together or taken apart. It will be seen that the parts are slipped one over the other." Under MPEP 2129 / In re Nomiya, 509 F.2d 566 (CCPA 1975), an applicant's characterization of what is old is treated as admitted prior art. Wetmore has thus conceded the operative mechanism (transformer with one-turn shorted secondary) and characterized his own contribution as assembly convenience.

4. Grounds of obviousness

Ground 1 — Claim 1 obvious over PA-1 (Colby) in view of PA-2 (Abshagen) and CK-1/CK-2

• C1-A, C1-B, C1-C are met on the face of PA-1: AC primary coil P, metallic trough S constituting the induction-coil secondary inside it, iron-wire core F inside that.
• C1-E is met by PA-1's annular receptacle holding material to be heated, expressly including "liquids." The only arguable delta is that Wetmore's fluid is the end product rather than a charge to be melted, and that his secondary is a cylinder wall that both generates heat and bounds the fluid chamber. But PA-1 already teaches the dual-function structure (the secondary trough is itself the receptacle). Adding PA-2 — an electric water-heating vessel with a closed compartment "in contact with the water" — supplies the fluid-handling purpose and vessel architecture.
• C1-D (removable heads with ports) is a routine mechanical closure: PA-1's removable disk F′ with openings; PA-2's water vessel; and, per ADM-1, Wetmore's own stated advantage is that "the parts are slipped one over the other."

Motivation to combine (KSR rationales):

1. Same field of endeavor / reasonably pertinent art. All three are electric heating of a load by a coil or resistance; PA-1 and 500520 are both "coil outside, body inside, iron core in the middle." KSR Int'l Co. v. Teleflex Inc., 550 U.S. 398 (2007) ("familiar elements according to known methods").
2. Explicit teaching toward the result. PA-1 states the secondary receptacle receives "liquids" and that a conductive charge "may itself constitute … the secondary coil" — i.e., use the heated body as the shorted secondary. That is precisely Wetmore's C1-B/C1-E combination.
3. Design incentive supplied by the art itself: a working fluid passed through a chamber is the known way to carry away induced heat; a person of skill heating water rather than melting steel would naturally retain the coil/core geometry and route fluid through the annular space, because that eliminates an intermediate heat-transfer step.
4. No teaching away. Nothing in PA-1 dissuades using a non-melting load; indeed its listed materials include non-conducting substances and liquids.
5. Predictable result. The claim states no numerical or unexpected result; the specification promises only "simplicity and cheapness in construction." Under KSR, where the result is predictable and the only asserted advantage is manufacturability, the claim is obvious.

Ground 2 — Claim 1 obvious over ADM-1 (admitted prior art) + CK-1 in view of the ordinary skill of a mechanic

Take the specification's admissions as the primary reference: the transformer-with-one-turn-shorted-secondary is conceded old, and the secondary construction is conceded to be the entire point. What remains in claim 1 unaccounted for by ADM-1 is only: (i) an iron core — the most conventional transformer element conceivable; (ii) threaded/removable end caps with ports — conventional closures for a tubular vessel; (iii) an annular gap — an inevitable geometric consequence of placing a smaller cylinder inside a larger one. Under the pre-1952 standard, this is the paradigm of "success through skillful use of mechanical details, and nothing more" — the phrase used by the court in the contemporaneous electric-heater case reported at 82 F. 993 (Consolidated Car Heating Co. v. American Electric Heating Co., https://law.resource.org/pub/us/case/reporter/F/0082/0082.f1.0993.pdf), where a resistance heater was nonetheless upheld only because of "the first practical successful heater," 29 prior patents going back to 1859, long-felt need, and "immediate use … very extensively availed of." That case is a useful calibration: in this exact art and era, claims of this character lived or died on secondary considerations, not on the mechanical novelty of arrangement. (I did not verify the full citation details of that case beyond the reported pages surfaced; treat the court/date identification as provisional.)

Ground 3 — Claim 2 obvious over claim 1 in view of CK-2

Claim 2 adds only "a spiral partition in said chamber," which the specification promptly reduces to "preferably made of a wire wound continuously from end to end."

• The function recited is purely hydraulic: force the fluid to "take a spiral course around the core, and thus be a comparatively long period in contact with the core and cylinder."
• A helical baffle/spiral passage to lengthen fluid residence time and increase heat-transfer surface contact is a long-standing, ubiquitous heat-exchange expedient — the "known technique" branch of KSR ("a court must ask whether the improvement is more than the predictable use of prior art elements according to their established functions"). The very durability of the expedient is illustrated by later, non-prior-art practice: US 1,746,522 (1938) claims a fluid heater with "spirally curved" plates and "means for causing the fluid to flow between said plates in the direction of the spiral" (https://patentimages.storage.googleapis.com/a6/9a/84/a322b17ae26f4b/US1746522.pdf) — cited only to show the expedient was conventional, not as §102 art.
• Motivation: prevent channeling and hot spots; maximize ΔT extraction per unit length; reduce size. All are ordinary engineering objectives; the result (more heat transferred over a longer path) is exactly what one expects.

Caveat I must flag: I was not able to verify a specific pre-Dec. 23, 1892 US patent for a spiral/helical partition in an annular chamber; the most direct spiral-baffle references surfaced to me post-date the patent. Ground 3 therefore rests on common technical knowledge (CK-2) rather than a specific documentary reference. That is a legitimate §103 basis, but it is the weaker link in the chain, and I have not manufactured a reference to fill it.

5. Anticipated counterarguments and their assessment

6. Conclusion and confidence

• Claim 1: Likely obvious in a modern §103 re-application, with moderate-to-high confidence, primarily over US 428,552 (Colby) in view of US 442,933 (Abshagen) and the admitted transformer art. Confidence is not higher because (i) I am relying on a single anchor reference for the induction-heating core, and (ii) the pre-1952 "invention" standard was more forgiving than §103 toward combinations of old elements in a commercially successful device.
• Claim 2: Likely obvious as claim 1 + the conventional helical-baffle expedient, with moderate confidence, because the spiral-partition element rests on common technical knowledge rather than a verified pre-1892 document.
• On the page's own prior-art list alone: I could not construct a sound §103 rejection. None of the three pre-filing entries on the "Similar Documents" list (US 409170 A; US 484532 A — a curling iron; US 487285 A) is shown by anything on the page to disclose an induction heater, and I did not retrieve the specifications of US 409170 A or US 487285 A, so I make no claim about their contents.
• Structural point worth carrying forward: if claims 1–2 are indeed obvious over 1890s art, then 500520's enduring significance is as prior art, not as a property right — consistent with its classification at H05B 6/108 (induction heating using a susceptor for heating a fluid) and with its citation by four later US patents.

Explicit uncertainties: (1) I did not obtain any 1893–1910 Patent Office file, interference record, or examiner's citation list; the "no references cited" observation is based on the fetched page, not on the paper file. (2) US 409170 A and US 487285 A remain unread. (3) The 82 F. 993 citation is reported from the reporter text I retrieved; the precise court/date identification is provisional. (4) The pre-1952 "two-year" versus "one-year" statutory-bar nuance is not material to this analysis because all the art I rely on predates the filing date outright.