Lilienfeld 1925 Inspect the Assembled Geometry

# Inspect the Assembled Geometry: Verify Foil Position, Terminal Separation, and Mechanical Integrity

Step 15 is the first verification operation in the Lilienfeld 1925 sequence: inspect the completed physical assembly before interpreting any electrical behavior. The inspection must establish that the aluminum control foil remains where the design requires it, that the two main terminal coatings remain separated from each other and from the foil, and that the reassembled glass support has not cracked, shifted, opened, or delaminated during the later deposition, sulfurization, connection, and handling operations.

This is not yet a film-continuity test, a leakage measurement, or a gain test. Those questions belong to Steps 16, 20, and 27. Step 15 freezes the geometry that all of those measurements will assume. If the foil is misplaced, proud of the surface, buried too deeply, tilted, or mechanically loose—or if a terminal has crept into the intended clearance—later electrical data cannot be assigned uniquely to field control.

The primary patent describes two insulating pieces forming a glass block, a transverse fracture that retains a thin aluminum foil, the foil edge approximately flush with the upper glass surface, and terminal coatings on opposite sides in close proximity while remaining separate from the foil. Those words define inspectable geometric relationships. They do not supply modern tolerances, so a reconstruction must declare its own dimensions, measurement uncertainty, and acceptance limits rather than pretending the patent specified them.

## 1. Convert descriptive geometry into measured datums

Choose a coordinate system tied to stable features of the support. In plan view, let $x_L$ be the inner edge of the left terminal, $x_F$ the foil centerline or defined foil edge datum, and $x_R$ the inner edge of the right terminal. The two lateral clearances are

$$ c_L=x_F-x_L-\frac{w_F}{2}, \qquad c_R=x_R-x_F-\frac{w_F}{2}, $$

where $w_F$ is the observed foil width at the inspection plane. Both must remain positive after measurement uncertainty and edge roughness are included. The terminal-to-terminal gap is

$$ g_{TT}=x_R-x_L, $$

but $g_{TT}>0$ does not prove that either terminal clears the foil; all three conductors must be located independently.

In cross-section, define foil-edge height relative to the local glass surface as $h_F$. A positive value means the foil stands proud, a negative value means it is recessed, and zero is nominally flush. Because the glass surface and fracture line may not be perfectly flat, report a height map or several registered sections rather than one favorable point. Foil tilt across its exposed length can be represented by a fitted slope and the peak-to-valley residual from that fit.

Dimensional inspection of the assembled Lilienfeld geometry Plan and cross-sectional views define terminal edges, foil position, conductor clearances, foil flushness, tilt, and fracture closure. STEP 15 TURNS “CLOSE” AND “FLUSH” INTO MEASURED GEOMETRY Three conductors, two clearances, one fracture and a foil edge that must stay mechanically registered PLAN VIEW — LOCATE EACH CONDUCTOR EDGE cL cR terminal 11 terminal 12 foil 13 Inspect the full overlap length: a positive center clearance does not exclude an edge bridge elsewhere. CROSS-SECTION — FLUSHNESS AND FRACTURE CLOSURE hF > 0: proud inspect crack extension inspect open seam / debris local glass surface is the height datum REPORT, DO NOT EYEBALL POSITIONxL, xF, xRCLEARANCEmin(cL, cR)HEIGHThF map + tiltDAMAGEcrack, chip, seam, liftTRACEimage + scale + datum

## 2. Inspection has to cover the assembled device, not one photogenic field

The foil and terminal edges may wander along their length. Copper deposition can create edge beads, islands, or lateral overreach. Sulfurization can change film volume and contrast. Lead attachment can pull on thin coatings or the retained foil. Handling can open the original glass fracture or extend it beyond the intended line. Inspect the entire active region, terminal transitions, foil exit, connection points, support perimeter, and both ends of every critical clearance.

At minimum, acquire calibrated plan-view images at low magnification for global registration and higher magnification for the minimum-clearance locations. Use oblique illumination or focus variation to reveal lifted edges and surface relief. A reconstruction may add optical profilometry, confocal imaging, interferometry, or X-ray methods, but the record should distinguish historically plausible visual inspection from modern nondestructive characterization.

If a cross-section is sacrificed, it provides valuable evidence about foil depth, fracture closure, trapped debris, glass chipping, and the relationship between visible top-surface edges and buried geometry. It cannot certify every device unless the sampled location and destruction bias are considered. Nondestructive surface measurements remain necessary for the devices that proceed.

## 3. Mechanical integrity is functional geometry

A crack is not rejected merely because cracks are aesthetically undesirable; the design already uses a deliberate fracture to retain the foil. The relevant distinction is between the intended closed fracture plane and damage that changes alignment, support stiffness, dielectric path length, or conductor clearance.

Record:

  • fracture extension beyond the designed line;
  • chips or missing glass near terminal and foil edges;
  • an open seam, rocking, or relative displacement between the two support pieces;
  • foil wrinkling, tearing, pull-out, buckling, or motion under gentle fixture loading;
  • terminal-coating lift, blistering, peeling, or cracking;
  • lead-induced stress, coating damage, or movement at attachment points;
  • debris trapped in the fracture or lodged between nominally separated conductors.

Mechanical probing must not create the defect it is intended to detect. Use a defined, low-force handling or fixture check and document the force or displacement limit. “Survived handling” is not repeatable unless the handling is controlled.

## 4. Acceptance limits must include measurement uncertainty

Suppose the minimum measured clearance is $\hat c_{min}$ with expanded measurement uncertainty $U_c$, and the design requires true clearance of at least $c_{req}$. A conservative acceptance rule is

$$ \hat c_{min}-U_c\ge c_{req}. $$

Likewise, if allowed foil flushness is $|h_F|\le h_{max}$, accept only when

$$ |\hat h_F|+U_h\le h_{max}. $$

These guards prevent a barely positive measured clearance or apparently flush foil from passing when the instrument cannot resolve the margin. Pixel calibration, focus, threshold choice, edge roughness, perspective, sample tilt, illumination, and operator selection all contribute uncertainty.

The patent’s qualitative “close proximity” is not an acceptance number. $c_{req}$ and $h_{max}$ must come from the reconstruction’s electrical-field, isolation, coating-step-coverage, and mechanical models, then be validated experimentally. Step 15 reports whether the fabricated geometry matches those declared limits; it does not invent limits after seeing the sample.

Geometry inspection workflow and defect disposition A coordinate-preserving inspection workflow measures global alignment, local minimum clearance, foil height, fracture damage, and coating adhesion before releasing the device to film-coverage inspection. GEOMETRY RELEASE REQUIRES A MAP, A MARGIN, AND A DISPOSITION Preserve coordinates so Step 16 can correlate film continuity with the physical assembly beneath it COORDINATE-PRESERVING INSPECTION GLOBAL VIEWsupport + all leads EDGE MAPfoil + terminals HEIGHT MAPflushness + tilt DAMAGE MAPcrack + lift + chip MARGINlimit − uncertainty Use one device coordinate system and retain every image, edge trace, height sample, scale calibration and exception. Output: an inspection package Step 16 can overlay with film-coverage and continuity findings. DEFECT SIGNATURE → CONSEQUENCE foil proud/recessedfield + coverage changeshold / quantifyclearance lowshort/leakage riskreject or reworkcrack extensionmotion/breakdown pathrejectterminal liftunstable contact/filmhold / inspectdebris in seamunknown spacing/pathclean or reject RELEASE GATE all required regions inspected? lower-bound margins pass? STEP 16 HOLD

## 5. Do not contaminate Step 15 with later conclusions

Visual continuity of the copper–sulfur film belongs to Step 16. Composition, thickness, uniformity, and resistance belong to Step 17. Direct electrical shorts belong to Step 18. Baseline terminal I–V and control leakage belong to Steps 19 and 20. Step 15 may identify geometry that threatens those tests, but it should not claim their results.

That separation protects causal diagnosis. If Step 20 later finds control leakage, the Step 15 map can show whether the leakage location coincides with minimum foil clearance, a proud foil edge, a crack, or debris. If Step 16 finds an open film, its coordinates can be compared with a terminal edge, fracture step, or delamination already recorded here. Without preserved coordinates, each later failure becomes an isolated anecdote.

## 6. Minimum inspection record

For each assembly, retain:

  • device identifier and fabrication genealogy through lead attachment;
  • calibrated global plan view and scale;
  • traced terminal and foil edges over the complete active length;
  • $c_L$, $c_R$, $g_{TT}$, foil width, symmetry, and minimum-clearance coordinates;
  • foil-height map, tilt, and the datum used to define flushness;
  • fracture-line, chip, seam, delamination, and coating-lift annotations;
  • inspection method, lighting, magnification, pixel scale, calibration, operator or algorithm version;
  • measurement uncertainty and the stated acceptance rules;
  • pass, hold, rework, or reject disposition with reason codes;
  • links between every anomaly and its image coordinates for later overlay.

The primary geometry source is [J. E. Lilienfeld, US Patent 1,745,175](https://patents.google.com/patent/US1745175A/en), with Canadian priority dated October 22, 1925. The patent supports the arrangement and relative positions; the numerical metrology and release framework here are modern engineering controls for testing whether a reconstruction actually conforms to that arrangement.

## Inspect the Assembled Geometry’s Place in the Process Lineage

Inspect the Assembled Geometry begins the roadmap’s verification phase as Step 15. It examines the physical relationships created by the preceding construction operations before Step 16, Inspect Film Coverage, asks whether the compound film spans the intended current path. Its release decision is deliberately geometric: the foil is registered and acceptably flush, the terminal and foil clearances retain uncertainty-aware margin, the glass and coatings remain mechanically intact, and every observation is mapped so later film and electrical failures can be correlated to their physical origin.

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