Many of of the computers built in the 1940s used relays for logic (see here and here):

  • Bell Labs Model I, 1940
  • Bletchley Park Bombe, 1940
  • Zuse Z2, 1940
  • Zuse Z3, 1941
  • Bell Labs Model II, 1943
  • Bell Labs Model III, 1944
  • Harvard Mark I, 1944
  • Bell Labs Model IV, 1945
  • Zuse Z4, 1945

  • Bell Labs Model V, 1946
  • Harvard Mark II, 1947
  • IBM SSEC, 1948
  • Bell Labs Model VI, 1949
  • Office of Naval Research Relay Computer, 1949

However, vacuum tubes were already available in this era. The triode was invented in 1906, various tetrodes from 1913 to 1927, and the pentode in 1926. Some computers were starting to be made with vacuum tubes during the 1940s:

  • Atanasoff–Berry Computer, 1942
  • Bletchley Park Colossus, 1943
  • ENIAC, 1945

  • Manchester Baby, 1948
  • Manchester Mark 1, 1949
  • EDSAC, 1949
  • BINAC, 1949
  • CSIRAC, 1949

The divider lines above split the wartime and postwar halves of the decade. Late 1940s computer models appear to be equally divided between relay and vacuum tube, but the early half of the decade had three times as many relay models as vacuum tube. Why were early 1940s computers predominantly made from relays?

("Because that was what was available" may possibly be an answer, but it needs to be supported by a reputable source, rather than speculation or argument.)

Possibly related: Have there been any studies comparing the reliability of relay versus vacuum tube computers?

  • 4
    Vacuum tubes may have been produced in the 1930s & 1940s, but they may not have been produced in numbers necessary. Their cost may have been a factor, but I suspect the experience of using them was a main factor. Some of the tubes were being used in newly developed radar systems.
    – Fred
    May 16, 2021 at 11:16
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    One reason that comes to mind is that Vaccuum tubes were unreliable in terms of continuous operation, they often broke and had to be replaced. From what I remember, you could count on this happening daily, if not multiple times per day.
    – Issel
    May 16, 2021 at 19:44
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    Price and reliability. For Zuse it was also a question of availability. His relays were recycled telecom relays he got cheaply as they were decomissioned. May 17, 2021 at 7:02
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    What about the question of voltages in the power system? For relays the exact voltage is normally not that critical. A modern 12 V relay can handle about 9V up to about 20V and work properly. On the other hand, as I understand, the filament voltage on a valve is quite critical. Just 10% plus and you decrease the life time very much. Having a even voltage for all those valves can't have been a easy job.
    – UncleBod
    May 17, 2021 at 11:38
  • Beside the huge difference in heat dissipation (energy requirements) of the two technologies, I suspect that the older tech would have been a lot cheaper, due to the abundant sources. I also suspect the material requirements for manufacturing, were more easily met for relays, than for vacuum tubes, particularly during war time. But a quick search did not turn up any useful price references.
    – jwdonahue
    May 17, 2021 at 20:56

8 Answers 8


Using relays to implement logic functions was already quite well understood at that time, and in fact the Post Office type relays were designed to do just that as part of the telephone system. The fact that relays were also very reliable, could run on relatively low voltages, and were relatively power efficient were also marks in their favour.

The major downside was that relays switch rather slowly, and this limits the speed of the resulting computer; this could be addressed in part by using smaller relays, but note that even the "one instruction per second" Harwell Witch was considered fast enough to be useful, largely because it was reliable and, unlike a human with a mechanical desk calculator, could work day and night without pause and without mistakes.

By contrast, thermionic valves were primarily used for analogue functions at that time, and their use for implementing digital logic was not immediately obvious to people who hadn't already done it as part of (top secret) wartime work. Even the analogue computers of the time, primarily built for fire control, were mainly mechanical with only a few electronic components. These valves typically required over a hundred volts of bias and a continuous cathode heater current, the latter of which made using more than about a dozen of them in any given device a daunting prospect from a heat dissipation standpoint.

It was also believed at the time, and not without reason, that valves were simply not reliable enough to be used in their thousands in a single machine, even assuming the power supply and heat problems were solved. The quality of valve manufacturing had improved dramatically during the war due to the sophisticated radio equipment needed in quantity. It was later discovered that running the cathode heaters below rated power and avoiding the thermal cycling of switching the machine on and off could improve reliability markedly, which helped to encourage experimentation with valves in digital computers.

  • 24
    @RobertHarvey, "here?" as in, "here on the Internet?" May 16, 2021 at 15:18
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    @RobertHarvey Here "not in Britain" is called "there". May 16, 2021 at 16:47
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    @RobertHarvey Since the early history of digital computing centred on Britain, the use of British terminology to describe it is simply correct.
    – Chromatix
    May 16, 2021 at 17:03
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    @RobertHarvey here we call it "válvula termoiónica" ;)
    – Aaron F
    May 16, 2021 at 22:22
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    @RobertHarvey I love how the British term describes how the device operates, and what is its function... while the American term is basically "uh, it's that glass thing with no air inside" :D And just for fun, here it's called "elektronka", which refers to it conducting electricity through a vacuum, or informally, "lamps", so again, describing how it works. It comes from Germany, another pioneer of the technology. Most of the world calls them after what their inventors and developers did, and then there's the US, like usual, and Edison claiming the discovery in the US, as usual. Just fun :)
    – Luaan
    May 17, 2021 at 7:44

Why were early 1940s computers predominantly made from relays?

It was the technology proven to work reliably in large scale. A computer is large scale application, something a radio isn't. Building a new application is usually done by using a proven tools, not by inventing horse and carriage at the same time one wants to introduce a delivery service.

"Because that was what was available"

Well, that's exactly the point. Plus:

It's good enough for the job

as the task at hand was not creating a fantasy device with the most advanced technology possible, but a device that solves the task of creating a working computer.

Relays were not only already operating, binary and well proven, but equally importantly, proven to be reliable in large scale applications. Valves in contrast are analogue devices that were only used in small numbers per application. A radio may need just one valve, but already the ABC, a non-programmable fixed-function calculator, used 300.

Beside the much higher effort to control 300 valves instead of a similar number of relays, its power consumption, the introduced failure rate goes to the power of devices used.

An all to gain something that isn't asked for?

At that point it's always important to keep in mind that the most significant gain in speed is not making the fastest computer possible, but making a computer at all. Having a Z3-like machine with 20 multiplications a minute will be many thousand times faster than doing it by hand. Making it twice or even 10 times faster isn't as much of a gain, and not worth the additional effort and delay.

A great example here is Zuse and his friend Helmut Schreyer's attempt to build a valve based-computer. Schreyer built basic digital circuits (logic gates, flip-flop *1) with valves as early as 1937. In 1938, Zuse and Schreyer demonstrated a basic valve setup for a computer at the TH Berlin (Berlin Institute of Technology) - in part as attempt to get funding for a 2000 valve machine. It got rejected as unrealistic idea (Phantasterei), but lead to acquiring funds for the later (relay-based) Z3. This was obviously seen as more realistic than a valve based computer (*2). See here for some comments by Konrad Zuse made in 1989 (Sorry, only German :))

Bottom line:

Let's first build a working computer and use it before going about science fiction devices :)

Also known as avoiding premature optimization.

may possibly be an answer, but it needs to be supported by a reputable source, rather than speculation or argument.)

Sorry, but that's asking exactly for argumentation done in hindsight. Why should have someone reasoned for (i.e. defended) not using an upcoming technology (valves vs. relays), when the primary concern is to create/prove an application (i.e. a computer), not proving that new technology?

Schreyer's developments do very clearly show that the implications of valves as switches were understood, but simply deemed too far-fetched, no matter how hard their proponents pushed. There was no need to use them.

"Why haven't they done X when its so obvious that Y is better' is quite related to a hindsight bias. Asking for "reputable sources, not argument" doubles down on this, as any such must be contemporary to (or before) the design in question, a point where this reasoning isn't of any concern. Or would anyone today make a (serious) reasoning about not using personal flight devices for a pizza delivery service?

Such reasoning will (and needs) only be made when the new(er) technology is proven to be practical and deliver a relevant benefit, outweighing any drawback.

*1 - Schreyer received a patent for valve based computational memory in 1943.

*2 - Schreyer did continue to work with valves and got his doctors degree in 1942 with "Das Röhrenrelais und seine Schaltungstechnik" - literally vacuum tube relay and its circuit technology. The same year he finished an experimental setup with 100 valves, somewhat comparable to the ABC.

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    @DrSheldon So is the question - it's not doing itself a favour by forcing a certain answer. For more details, just click on his (linked) wiki entry, or any of the many pages filled with information to his (and Zuse's) work. (also it's Schreyer, Speyer is a city in southwestern Germany :))
    – Raffzahn
    May 16, 2021 at 14:43
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    @DrSheldon now you have the thing to search you can improve your own research.
    – Solar Mike
    May 16, 2021 at 15:11
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    If you want to see some early designs demonstrating the complexity of relay-based technology, look at 19th-century pipe organ consoles. These were basically a 3-dimensional network connecting a few hundred inputs from the keyboards, a hundred or so inputs from the stop controls, and the wind supply to several thousand pipes. The network topology was many-to-many, not just a branching tree structure. Oh, and they also contained "user-editable non-volatile stored program memory" as well. In fact there were a few individuals who were pioneers in both organ design and telephone exchange design,
    – alephzero
    May 16, 2021 at 19:46
  • @alephzero: Were any significant number of electropneumatic pipe organs built prior to 1900? Certainly they were widespread by the 1950s, but before 1900? Also, the large relays on pipe organs were seldom operated by electomagnets or solenoids, but instead operated by bellows with an air (or possibly vacuum) supply that was switched using a small electromagnetic valve. I don't know that it would be worth a standalone question, but I'm curious whether electropneumatic relays were used in any other applications.
    – supercat
    May 17, 2021 at 16:54

An couple of under-appreciated advantages of relays over tubes is that a single relay coil could operate multiple sets of contacts, and relay contacts could easily be wired into a variety of series and parallel arrangements that don't really work with tubes. Although relays that go beyond DPDT are today somewhat uncommon, and those that go beyond 4PDT are even more so, back in the days when relays were used for signal switching rather than power switching, they were commonly available in much wider configurations. Until pinball machines started being controlled by microprocessors, they would routinely use relays that could have up to eight DPDT sets of contacts installed (contacts that weren't used would typically be omitted, but nearly all of the actuating mechanisms would have slots for all eight contacts). Further, it was common to have latching relays which could be arranged in banks, with one activating coil per relay and one release coil per bank. When using ordinary tubes, from what I can tell, a group of N valves (of which there could typically be two in each glass tube) would behave as a combination of N two-input AND gates feeding an NOR gate. While a single output from such a tube-based gate could drive the inputs of many others, each such gate could only switch one output. Thus, the number of valves required to perform many functions would exceed the number of relay coils needed to achieve them.

If anything, what I find curious is that there wasn't more use of electromechanical logic within tube-based computers, outside of the Atanasoff Engine. Something like the Eniac had thousands of digits of registers whose contents could be rapidly read in arbitrary sequence, but could only be set by hand. I would think it would have been more useful to have a means by which either a punch-card or punch-tape operated device could be used to set the state of all the switches, or else replace each group of 72 digits worth of switches with a set of spring plungers located above a card slot, so that the machine could be configured simply by placing cards in all of the appropriate slots.

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    The ENIAC architecture was essentially an analog computer implemented with digital "accumulators" replacing analog integrators. It was to be set up to solve problems just like an analog computer, with patch cords directing data flows. The "function tables" were the equivalent of the cams that one would fabricate to evaluate functional relationships in an analog computer. There were many features of the machine that seem inefficient in retrospect, but of course future knowledge was not available to its designers.
    – John Doty
    May 16, 2021 at 23:35
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    @JohnDoty The ENIAC is also a wonderful study in how expensive and almost fruitless trying to jump a technology gap is; instead of incremental designs and quick feedback cycles, it was a massive project that cost a fortune and was known to be technologically far behind three years before completion. It's obvious it wouldn't have been made if not for the war. That doesn't mean it wasn't useful, of course, especially considering the wartime conditions. But it was also a show of just how much spare industrial capacity the US had in the war, while e.g. Zuse's work was cut as "unimportant".
    – Luaan
    May 17, 2021 at 7:56

A large number of engineers had been building telephone switching networks for half a century, and phone company suppliers thus had lots of experience manufacturing good (telco quality) relays in vast numbers. Whereas vacuum tubes had never been used for digital logic circuits until Atanasoff–Berry, Tommy Flowers' team at Bletchley Park, and Eckert and Mauchly's team on the ENIAC (influenced by Atanasoff) experimented. The tubes themselves, having never been originally designed for digital purposes, their reliability at first was quite low. (I would not be surprised if digital square waves put a lot more thermal cycling stress on these new vacuum devices than the same amplitude of more sinusoidal-like radio signals.)

So there was both a learning curve, and a manufacturing cost and reliability curve, that had to be climbed before a more sufficient number of engineers knew enough, and could get good enough components at a reasonable price, to design digital logic with vacuum tubes with equally reliable to relay switching networks.


Relays had been around for a century. They are electrically controlled switches, naturally adapted to logical operations. There was a rich culture of using them as logical elements in telegraph, railway signaling, and telephone switching systems.

On the other hand, use of tubes as logical switching elements was new and exotic. Rossi's coincidence circuit dates from 1930. Being a pioneer in both architecture and implementation technology at the same time was too great a leap for early computer designers.


Here is a relay computer that was built in Japan in 1958 and is still going... https://www.youtube.com/watch?v=_j544ELauus

I have wound relays. It is not hard. I have never made a valve.

Early computing missed the plasma display. A neon lamp may take 20 volts to strike it, but will keep going with 5 volts. If you have a set of X wires at 0V and Y wires at 10V then each lamp joining an X and a Y will stay lit or stay off. If you drive one X wire to -6 volts and one Y wire to +16 volts you light the lamp at the junction of these two lines. If you set the same two lines to +4 volts and +6 volts you will put it out. You could potentially stick a lot of memory inside a single valve. There were counting tubes called decatrons that counted to the base 10, but they never made large chunks of X-Y addressable memory.

  • The question is asking about what happened during the 1940s, not other decades.
    – DrSheldon
    May 18, 2021 at 8:12
  • Yes, but relays were still a good choice a decade on. May 18, 2021 at 10:20
  • From what I understand, neon would be much slower than thermionic tubes or magnetic cores. On the other hand, I find myself curious whether it may have been practical to design a write-only "bitmap" plasma display with the ability to set or clear an arbitrary subset of pixels on a line at a time (leaving the remainder undisturbed) using one pin per row or column. Such a display would require a resistor per pixel inside the glass, but I think there should be some way of mass-producing that such as by fabricating a "comb" from ceramic dipped in conductive material.
    – supercat
    May 18, 2021 at 17:28
  • The early plasma displays did have the ability to blank rows and columns, and you could read from them as well as write to them. I don't know about the switching speed, but I feel it must have beaten relays. May 19, 2021 at 18:32
  • @RichardKirk: You're saying that there were no devices that used addressable plasma for large chunks of X-Y addressable memory, but also saying there were plasma displays that were readable as well as writable. Did the displays that allowed blanking of rows and columns as well as readback use the neon for data storage, or merely for presentation?
    – supercat
    May 25, 2021 at 15:55

The question asked why early 1940s computers were predominantly made from relays. The answer to this question might be simplified if limited to discussion of factual commercial and military activities in the United States from about 1940 to 1950.

Relays were durable, and even as scrap many reclaimed relays worked ok. Tubes were more difficult to find. And tube radios were relatively expensive because tubes weren’t cheap. In essence, with the country in the grip of the depression, there were not many useable surplus tubes prior to 1940. One would not expect to find surplus or used tubes in good working condition because the usual case was such tubes were replaced as they burned out and were thrown away. One may keep a spare tube or two at hand to maintain a working radio. But then after the war began, there were virtually no tubes available at all except those already available in commercial stock. Tubes were allocated for wartime uses with few available for domestic needs. But the relays were good and in computation, using relays was a different story. But tubes, nobody thought much of using them in computing; hard to get, would burn out. Nevertheless, the country was making preparations for going to war.

However one reads the question, given the current retrospective view, the premise to the question and its answer may be biased somewhat, or even considerably, by presently held impressions. In fact, aspects regarding the early development of the computer seem to be clouded by interpolation, conjecture, and speculation, hagiography, and character assassination. The interpersonal dynamics in development of the computer included end-running, ignoring valuable contributions while taking credit inappropriately, refusing to act on a reasonable request, back-biting, etc., etc., etc. Even views of the much later patent case in Honeywell Inc vs Sperry Rand Corp., (i.e. asking too much for due royalties) and the decision itself, are filled with anger and controversy. People of this early 1940s time showing an innocent vital interest in then-current thinking regarding computational technology find themselves in corporate life 30 years later portrayed in conflict with their own corporate interests and now-current entrepreneurial intentions. Royalties were a corporate survival cornerstone. Looking at this question about relays from the viewpoint of the developments of the 1940s, and understanding the needs in extended computation necessary to solve certain war-time problems, a rather different perspective is seen. Maybe the counterpoint of the question should have been asked, namely, why weren't early 1940s computers predominantly made with tubes instead of relays. In fact, development of the computer relied on a simple triode tube circuit that would be used as a relay in electronic computation.

By the start of 1940, the number of functioning, non-analogue, electromechanical relay computers could be counted on one finger. Mechanical hand-assisted calculation with a Marchant or Monroe calculator was the name of the day. Or one skillfully used a slide rule. Prior to 1940, the country was within the grip of the Depression. There was no pressing need for extended computation. Before involvement of the US in the Second World War, the perfection of extended computation was in the development of efficient and innovative analogue devices such as the Bush differential analyzer. Ideas and prototypes were just emerging in the use of electromechanical relays as counting and switching devices useful in rudimentary computation. At least one analogue computational device, the M-9, would be perfected for antiaircraft ballistics use during the Second World War. And successive developments in relay computation would also progress.

Lets digress and examine that aspect. In the United States, the situation in developing calculating machine automata begins in 1937 with George Stibitz building the first prototype relay computation module on his home kitchen table. He worked for Bell Laboratories, and had access to their surplus scrap and reclaimed material. Consequently he was able to get access to reclaimed and salvaged relays that were perfectly functional. Company executives, although unimpressed at first, realized the potential value in Stibitz' device and agreed to finance building a larger version. Bell needed a way to solve their math problems that largely involved math with complex numbers. With the design Stibitz completed in 1938, work began on the requested machine in April 1939 and it was operable in January 1940. Although Stibitz was given leave from Bell Labs and would work for the National Defense Research Committee for a while, and then Office of Scientific Research and Development, he continued in design and development of relay computational equipment with the support of Bell Labs until well after the war ended. Several more advanced relay-based devices were designed and completed under his direction, the last of the most advanced series incorporated error checking, an assembly language, and floating point arithmetic. His general purpose devices were used by the US Navy for ballistics calculations and gun control. And his most advanced systems, the last two constructed, were operable and productive until 1960. They were essentially in fully functional working condition when they were retired.

Also in 1937, Howard Aiken, at Harvard, would propose construction of a large scientific calculator for the extended calculation of problems that were beyond the reach of hand computation. Aiken was a seasoned engineer and was back at Harvard to pursue an advanced degree. But Harvard astronomer Harlow Shapley sent Aiken to IBM, instead. Astronomy requires some hard-to-do calculations. The year was now 1938; Thomas J. Watson, Jr., president of IBM, was sufficiently impressed with Aiken's ideas, that he assigned IBM engineer Clair D. Lake to supervise the design and construction of the Harvard machine. Along with Aiken of Harvard, and Lake of IBM, Francis Hamilton and Benjamin Durfee, both of IBM, would lead in the development, construction, and eventual completion of the Harvard machine. Commissioned by the Navy, the Harvard machine, when completed in 1944, was used in the calculation of ballistics problems. Electromechanical relays comprised the computing engine of the Harvard machine, a massive device. From concept to completion required seven years. When completed, utility of the machine has been characterized as already obsolete. After the war IBM would build one more-advanced relay machine of this type that was programmable; both machines were potentially useful.

So lets look again at the status of relay computation in the early 1940s. In the United States prior to January 1944, there were two working relay computational devices, both built by George Stibitz at Bell Telephone Laboratories. Zuse's work in Germany was not accessible for computation as he struggled to retain his foothold on constructing a working relay computer while trying stay ahead of the war. His prototype relay systems were war casualties, but luckily he was not. And as a citizen of an enemy country, his situation was quite tenuous. He was not able to secure an advanced working prototype until after the war ended.

However, in 1941 as the United States entered into war (ultimately in two world theaters), pressing needs in computation required faster and extended means of computation for ballistics calculations. The US Army Ballistics Research Laboratory required ballistics tables for various artillery applications being used during the war. These needs also could not be fully met by differential analyzers. Even though relays, as such, were very effective, they were only marginally effective at meeting the computational needs of the time. There were no fully assembled relay computers for use in extended computation until just before the war ended. The use of tubes as relay-switching devices really did not differ at all from using electromechanical relays. Using tubes as relays is more efficient because tubes are electronic, they have no mechanical parts, and they are incredibly fast.

In 1941, the attention of industry was directed solely toward supporting the war effort and meeting pressing domestic needs. Industry support for expanded commercial computation was not part of that picture. IBM, nevertheless, did provide businesses with various equipment designed to facilitate tabulating needs. By 1942, war restrictions and rationing were in place. Nevertheless, expansions in computation were necessary to support the computation of ballistics tables needed by the Army and Navy, required a prodigious amount of work in the hand calculation of projectile trajectories. If one knew math and how to efficiently use an electric calculator, one had a secure job. Typically, a skilled computer would require about 20 hours to hand-calculate a single, one-minute projectile flight trajectory for the ballistics table. But the problem was further complicated by factors such as air resistance, a variable number not easily specified or determined numerically. When multiple trajectory calculations were completed for various conditions given a specific use, the end results were compiled into firing tables comprised of hundreds of different trajectories. A computational team of 200 or more would require two to three months for the completion of such a compilation. Whenever a new combination of shell, gun, or firing propellant was used, a new set of firing tables specific to these new uses was necessary. The Army attempted to improve the calculation process using state-of-art differential analyzers, but the limit of using this device was quickly reached - they had hit the wall. Nevertheless, several significant events occurred that were all consequential to needing projectile trajectory determinations, thereby setting forth the future path of computation well into the second half of the 20th century. Somewhat ironically, the machine specially constructed to do the ballistics calculations, was not initially used for that specific purpose.

The US Army was already in a cooperative relationship with the University of Pennsylvania Moore School for Electrical Engineering for computational assistance regarding firing tables (Moore School had a differential analyzer and hired lots of skilled computers) when, in 1942, Dr. John Mauchly, who had recently secured a teaching position at the Moore School, suggested to the School's Army liaison officer Lt. Herman Goldstine, also a Ph.D ballistics mathematician, that calculation of firing tables might be more easily accomplished with an electronic computer. Dr. Goldstine was interested. Mauchly had been working with triode flip-flop counters in aspects of meteorological research and realized their potential use as a module in application to computation. He had also been talking with others (such as Atanasoff and Stibitz) and seeing what they were doing in computation because he saw a pressing wartime need, as the US was preparing for war, and he was in an unique position to evaluate the problem.

J. Presper Eckert, a graduate teaching fellow at the Moore School, had just recently met Mauchly at the School the previous year where Mauchly was taking a defense training course for electronics, and Eckert was helping instruct. Eckert was keenly interested in Mauchly's ideas, and the two became friends. Eckert was in his early twenties yet nevertheless a superb electrical engineer. He was very familiar with triode flip-flop circuits and how they could be used. These circuits had been around for more than 20 years. Eckert and Mauchly put their heads together and submitted their proposal to the Army in a memo dated April 2, 1943, for an electronic computer to produce ballistics calculation results like those "in the computation of a trajectory by hand." Their estimated time required to compute the compilation of trajectory tables for any particular application would be about two days. A contract with the Army was secured, assigning Goldstine as technical liaison, Mauchly in charge of project development, and Eckert in charge engineering design and construction. With Eckert in charge of engineering, a team of about 20 engineers would build this machine.

Although everything to be used was then currently available, fundamental to operation was use of the flip-flop circuit. The use of cross-connected triodes as counting devices had been known since about 1919, and their switching times are incredibly fast. This rather elementary circuit would form the basis for large-scale calculation in support of the war effort. The only problem encountered would be in building the machine. Parts were, at times, unavailable for a while. The design using triode tubes, nevertheless, was innovative, straightforward, essentially elementary, and required nothing that was new.

What, exactly, is a flip-flop? Two triode tubes were cross connected so that when one of the tubes was energized, the other was inactive. If the other tube was than energized, the first would become inactive. This flip-flop triggering scheme, in effect, makes the device an incredibly fast electronic relay. Operation of this device as a counter would be typically at 100,000 counts per second. By arranging these flip-flops into interconnected decade counters, an electronic adding machine is produced. The machine Eckert would construct had 200 such decade units giving the machine a memory of twenty numbers of ten digits each.

The project, classified as secret, took about a year to design and another year and a half to build in the basement of the Moore School. Securing the necessary materials was sometimes not possible due to war-time shortages, and delays in completion of the project were incurred. And the project was over budget, costing about $500,000. When the project was completed in November 1945, preliminary tests were completed. The machine dedication was February 1946. But the first task at hand was not calculation of ballistics tables. This machine, really a massive and fast electronic-relay calculator, was needed to do extended calculations for the Cold War effort in Los Alamos. The Los Alamos group (Teller) needed to know if certain design concepts would work in building an H-bomb.

By 1950, Eckert and Mauchly had already realized they needed leave the Moore School and start their own company if they were going to continue productively in developing large-scale computing machines. Their relationship with the Moore School had deteriorated as the School had pressed them, time and again after the war ended, to sign over their patent rights in the design of the ENIAC, which they steadfastly refused to assign to the University. Mauchly, on assignment from the University's Moore School to the ENIAC project, resigned. Eckert, also an university employee, was fired but technically he had already submitted his resignation, as well. Consequently, these two friends ultimately formed what would become the Eckert-Mauchly Computer Corporation to build and sell the first commercially available electronic computer. The rest is Univac history.

This was an excellent question. The answer provided herein may not have been exactly as sought, but then again, maybe it was. Discussion was limited to activities in the United States from just before 1940 to about 1950. Of course, Zuse’s important work in Germany was also mentioned in context. Information used in preparation of this answer came from personal recollections regarding Eckert speaking at Arizona State University in 1975. He was not embittered but philosophical about the outcome of the Honeywell patent-infringement case. Further information was derived from various sources in researching the answer to this question, including Charles and Ray Eames' book, A Computer Perspective, published by Harvard in 1973. This book is no longer in its original press. Additional information was derived from the reference cited in answering the ancillary question about relays and tubes asked by Dr Sheldon, linked with this question, above.

  • Do you know if any efforts were made to design and use tubes which were optimized for computation? I would think an 8-pin tube tube with a filament cathode that computed not ((A and B) or (C and D)) and its complement would be more useful for many computations than an 8-pin dual triode with isolated cathodes, but would be simple to construct (stack a triode and two tetrodes on top of each other, with a shared filament running through them; connect the two tetrode anodes to each other and the grid of the triode).
    – supercat
    May 25, 2021 at 20:59

If one looks at new tube types designed in the 1950s, explicitly for computer service, a few important innovations become apparent:

There were designs that dealt with the "cathode interface" effect that would destroy vacuum tubes meant for receiver service rather quickly if you used them for "hard" switching tasks ....

Also, if you look at "Eniac" era designs: Octal base tubes were apparently still prevalent, and construction techniques adapted to the size of these tubes, making tube circuitry rather larger than a relay based system. "Computer grade" tubes tended to use the newer 9-pin noval and 7-pin miniature designs, which are in practice half as large (if you go by volume, 1/5th to 1/10th :) ) the size of octal types... At least the 7-pin socket was known in the Eniac era, but apparently not very mainstream...

Without these details taken care of, building computers from existing common tube types probably was not very rewarding.

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