# John Atanasoff

### Physicist, Inventor — 1903–1995 — United States (Iowa)

> _"I had been forced to the conclusion that I would have to design and build my own computer. And it would have to be electronic."_

---

## Why This Matters

You cannot understand the history of computation without understanding John Atanasoff. Before ENIAC, before Colossus, before the corporate giants entered the field, a physics professor at Iowa State College conceived and built the first electronic digital computer. The Atanasoff-Berry Computer (ABC), completed in 1942, introduced the foundational elements that define all modern computing: electronic switching, binary arithmetic, regenerative memory, and the separation of memory from computation. When the courts finally adjudicated the patent disputes in 1973, they ruled definitively: Atanasoff invented the electronic digital computer. Every time you use a computer, you are using a machine built on principles Atanasoff first demonstrated in a basement in Ames, Iowa.

---

## Quick Reference

| Attribute | Value |
|-----------|-------|
| **Registry #** | 43 |
| **Born** | October 4, 1903, Hamilton, New York, United States |
| **Died** | June 15, 1995, Frederick, Maryland, United States |
| **Active Period** | 1930s–1940s (computing work); continued physics career through 1950s |
| **Fields** | Physics, Mathematics, Computer Engineering, Electronics |
| **Known For** | Atanasoff-Berry Computer (ABC) — first electronic digital computer |
| **Influenced By** | Classical physics, electrical engineering traditions, necessity of solving large systems of linear equations |
| **Influenced** | John Mauchly (ENIAC), all subsequent electronic digital computing |

---

## Table of Contents

1. [Origins & Formation](#1-origins--formation)
2. [Intellectual Genealogy](#2-intellectual-genealogy)
3. [The Work: Chronological](#3-the-work-chronological)
4. [Core Ideas & Contributions](#4-core-ideas--contributions)
5. [Impact & Legacy](#5-impact--legacy)
6. [Study Guide: The Mental Model](#6-study-guide-the-mental-model)
7. [Going Deeper: Sources](#7-going-deeper-sources)

---

## 1. Origins & Formation

### A Note on Historical Sources

> **On Controversy:** John Atanasoff's place in computing history was obscured for decades by patent disputes, institutional neglect, and the fame of later machines like ENIAC. Only through the landmark 1973 court case _Honeywell v. Sperry Rand_ was his priority legally established. The biographical details below draw from court testimony, Atanasoff's own accounts, and subsequent scholarly investigation. Unlike some figures in computing history, Atanasoff's contributions are well-documented but were long overlooked.

### Early Life & Context

> _Etymology: **Atanasoff** is a Bulgarian surname meaning "son of Atanas" (from the Greek Athanasios, "immortal"). His father, Ivan Atanasoff, emigrated from Bulgaria in 1889._

John Vincent Atanasoff was born on **October 4, 1903**, in Hamilton, New York, the eldest of seven children. His father was an electrical engineer who had immigrated from Bulgaria, and his mother, Iva Lucena Purdy, was a mathematics teacher. The household valued education and technical skill — young John had access to engineering manuals and tools from an early age.

**The Atanasoff Household:**
- Father worked as an electrical engineer for a phosphate mining company
- Mother held a degree in mathematics — unusual for women of that era
- Family relocated to Florida when John was nine
- Strong emphasis on practical problem-solving and mechanical understanding

**Florida in the 1910s:**
- The family moved to Brewster, Florida, a small town in the phosphate mining region
- John attended a one-room schoolhouse, then high school in Mulberry
- Rural isolation meant self-directed learning and improvisation
- At age nine, John helped his father install electrical systems, developing hands-on electrical knowledge

### Education & Training

| Period | Institution | Focus | Achievement |
|--------|-------------|-------|-------------|
| 1921–1925 | University of Florida | Electrical Engineering | B.S. with highest honors |
| 1925–1926 | Iowa State College | Mathematics | M.S. |
| 1926–1930 | University of Wisconsin | Theoretical Physics | Ph.D. |
| 1930–1942 | Iowa State College | Physics Faculty | Associate Professor; ABC development |

**University of Florida (1921–1925):**

Atanasoff entered the University of Florida at age 17, intending to study electrical engineering. He excelled in mathematics and physics, graduating with the highest honors. Here he first encountered the problem that would drive his later work: solving large systems of linear simultaneous equations. The methods available — hand calculation, mechanical calculators — were painfully slow for systems of any significant size.

**Iowa State College (1925–1926):**

For his master's degree, Atanasoff studied mathematics, focusing on the mathematical techniques used in physics. Iowa State would become his intellectual home — he would return as faculty and build his computer there.

**University of Wisconsin (1926–1930):**

Atanasoff's doctoral work in theoretical physics involved quantum mechanics and the behavior of helium atoms. His dissertation, "The Dielectric Constant of Helium," required extensive numerical computation. The inadequacy of existing computational tools became viscerally clear: weeks of hand calculation for results that could be wrong due to a single arithmetic error.

### Formative Influences

**The Problem of Simultaneous Equations:**

Physics and engineering constantly require solving systems of linear equations. A system of _n_ equations requires roughly _n_^3 arithmetic operations to solve. For large systems (10+ equations), hand calculation became prohibitive. Existing mechanical calculators could only handle limited problems. Atanasoff experienced this frustration directly in his thesis work.

**Electronic Technology:**

By the late 1930s, vacuum tube electronics had advanced significantly. Tubes were used in radios, amplifiers, and experimental circuits. Atanasoff recognized that electronic switching — far faster than mechanical gears — could transform computation. The key insight: use electronics not just to amplify signals, but to perform logical and arithmetic operations.

**The Night in the Tavern (Winter 1937–1938):**

By late 1937, Atanasoff had spent years considering how to build a computing machine. Frustrated by the limitations of his designs, one winter evening he drove two hundred miles at high speed — a habit he had for clearing his mind — and stopped at a roadside tavern in Illinois. Over bourbon, the fundamental concepts crystallized:

1. **Electronic switching** — use vacuum tubes, not mechanical parts
2. **Binary arithmetic** — base-2 is natural for on/off electronic states
3. **Regenerative memory** — capacitors that periodically refresh to prevent charge decay
4. **Separation of memory and computation** — distinct units for storage and processing

He drove back to Iowa the next morning with the architecture of the ABC clear in his mind.

---

## 2. Intellectual Genealogy

### The Lineage: Who Influenced Atanasoff

```
Mechanical Computing Tradition
(Babbage, Monroe calculators, etc.)
        |
        v
+---------------------------------------+
| Electrical Engineering               |
| (Vacuum tubes, electronic switching)  |
+---------------------------------------+
        |
        v
    +-----------+
    | ATANASOFF |
    +-----------+
        |
        v
+-------------------------------------------------------------------+
| Mauchly (visited 1941) --> ENIAC --> Modern Electronic Computing  |
|                                                                   |
| --- Contested transmission of ideas ---                           |
|                                                                   |
| Court ruling (1973): Atanasoff's priority established             |
| ABC recognized as first electronic digital computer               |
+-------------------------------------------------------------------+
```

**Direct Influences on Atanasoff:**

- **Mechanical Calculator Tradition:** Monroe calculators, Marchant calculators — showed what computation required, but also the limitations of mechanical approaches
- **Analog Computing:** Bush's differential analyzer at MIT — powerful but limited to specific problem types, imprecise
- **Vacuum Tube Technology:** Radio engineering demonstrated that tubes could switch rapidly and reliably
- **Mathematics Faculty at Iowa State:** Colleagues who shared the computational burden and understood the need

**Contextual Influences:**

- **Physics Community:** The demand for numerical solutions to quantum mechanics problems drove the need
- **Academic Computing Culture:** Universities were centers of computational work; faculty often built their own tools
- **Pre-War Technical Environment:** The late 1930s saw increasing government interest in technical capabilities

### The Lineage: Who Atanasoff Influenced

**The Mauchly Visit (June 1941):**

| Date | Event | Significance |
|------|-------|--------------|
| December 1940 | Mauchly reads paper by Atanasoff at AAAS meeting | First learns of Atanasoff's work |
| June 13–18, 1941 | Mauchly visits Iowa State | Sees ABC, reads 35-page description, extensive discussions |
| 1943–1946 | Mauchly and Eckert build ENIAC | Electronic digital computer using concepts from ABC |
| 1964–1973 | _Honeywell v. Sperry Rand_ | Court determines ENIAC patents invalid; Atanasoff priority established |

John Mauchly, then a physics professor at Ursinus College, visited Atanasoff at Iowa State for five days in June 1941. He saw the ABC, read Atanasoff's detailed description, and discussed the concepts extensively. Two years later, Mauchly and J. Presper Eckert began building ENIAC at the University of Pennsylvania, incorporating the foundational concepts Atanasoff had developed.

**Subsequent Electronic Computing:**

- **ENIAC (1945):** First large-scale electronic digital computer; incorporated ABC concepts
- **EDVAC, UNIVAC, and descendants:** All electronic digital computers trace lineage through ENIAC
- **Modern computing architecture:** Binary representation, electronic switching, memory separation — all from ABC

**Ideas That Persist:**

| Atanasoff Concept | Modern Manifestation |
|-------------------|---------------------|
| Binary representation | Universal in digital computing |
| Electronic switching | Transistors replaced tubes; principle identical |
| Regenerative memory | DRAM refresh cycles |
| Separation of memory/computation | Von Neumann architecture |
| Parallel processing elements | Modern parallel computing |

---

## 3. The Work: Chronological

### Master Timeline

| Period | Work | Type | Significance |
|--------|------|------|--------------|
| 1937–1938 | Conceptual design | Theoretical | Fundamental architecture defined |
| 1939 | Prototype construction begins | Engineering | With graduate student Clifford Berry |
| 1939 | Iowa State grant: $650 | Funding | First institutional support for electronic computing |
| 1940 | Working prototype demonstrated | Milestone | Add/subtract unit functional |
| 1941 | Full ABC operational | Completion | 29 equations solvable; electronic digital computing demonstrated |
| 1942 | Atanasoff leaves for war work | Interruption | ABC development ends; machine never patented |
| 1942 | 35-page manuscript | Documentation | Detailed description; later crucial in court case |

### The Central Achievement: Atanasoff-Berry Computer (ABC)

> _Etymology: **ABC** — Atanasoff-Berry Computer, named for its two creators. Clifford Berry was the graduate student who translated Atanasoff's concepts into working hardware._

**What It Was:**

The ABC was a special-purpose electronic digital computer designed to solve systems of linear simultaneous equations. It was not a general-purpose programmable computer (that would come with later machines), but it was the first machine to use electronic digital computation — the foundation of all modern computing.

**Physical Description:**

- Size: Roughly desk-sized (about the size of a large desk)
- Weight: Approximately 700 pounds
- Components: Over 300 vacuum tubes, capacitors, rotating drums for memory
- Location: Basement of the Physics Building, Iowa State College, Ames, Iowa

**Key Technical Features:**

| Feature | Description | Significance |
|---------|-------------|--------------|
| **Electronic Logic** | Vacuum tubes performed arithmetic | First use of electronics for computation |
| **Binary Arithmetic** | Base-2 number representation | Natural fit for electronic on/off states |
| **Regenerative Memory** | Capacitors on rotating drums; refreshed each rotation | Solved charge decay problem; precursor to DRAM |
| **Parallel Processing** | 30 simultaneous add/subtract operations | Early parallelism |
| **Separation of Memory/Computation** | Distinct subsystems | Architectural principle still used |

**What It Could Do:**

The ABC could solve systems of up to 29 linear equations with 29 unknowns — a computation that would take weeks by hand. It used a punch-card-based intermediate storage system for the elimination process.

**What It Could Not Do:**

- Not programmable — designed for one specific task
- Card reader had reliability issues (punching sometimes failed)
- Never completed to fully production-ready state before Atanasoff left for war work

### Supporting Work

**The 35-Page Manuscript (1942):**

Before leaving Iowa State, Atanasoff wrote a detailed description of the ABC's design and operation. This document, never formally published, became crucial evidence in the 1973 court case. It demonstrated that Atanasoff had clearly articulated all the fundamental concepts before Mauchly built ENIAC.

**Patent Application (Never Completed):**

Iowa State College hired a patent attorney to file for protection, but institutional bureaucracy and wartime priorities meant the application was never completed. This failure to patent allowed the ENIAC patents to proceed — and led to decades of misattributed credit.

---

## 4. Core Ideas & Contributions

### The Central Insight

Atanasoff understood that computation could be transformed by switching from mechanical to electronic components, and from decimal to binary representation. Electronic switches (vacuum tubes) could perform logical operations millions of times faster than mechanical gears. Binary representation made electronic implementation natural: on/off states map directly to 1/0 digits. Together, these insights created the foundation for all digital computing.

This is the insight that underlies:
- Every digital computer ever built
- Binary representation in all computing systems
- Electronic switching from tubes to transistors to integrated circuits
- The separation of memory and processing units

Atanasoff didn't just build a calculator. He invented the architecture of electronic digital computation.

### Key Concepts

#### Electronic Switching for Computation

> _Technical Basis: Vacuum tubes (triodes) can act as switches — a small voltage on the grid controls a large current flow. Tubes can switch in microseconds; mechanical relays require milliseconds._

**Definition:** Using electronic components (vacuum tubes) rather than mechanical parts (gears, relays) to perform computational operations. The key insight is that tubes don't just amplify — they can compute.

**Implementation in ABC:** Vacuum tubes performed binary addition and subtraction through logical operations. The carry mechanism was electronic, not mechanical.

**Modern Application:** Transistors replaced tubes; integrated circuits pack billions of transistors; but the principle — electronic switching for computation — remains unchanged.

#### Binary Representation

> _Mathematical Basis: Any number can be represented in base-2 using only digits 0 and 1. Electronic circuits naturally represent two states (on/off, high/low voltage)._

**Definition:** Representing numbers and data in base-2 (binary) rather than base-10 (decimal). Each binary digit (bit) corresponds to an electronic state.

**Example:** The decimal number 13 is represented as 1101 in binary (8 + 4 + 0 + 1).

**Why It Matters:** Decimal representation requires distinguishing ten voltage levels — error-prone and complex. Binary requires only two states — robust and simple. This simplicity enables reliable computation at high speeds.

**Modern Application:** Universal. Every computer, phone, and digital device uses binary representation internally.

#### Regenerative Memory

> _Technical Problem: Capacitors store charge, but charge leaks over time. How do you maintain memory when the physical medium decays?_

**Definition:** A memory system where stored data is periodically refreshed (rewritten) to prevent loss from physical decay. The ABC used rotating drums with capacitors; each rotation the values were read and rewritten.

**Implementation in ABC:** Two rotating drums, each with 1,600 capacitors arranged in 32 bands of 50. As the drum rotated, each capacitor was read, amplified, and rewritten — regenerating the stored value before it could decay.

**Modern Application:** Dynamic RAM (DRAM) in every computer uses the same principle. DRAM cells are capacitors that must be refreshed thousands of times per second to maintain data.

#### Separation of Memory and Computation

> _Architectural Principle: Memory (where data is stored) and computation (where operations occur) are distinct subsystems that communicate._

**Definition:** Dividing the computer into separate units for storage and processing, rather than combining them. Data moves between memory and computational units.

**Implementation in ABC:** The rotating drums stored numbers; the vacuum tube circuits performed arithmetic; the card punch/reader provided intermediate storage.

**Modern Application:** The von Neumann architecture (memory separate from CPU) is the standard design. This principle enables flexible, general-purpose computing.

### Theoretical Framework

Atanasoff's design operated through a systematic process:

```
INPUT:  Coefficients of linear equations (via keyboard/cards)
           |
           v
+-------------------------------------+
| Store in regenerative memory        |
| (rotating drum capacitors)          |
+-------------------------------------+
           |
           v
+-------------------------------------+
| Perform elimination operations:     |
| 1. Read two rows                    |
| 2. Electronic add/subtract          |
| 3. Store result                     |
| 4. Repeat until solution            |
+-------------------------------------+
           |
           v
OUTPUT: Solution values (displayed/recorded)
```

The process was **deterministic** — given the same input, the same output would result. The electronic operations were **reliable** — vacuum tube logic was more consistent than mechanical linkages.

### Innovations & Firsts

| Innovation | Description | Prior State | What Changed |
|------------|-------------|-------------|--------------|
| Electronic computation | Vacuum tubes perform arithmetic | Mechanical gears/relays | Million-fold speed increase |
| Binary digital representation | Base-2 for all operations | Decimal (analog or digital) | Robustness, simplicity |
| Regenerative memory | Self-refreshing capacitor storage | Static memory only | Dynamic storage enabled |
| Parallel addition | 30 simultaneous operations | Serial operation | Throughput increase |
| Memory/computation separation | Distinct architectural units | Integrated designs | Flexibility, scalability |

---

## 5. Impact & Legacy

### Immediate Impact

**At Iowa State (1939–1942):**

The ABC was a working demonstration of electronic digital computing principles. Faculty and students witnessed its operation. The machine successfully solved systems of equations. However, the card reader's reliability issues and Atanasoff's departure for wartime work at the Naval Ordnance Laboratory meant the ABC was never fully completed or widely publicized.

**The Mauchly Visit and Its Consequences:**

John Mauchly's June 1941 visit created the transmission path to later computing development. Mauchly saw the ABC, read Atanasoff's documentation, and spent five days in technical discussions. Two years later, Mauchly and Eckert proposed ENIAC. The concepts from Iowa appeared in Philadelphia.

**The Lost Decades:**

For thirty years, Atanasoff received little credit. The ABC was dismantled during World War II (though the drums were found decades later). The ENIAC team held patents and received recognition. Textbooks attributed electronic computing to Mauchly and Eckert.

### The Court Case: _Honeywell v. Sperry Rand_ (1967–1973)

| Year | Event |
|------|-------|
| 1967 | Honeywell sues Sperry Rand over ENIAC patents |
| 1971 | Trial begins; Atanasoff testifies |
| 1973 | Judge Earl Larson rules: ENIAC patents invalid; Atanasoff invented the electronic digital computer |

**The Ruling:**

> "Eckert and Mauchly did not themselves first invent the automatic electronic digital computer, but instead derived that subject matter from one Dr. John Vincent Atanasoff."

Judge Larson's 319-page decision was unambiguous. The court found that Mauchly had access to Atanasoff's work, that the key concepts in ENIAC derived from the ABC, and that the ENIAC patents were therefore invalid.

### Long-Term Influence

**In Computer Science:**

- The ABC demonstrated that electronic digital computing was possible
- Binary representation became universal
- Regenerative memory principles persist in modern DRAM
- The architecture (separate memory and computation) became standard

**In Technology History:**

- The court case established legal precedent for intellectual property in computing
- Historical scholarship now recognizes Atanasoff's priority
- The "invention of the computer" narrative was corrected

**In Education:**

- Iowa State University honors Atanasoff's legacy
- Computer science history courses now include the ABC
- The reconstructed ABC (completed 1997) serves as an educational artifact

### The Counterfactual

> What if Atanasoff had completed and patented the ABC?

Computing would have developed differently — but perhaps not faster. The ABC was a special-purpose machine; general-purpose programmability came from other sources (Turing, von Neumann). However, Atanasoff might have led wartime computing efforts, and the institutional credit would have flowed to Iowa State rather than Penn and the corporate successors.

The key concepts — electronic switching, binary representation, regenerative memory — would have emerged regardless; they were implicit in the technology. But Atanasoff was first to synthesize them into a working system.

### Recognition & Honors

| Year | Recognition |
|------|-------------|
| 1973 | Vindicated by Honeywell v. Sperry Rand ruling |
| 1981 | Honorary doctorate from Iowa State |
| 1990 | National Medal of Technology from President Bush |
| 1995 | Died in Frederick, Maryland; legacy secured |
| 1997 | ABC reconstructed at Iowa State; now in Computer History Museum |

---

## 6. Study Guide: The Mental Model

### The One Sentence

> **Atanasoff built the first electronic digital computer — the ABC — in a basement in Iowa, introducing binary representation, electronic switching, and regenerative memory decades before these became the foundation of all computing.**

### The Three Things to Remember

1. **First Electronic Digital Computer:** The ABC was the first machine to use electronic switching for digital computation. Before Colossus, before ENIAC, there was the ABC.

2. **The Four Principles:** Binary representation, electronic switching, regenerative memory, separation of memory and computation — Atanasoff introduced all of these. They define modern computing.

3. **The Lost Credit:** For thirty years, Atanasoff was forgotten while others received recognition. The 1973 court case corrected history, but the lesson remains: priority in invention depends on documentation and persistence.

### The Visual

```
+------------------------------------------------------------+
|                   ATANASOFF'S ABC                          |
|              (First Electronic Digital Computer)            |
|                                                            |
|   MEMORY                COMPUTATION            OUTPUT      |
|  +------------+      +----------------+      +----------+  |
|  | Rotating   |      | 300+ Vacuum    |      | Card     |  |
|  | Drums      | <--> | Tubes          | ---> | Punch    |  |
|  | (capacitors)|     | (binary add/   |      | Results  |  |
|  | Regenerative|     |  subtract)     |      |          |  |
|  +------------+      +----------------+      +----------+  |
|       |                    |                              |
|       v                    v                              |
|   BINARY              ELECTRONIC                          |
|   (base-2)            (not mechanical)                    |
|                                                            |
+------------------------------------------------------------+
```

### Connecting to Other Figures

| If You Know... | Then Understand That Atanasoff... |
|----------------|----------------------------------|
| Charles Babbage | Moved from mechanical to electronic — the fundamental transition |
| Alan Turing | Demonstrated practical electronic computing while Turing theorized |
| 42-John von Neumann | Provided the electronic foundation von Neumann's architecture required |
| Claude Shannon | Applied Shannon's binary logic in hardware |
| Mauchly/Eckert (ENIAC) | Was their conceptual source, as the court confirmed |

### Common Misconceptions

| Misconception | Reality |
|---------------|---------|
| "ENIAC was the first electronic computer" | The ABC preceded ENIAC by years; the court ruled Atanasoff had priority |
| "Atanasoff just built a calculator" | The ABC introduced fundamental architectural principles, not just calculation |
| "The ABC didn't work" | It worked — it solved equations; only the card reader had reliability issues |
| "He was obscure because his work wasn't important" | He was obscure because he didn't patent and was overshadowed by better-funded successors |

### Test Your Understanding

1. **Technical:** Why does binary representation make electronic computing more reliable than decimal representation?

2. **Historical:** What were the specific concepts Mauchly could have learned from his 1941 visit to Iowa State?

3. **Architectural:** How does the ABC's regenerative memory relate to modern DRAM refresh cycles?

---

## 7. Going Deeper: Sources

### Primary Sources

| Source | Type | Access | Notes |
|--------|------|--------|-------|
| Atanasoff's 35-page manuscript (1942) | Technical Description | Court records; some archives | Detailed description of ABC design |
| _Honeywell v. Sperry Rand_ court records | Legal Documents | Federal court archives | 319-page decision with technical findings |
| Atanasoff's testimony (1971) | Court Testimony | Trial transcripts | First-person account of invention process |
| Reconstructed ABC | Physical Artifact | Computer History Museum | Faithful reconstruction completed 1997 |

### Essential Secondary Sources

| Source | Author | Type | What It Covers |
|--------|--------|------|----------------|
| _Atanasoff: Forgotten Father of the Computer_ | Clark R. Mollenhoff | Biography | Comprehensive account of Atanasoff's life and the court case |
| _The First Electronic Computer: The Atanasoff Story_ | Alice R. Burks & Arthur W. Burks | Technical History | Detailed technical analysis by historians who knew the principals |
| _ENIAC: The Triumphs and Tragedies of the World's First Computer_ | Scott McCartney | History | Context on ENIAC with Atanasoff's role |
| _The Computer from Pascal to von Neumann_ | Herman Goldstine | History | Contemporary account (though reflects pre-1973 perspective) |

### Modern Introductions

- **For general readers:** Mollenhoff's biography provides accessible narrative
- **For technical readers:** Burks and Burks offer detailed technical analysis
- **For legal/historical perspective:** The court decision itself is remarkably readable

### Online Resources

- [Computer History Museum: Atanasoff-Berry Computer](https://computerhistory.org) — Includes reconstructed ABC
- [Iowa State University Atanasoff Archive](https://www.cs.iastate.edu/atanasoff) — Primary documents and history
- [Honeywell v. Sperry Rand decision](https://law.justia.com) — Full text of 1973 ruling
- Wikipedia: "Atanasoff-Berry Computer" — Good starting point with extensive references

---

## Appendix: Handling Uncertainty

> **Note on Historical Controversy:** For decades, the invention of the electronic computer was attributed to Mauchly and Eckert. The 1973 court decision resolved this definitively for legal purposes, but some historians still debate the extent of transmission from Atanasoff to Mauchly. The facts below reflect the court's findings and scholarly consensus.

| Claim | Confidence | Source |
|-------|------------|--------|
| ABC was first electronic digital computer | High | Court ruling; scholarly consensus |
| ABC used binary, electronic switching, regenerative memory | High | Physical evidence; documentation |
| Mauchly visited and saw ABC (June 1941) | High | Court testimony; letters; multiple witnesses |
| Mauchly derived ENIAC concepts from ABC | High (legally) | Court ruling; debated by some historians |
| ABC was fully operational | Medium | Card reader issues; never production-ready |
| Atanasoff's bourbon-tavern insight story | Medium | Atanasoff's account; no independent verification |

---

_Last updated: 2026-03-26. This is a living document._