Editing ZX81 (section)

== History ==
=== Background ===
[[File:SinclairExecutive-01.jpg|right|thumb|The Sinclair Executive "slimline" pocket calculator (1972)|alt=A black rectangular calculator being held in a person's right hand.]]
Clive Sinclair, a former radio kit salesman, established his first company, [[Sinclair Radionics]], in 1962.<ref>{{Cite web |last=Machkovech |first=Sam |date=2021-09-17 |title=RIP Sir Clive Sinclair, creator of UK's famed ZX Spectrum gaming computer |url=https://arstechnica.com/gaming/2021/09/rip-sir-clive-sinclair-creator-of-uks-famed-zx-spectrum-gaming-computer/ |access-date=2022-12-07 |website=Ars Technica |language=en-us}}</ref> The company made its name producing a wide range of cheap electronics aimed at the hobbyist market.<ref>{{Cite web |last=Manners |first=David |date=2021-09-20 |title=Uncle Clive |url=https://www.electronicsweekly.com/news/business/uncle-clive-2021-09/ |access-date=2022-12-06 |website=Electronics Weekly |language=en}}</ref><ref>{{Cite web |last=Price |first=Chris |date=2021-09-17 |title=Sir Clive Sinclair RIP – 10 things about the great man you may not know |url=https://www.techdigest.tv/2021/09/sir-clive-sinclair-rip-10-things-about-the-great-man-you-may-not-know.html |access-date=2022-12-06 |website=Tech Digest |language=en-GB}}</ref><ref name=":0">{{Cite web |title=Sinclair ZX81 - Computer - Computing History |url=https://www.computinghistory.org.uk/det/184/Sinclair-ZX81-Issue-One/ |access-date=2023-08-09 |website=www.computinghistory.org.uk}}</ref> Its products include [[amplifier]]s, radios, [[multimeter]]s and other items which were generally sold in kit form to hi-fi enthusiasts and other electronics hobbyists.{{sfn|Dale|1985|p=39}} The company entered a new market in 1972 when it launched the first "slimline" pocket calculator, the [[Sinclair Executive]].{{sfn|Adamson|Kennedy|1986|p=31}} Radionics followed up by launching a wide range of pocket calculators. The company's subsequent expansion made it Europe's biggest calculator manufacturer by 1975.{{sfn|Adamson|Kennedy|1986|p=35}}<ref name=":0" />

By the late 1970s, however, Sinclair Radionics was experiencing serious difficulties{{Opinion|date=March 2023}}.{{Citation needed|date=March 2023}} It lost its ability to compete effectively in the calculator market following the launch of a new generation of Japanese-produced calculators with [[liquid-crystal display]]s, which were much more capable and power-efficient than Sinclair's [[light-emitting diode|LED]] calculators.{{sfn|Adamson|Kennedy|1986|p=36}} Projects to develop a pocket television and digital watch turned out to be expensive failures. The company made losses of more than £350,000 in 1975–76, bringing it to the edge of bankruptcy.{{sfn|Adamson|Kennedy|1986|p=39}} In July 1977 Radionics was rescued by a state agency, the [[National Enterprise Board]] (NEB), which recapitalised it, provided a loan facility and took effective control of the company by acquiring a 73% stake.{{sfn|Adamson|Kennedy|1986|p=35}}

Clive Sinclair's relationship with the NEB was fraught due to conflicting notions about which direction the company should go. Radionics had begun a project to develop a home computer but the NEB wanted to concentrate on the instrument side of the business, which was virtually the only area where Radionics was profitable. Sinclair disagreed vehemently with what he characterised as the view "that there was no future in consumer electronics".{{sfn|Adamson|Kennedy|1986|p=65}} This and other disputes led to Sinclair resigning from Radionics in July 1979.{{sfn|Adamson|Kennedy|1986|p=66}}

While he was struggling with the NEB, Clive Sinclair turned to a "corporate lifeboat" in the shape of an existing corporate shell under his exclusive control – a company called Ablesdeal Ltd, which he had established in 1973 and later renamed Science of Cambridge. It became a vehicle through which he could pursue his own projects, free of the interference of the NEB.{{sfn|Adamson|Kennedy|1986|p=68}} Despite his later success in the field, Sinclair saw computers as merely a means to an end. As he told the ''[[Sunday Times]]'' in April 1985, "We only got involved in computers in order to fund the rest of the business", specifically the development of the ultimately unsuccessful [[TV80]] pocket television and [[Sinclair C5|C5]] electric vehicle.{{sfn|Adamson|Kennedy|1986|p=86}} In an interview with ''[[Practical Computing]]'', Sinclair explained:

{{blockquote|I make computers because they are a good market, and they are interesting to design. I don't feel bad about making them or selling them for money or anything, there is a demand for them and they do no harm; but I don't think they are going to save the world.<ref name="Hayman" />}}

=== Precursors: the MK14 and ZX80 ===
[[File:Sinclair - Science of Cambridge MK14.jpg|right|thumb|The Science of Cambridge MK14 (1978), sold in kit form, was Sinclair's first computer.]]

By the late 1970s, American companies were producing simple home computer kits such as the [[MITS Altair]]<ref>{{Cite web |last=Cass |first=Stephen |date=2018-02-23 |title=Build Your Own Altair 8800 Personal Computer |url=https://spectrum.ieee.org/build-your-own-altair-8800-personal-computer |access-date=2022-12-07 |website=IEEE Spectrum |language=en}}</ref> and [[IMSAI 8080]].<ref>{{Cite web |last=Lendino |first=Jamie |date=2022-04-15 |title=45 Years Ago, Apple Kickstarted the Personal Computer Industry |url=https://www.pcmag.com/news/45-years-ago-apple-kickstarted-the-personal-computer-industry |access-date=2022-12-07 |website=PCMAG |language=en}}</ref> This aroused interest among electronics hobbyists in the UK but relatively high prices and lower disposable income reduced the appeal of the American products. ''[[New Scientist]]'' stated in 1977 that "the price of an American kit in dollars rapidly translates into the same figure in pounds sterling by the time it has reached the shores of Britain".{{sfn|Valéry|1977}} Off-the-shelf personal computers were also available for the high end of the market but were extremely expensive; Olivetti's offering cost £2,000, and the Commodore PET, launched in 1979, sold for £700. There was nothing for the hobbyist at the low end of the market. Sinclair realised that this provided a useful commercial opportunity.{{sfn|Dale|1985|p=95}}

Sinclair's first home computer was the [[MK14]], which was launched in kit form in June 1978.{{sfn|Adamson|Kennedy|1986|p=80}} It was a long way from being a mass-market product. Its very name – MK standing for "Microcomputer Kit" – was indicative of its origins as a product developed by, and for, hobbyists.

It had no screen but instead used an LED [[segment display]] (though Science of Cambridge did produce an add-on module allowing it to be hooked up to a UHF TV); it had no case, consisting of an exposed circuit board; it had no built-in storage capabilities and only 256 [[byte]]s of memory; and input was via a 20-key [[hexadecimal]] keyboard.<ref>[[#MK14advert|MK14 advertisement (1978)]]</ref>

Despite the limitations of the machine it sold a respectable 10–15,000 units;{{sfn|Adamson|Kennedy|1986|p=81}} by comparison, the much more expensive [[Apple II]] had only sold 9,000 units in the United States, a much bigger market, in 1978.{{sfn|Adamson|Kennedy|1986|p=61}} This success convinced Clive Sinclair that there was an untapped market for low-cost computers that could profitably be exploited.

Sinclair followed up the MK14 by producing the ZX80, at the time the world's smallest and cheapest computer, which was launched in January 1980 costing £99.95 (equivalent to £390 at 2021 prices.<ref>{{cite web |title=Measuring Worth (retail price index) |url=https://www.measuringworth.com/calculators/ukcompare/result.php?year_source=1980&amount=99.95&year_result=2021 |access-date=19 November 2022 |website=measuringworth.com}}</ref>) The company conducted no market research whatsoever prior to the launch of the ZX80; according to Clive Sinclair, he "simply had a hunch" that the general public was sufficiently interested to make such a project feasible and went ahead with ordering 100,000 sets of parts so that he could launch at high volume.<ref name="Lorenz" />

[[File:ZX80.jpg|left|thumb|The Sinclair ZX80 (1980) is the immediate predecessor of the ZX81 and shares many design features.|alt=View of the ZX80, a white rectangular computer with a black keyboard with blue keys and the words "SINCLAIR ZX80" on the centre left of the case, in front of a large bulge at the rear of the machine.]]

The ZX80's design introduced many key features that were carried over to the ZX81; as Sinclair himself later said, "the ZX80 was very much a stepping stone to the ZX81".<ref name="YC-Interview">[[#YCAugSept81|Scot (Aug/Sept 1981)]]</ref> The design was driven entirely by the desired price – the machine had to cost less than £100 but still make a healthy profit.{{sfn|Dale|1985|p=95}} Its distinctive wedge-shaped white case concealing the circuitry and the touch-sensitive membrane keyboard were the brainchild of [[Rick Dickinson]], a young British [[industrial design]]er who had recently been hired by Sinclair. As he later recalled of Sinclair's approach, "Everything was cost driven. The design was the face of the machine."<ref name="BBC23-4-07">[[#BBC230407|BBC News (23 April 2007)]]</ref> The unconventional keyboard was the outcome of Sinclair's cost-cutting. It made use of a sheet of plastic, on which the keys were printed, overlaying a metallic circuit that registered when a key was pressed. This avoided the expense of providing a typewriter-style keyboard, though the design had many drawbacks when it came to usability and "feel".{{sfn|Adamson|Kennedy|1986|p=90}}

Inside the case, there were many more similarities with the ZX81. Like its successor, it used the Z80A microprocessor and had only 1&nbsp;KB of on-board RAM. It came with a specially written BASIC interpreter on a dedicated ROM chip and could use a television as a display. It relied on an ordinary cassette tape recorder for data storage. The main difference between the two machines lay in the internal software; when the ZX81 was released, ZX80 owners were able to upgrade by the relatively simple expedient of plugging a new ROM onto the circuit board.<ref name="FT 6 Mar 1981" />

The ZX80 was an immediate success, selling 20,000 units over the following nine months.{{sfn|Dale|1985|p=97}} Science of Cambridge was producing ZX80s at the rate of 9,000 a month by the end of 1980{{sfn|Dale|1985|p=102}} and within 18 months of its launch the company had sold 100,000 units.<ref name="FT 20 Mar 1982">[[#FT200382|Crisp (20 March 1982)]]</ref> The commercial success of the ZX80 made a follow-up product inevitable. The company was renamed Sinclair Computers in November 1980, reflecting its new focus, and became Sinclair Research in March 1981.{{sfn|Adamson|Kennedy|1986|p=256}}

=== BBC Micro bid ===
The launch of the ZX81 was catalysed in part by the [[BBC]] Computer Literacy Project's plan to produce [[The Computer Programme]] TV series, to be broadcast in 1982, aimed at popularising computing and programming. The BBC intended to commission an existing manufacturer to provide it with a BBC-branded home computer to tie in with the series. When Clive Sinclair heard of the project in December 1980, he wrote to the BBC informing them that he would be announcing a new version of the ZX80, to be called the ZX81, in early 1981. It would remedy some of the ZX80's deficiencies and would be both cheaper and more advanced.{{sfn|Dale|1985|p=100}} Sinclair wanted the ZX81 to be a candidate for the BBC contract and lobbied for its adoption. He pointed out that there were already 40,000 users of the ZX80 and that by the time the series was broadcast there were likely to be upwards of 100,000 ZX81 users.{{sfn|Dale|1985|p=103}}

A prototype ZX81 was demonstrated to BBC representatives in January 1981,{{sfn|Dale|1985|p=104}} while Sinclair's local rival [[Acorn Computers]] put forward their proposed Proton computer, a design – of which a prototype did not yet exist – based on the [[Acorn Atom]].{{sfn|Adamson|Kennedy|1986|p=104}} To Sinclair's dismay, the contract to produce the [[BBC Micro]] went to Acorn, which launched the machine in January 1982.{{sfn|Adamson|Kennedy|1986|pp=116–117}} Paul Kriwaczek, the producer of ''The Computer Programme'', explained his reservations in a March 1982 interview with ''[[Your Computer (British magazine)|Your Computer]]:''

{{blockquote|I would have been very reluctant for the BBC to sell something like the Sinclair [ZX81] because it is so limited. The Sinclair cannot be expanded; it is fundamentally a throw-away consumer product. Its usefulness lies in learning about programming, but I do not believe that the future of computers lies in everyone learning to program in BASIC.<ref>[[#Gore|Gore (March 1982)]]</ref>}}

Sinclair was critical of the BBC's decision, accusing it of incompetence and arrogance.<ref name="ET-interview">[[#ET220282|''Engineering Today'' (22 February 1982)]]</ref> Shortly after [[Acorn Computers|Acorn]] won the BBC contract the Government issued a recommended list of computers, including the [[BBC Micro]] and [[Research Machines 380Z]], that schools could purchase, with the aid of a grant, for half price; Sinclair's computers were not included on the list. Sinclair responded by launching his own half-price deal, offering schools the chance to buy a ZX81 and 16&nbsp;KB RAM pack for £60, plus a ZX Printer at half price, for a total cost of £90. As the cheapest Government-approved system was £130, this was an attractive offer for some schools and about 2,300 bought Sinclair's package, although 85% adopted the BBC Micro.{{sfn|Dale|1985|pp=106–7}}{{sfn|Campbell|1983}}{{sfn|McClelland|2011}}

=== Development and manufacture ===
[[File:ZX81 Leiterkarte.jpg|thumb|The ZX81 motherboard, Issue One version. The Ferranti ULA is on the left of the image and the NEC Z80 processor is in the centre. The TV output modulator is on the top left. At <!--in, on, at ... wondering which is best-->the bottom right is a ribbon cable connecting to the membrane keyboard.|alt=Top-down view of the ZX81 motherboard showing the layout of the components. Four chips are prominent, along with a TV modulator on the top left and a ribbon cable on the bottom right.]]
[[File:Sinclair ZX81 PCB Top.JPG|thumb|Revised Sinclair ZX81 PCB Revision 3 Top Side]]
[[File:Sinclair ZX81 PCB Revision 3 Bottom Side.JPG|thumb|Sinclair ZX81 PCB Revision 3 Bottom Side]]
The development of the ZX81 got under way even before the ZX80 had been launched. Sinclair's chief engineer, [[Jim Westwood]], was given the task of improving the ZX80's hardware to reduce the number of components and thus bring down the cost.<ref>{{Cite web |title=Sinclair ZX81 |url=https://www.timexsinclair.com/computers/sinclair-zx81/ |access-date=2023-03-15 |website=timexsinclair.com |language=en-US}}</ref> He also sought to fix some of the more annoying problems with the ZX80. Westwood and his colleagues found that the component count could be reduced greatly by combining eighteen of the ZX80's chips into a single [[Gate array|uncommitted logic array (ULA)]], a type of general-purpose chip full of logic gates that were connected up as the customer required during chip manufacture. This short-lived technology of the day was cheaper and quicker than the design of a customised logic chip, which typically required very high volumes to recoup its development cost.{{sfn|Adamson|Kennedy|1986|p=103}} [[Ferranti]] produced the new chip for Sinclair,{{sfn|Adamson|Kennedy|1986|p=104}} who hailed Westwood's design as a triumph of innovation: "The ZX81 had four chips when our nearest competitor in this respect, the TRS-80, had 44."<ref>[[#NS260686|Sinclair (26 June 1986)]]</ref> Only 70% of the [[logic gates]] on the ULA were supposed to be used, but Sinclair decided to use them all to squeeze more functions in. This resulted in the machine becoming uncomfortably warm during usage.<ref>[[#TSS-CRASH|''CRASH'' (May 1985)]]</ref> Computing folklore held that the ZX81 had to be refrigerated by balancing a carton of cold milk on top of the case.<ref>[[#Goodwin|Goodwin (Jan 1988)]]</ref><ref>[[#cw|Gillies-Jones (22 June 2000)]]</ref>

The ZX81's ROM was doubled to 8&nbsp;KB, from the ZX80's 4&nbsp;KB ROM. This enabled a fuller implementation of a version of [[BASIC#Standards|ANSI Minimal BASIC]] (termed [[Sinclair BASIC]] by the company). Clive Sinclair re-commissioned a company called Nine Tiles, which had produced the ZX80 ROM, to develop the new ROM software for the ZX81.{{sfn|Adamson|Kennedy|1986|p=104}} The code was written by John Grant, the owner of Nine Tiles, and [[Steve Vickers (computer scientist)|Steve Vickers]], who had joined the company in January 1980. Grant concentrated on the software that drove the ZX81's hardware, while Vickers developed the new BASIC and the accompanying manual. Sinclair's brief to the pair was fairly non-specific but primarily concerned remedying a key defect of the ZX80 so that the new machine could be used for practical programming and calculations. Vickers later recalled:

{{blockquote|As far as Clive was concerned, it wasn't a question of what the machine ought to be able to do, but more what could be crammed into the machine given the component budget he'd set his mind on. The only firm brief for the '81 was that the '80s math package must be improved.{{sfn|Adamson|Kennedy|1986|pp=89–90}}}}

The new ROM incorporated [[trigonometry|trigonometric]] and [[floating-point arithmetic|floating-point]] functions, which its predecessor had lacked – the ZX80 could only deal with whole numbers. Grant came up with one of the ZX81's more novel features, a syntax checker that indicated errors in BASIC code as soon as it was entered (rather than, as was standard at the time, only disclosing coding errors when a program was run).{{sfn|Adamson|Kennedy|1986|p=105}} Unfortunately for Vickers, he introduced a briefly notorious error – the so-called "square-root bug" that caused the square root of 0.25 to be returned erroneously as 1.3591409 – as a result of problems with integrating the [[ZX Printer]] code into the ROM. Although it was eventually fixed, the bug became the subject of controversy and Sinclair was forced to replace some of the ZX81s sold to early customers.{{sfn|Adamson|Kennedy|1986|pp=107–8}} On a more positive note, Vickers' work on the manual was received favourably, being described in 1983 as "one of the classic texts on BASIC".{{sfn|Adamson|Kennedy|1986|p=106}} Max Phillips commented in a ''What Micro?'' retrospective:

{{blockquote|It does a reasonable job and sensibly provides lots for the reader to do. It's quite honest about the [ZX81]'s shortcomings and provides hints and tips for ways round them ... Best of all, the manual is complete and comprehensive. There's some fairly advanced and often undisclosed information in there. The beginner won't understand it for a long time but if he or she learns some more advanced ideas, the manual is ready for them.<ref>[[#Phillips|Phillips (April 1983)]]</ref>}}

The task of designing the ZX81's case again fell to Rick Dickinson, who produced an updated version of the ZX80's wedge-shaped case. This time round, the design team were able to use [[injection moulding]],{{sfn|Laing|2004|p=61}} which enabled them to deliver a higher-quality case. Dickinson originally envisaged the ZX81 as "an expandable range of boxes following a vaguely modular approach with a common width", though this approach was eventually dropped.<ref>[[#Dickinson|Dickinson (24 August 2007)]]</ref> From start to finish, the design process took about six months.<ref>[[#Cooke|Cooke (August 1982)]]</ref>

[[File:ZX81 concept design.jpg|left|thumb|One of Rick Dickinson's original concept drawings for the ZX81, envisaging the machine as part of "an expandable range of boxes following a vaguely modular approach"|alt=Sketch of two concept designs for the ZX81, showing the computer with a series of similarly shaped boxes stacked behind it in a row.]]
The ZX81 was launched on 5 March 1981 in two versions (though with identical components) – a pre-assembled machine or a cheaper kit version, which the user could assemble themself. Both versions were manufactured in [[Dundee]], [[Scotland]] by [[Timex Corporation]]<ref name="FT 6 Mar 1981" /> at the company's [[Dryburgh (Dundee district)|Dryburgh]] factory.<ref name="McManus">[[#McManus|McManus Galleries]]</ref> Timex had not been an obvious choice of manufacturing subcontractor, as the company had little previous experience in assembling electronics. It was a well-established manufacturer of mechanical watches but was facing a crisis at the beginning of the 1980s. Profits had dwindled to virtually zero as the market for mechanical watches stagnated in the face of competition from the digital and quartz watches. Recognising the trend, Timex's director, [[Fredrik Olsen|Fred Olsen]], determined that the company would diversify into other areas of business.{{sfn|Adamson|Kennedy|1986|p=94}}

This shift by Timex came at an ideal time for Sinclair. The ZX80 had proved more popular than expected and Sinclair's existing manufacturer, a small electronics company in [[St Ives, Cambridgeshire|St Ives]], lacked the resources to deal with the demand. Timex took over production of the ZX80 late in 1980.{{sfn|Adamson|Kennedy|1986|p=94}} The arrangement worked well for both companies and Timex took on the manufacture of the ZX81, aided by capital investment in its Dundee plant.<ref name="Church">[[#SU05-82|Church (May 1982)]]</ref> Sinclair initially planned to produce 10,000 ZX81s a month, rising to 30,000 a month within a year.<ref name="FT 6 Mar 1981" /> However, Timex initially had significant problems in producing enough ZX81s to satisfy demand. As a consequence, it took up to nine weeks for ZX81s to be delivered by mail order. It was not until September 1981, five months after the launch of the ZX81, that the delivery times finally came down to the promised twenty-eight days.{{sfn|Adamson|Kennedy|1986|p=111}} Those who already owned or had recently ordered the ZX80 were not excluded; anyone who had ordered a ZX80 in the two weeks before the ZX81's launch would receive the newer machine, while existing owners were able to upgrade their ZX80s by plugging an extra £20 ROM chip into the circuit board.<ref name="FT 6 Mar 1981" />

The reliability of the ZX81 was controversial. W.H. Smith, one of the machine's key distributors, had a company policy of ordering a third more ZX81s than were actually required for sale, so that it would have enough replacements for faulty machines.{{sfn|Adamson|Kennedy|1986|p=112}} Similar problems were reported in the US market, where contemporary reports suggested that only a third of the ZX81s shipped actually worked.{{sfn|Adamson|Kennedy|1986|p=133}} However, figures released by Sinclair claimed that only 2.4 per cent of pre-assembled machines were returned, although 13 per cent of kits were returned.<ref name="Church" /> Clive Sinclair strongly denied any problem with reliability:

{{blockquote|We have a lower rate of failure on our computers than anybody else in the world, and the reason for that is that we do everything to keep the quality right. The ZX81 production line is a miracle of efficiency; after all, one is made every 10 seconds. They go through the most amazing quality control. Also we have a far lower component count than anyone else. We have only four chips where everyone else has 40.<ref name="Hayman">[[#Hayman|Hayman (June 1982)]]</ref>}}

Sinclair attributed the higher failure rate of the kits to customers breaking the components by inserting or soldering them the wrong way, though the company admitted that there was a persistent problem with power supplies that affected both kits and pre-assembled ZX81s.<ref name="Church" /> The bigger problem was perhaps Sinclair's lack of after-sales service, which Robin Clarke of ''New Scientist'' described as "one of the worst after-sales performance records of any company ever established."<ref name="Clarke" /> The ''Financial Times'' observed that "Clive Sinclair's offices are filled with returned computers which can take months to be repaired."<ref name="FT 20 Mar 1982" /> The company's slowness in replacing returns and delivering freshly ordered machines meant that Sinclair Research gained a reputation for poor customer service.{{sfn|Adamson|Kennedy|1986|p=108}}

=== Marketing ===
The marketing of the ZX81 was handled by Sinclair's long-standing marketing agency Primary Contact (now part of [[Ogilvy & Mather]]), which had provided marketing services for Sinclair since 1971 and was to continue doing so until 1985. Sinclair's entry into the nascent home computing market gave Primary Contact a major challenge – how to market a product simultaneously at hobbyists and at the "man on the street", who probably had little or no computer literacy.{{sfn|Adamson|Kennedy|1986|p=97}} The answer was to pursue what the journalist David O'Reilly of ''[[MicroScope]]'' magazine described as a single-minded "user-friendly strategy." Chris Fawkes, one of Primary Contact's directors, explained: "We brought personal computers to the mass market by showing that you didn't have to be a whizzkid to use one."{{sfn|Adamson|Kennedy|1986|p=99}} As Clive Sinclair put it in a 1982 interview with ''Your Computer'',

{{blockquote|There are two big markets. There is the hobbyist and the man in the street. The hobbyist was a dead certainty. We knew we could sell to him because we have so much experience of it and we were offering a better product. The much less certain prospect was the man in the street. There the view was that if we offered him a computer plus a self-training book at a keen enough price he would buy by mail order – which, of course, he has.<ref name="YC-Interview" />}}

According to [[Ben Rosen]], by pricing the ZX81 so low, "Sinclair has opened up a completely new market among people who had never previously considered owning a computer." Clive Sinclair acknowledged the role that guesswork had played in his decision to launch the ZX81 on such a large scale: "It was a surmise that the man in the street would want such a computer. He does, and our information is that a lot of people are using the machines avidly."<ref name="ET-interview" /> A ''New Scientist'' retrospective published in 1986 commented:

{{blockquote|Sir Clive's marketing achievement was to downgrade the "concept" of a computer to the point where he could claim to provide one for less than the magical £100 mark. To this end, efficient keyboards and monitors, useful amounts of memory, efficient filing and storage systems and the like were stripped away, to leave an affordable facsimile of a "computer". The market image was more important than what the computer could do, but the burgeoning industry in computer games provided an application which adolescents – young and old – eagerly seized on as the ''raison d'être'' for their new gadget. In the main, it was ignorance of genuine computer technology that fired the success of the ZX range, despite the availability of accessories that, albeit inefficiently, turned the Z80 processor chip at the heart of these up-market toys into the core of a useful machine.{{sfn|Adamson|Kennedy|1986}}}}

[[File:ZX81 Sinclair Research advert.jpg|right|thumb|upright=1.35|Sinclair Research's launch advertising for the ZX81. High-profile advertisements such as this were used to promote the benefits and value for money of the ZX81.|alt=A two-page advertising spread showing the ZX81 with a 16&nbsp;KB RAM pack and ZX Printer attached, next to the headline "Sinclair ZX81 Personal Computer – the heart of a system that grows with you"]]
High-profile advertising was central to the marketing campaign. Although Sinclair Research was a relatively small company, it had a long-standing policy of using large-scale advertisements that stood out in stark contrast to the more muted advertisements of other manufacturers. Superlatives, exhortations, appeals to patriotism, testimonials, eye-catching drawings and photographs on double-page spreads, varying from month to month, were used to drum up mail order business for Sinclair.{{sfn|Adamson|Kennedy|1986|p=21}} The launch advertising for the ZX81 illustrates this approach. A photograph of the ZX81 alongside the official Sinclair peripherals dominated the centre of a double-page spread. The value for money of Sinclair's products was emphasised by the prices being printed in larger type than any other text on the spread. The ZX81's benefits were promoted with the aspirational slogan "Sinclair ZX81 Personal Computer – the heart of a system that grows with you". The advertisement highlighted ''ZX81 BASIC Programming'', the manual written by Steve Vickers, as "a complete course in BASIC programming, from first principles to complex programs." The educational benefits of the ZX81 were stressed ("it's still very simple to teach yourself computing") and its technical advantages were explained in relatively non-technical terms. For instance, the ZX81's idiosyncratic method of typing commands with a single keystroke – the result of the memory-saving method of using one-byte tokens to represent keywords{{sfn|Nash|1984|p=102}} – was presented as "eliminat[ing] a great deal of tiresome typing". The ZX81's British character was emphasised; it was "designed by Sinclair and custom-built in Britain."<ref name="SRadvert">[[#SRadvert|''Everyday Electronics'' (April 1981)]]</ref> Sinclair's advertising in the United States provides an illustration of how the company perceived the ZX81's purpose:

{{blockquote|For less than $100, the Sinclair ZX81 will get you started in personal computing right now. Your children will gain an understanding of computers that will benefit them for the rest of their lives. And you will be prepared to make informed decisions about using and buying computers, both in your career and in your home.<ref>[[#PopSciadvert|''Popular Science'' (Oct 1982)]]</ref>}}

This approach to advertising was driven by Sinclair's reliance on mail-order marketing. It came with a high up-front cost in terms of purchasing space in publications but it had the advantage of ensuring that all sales were firm and pre-paid. A big splash on launch produced a large influx of cash at the outset of a campaign, though it did also depend on the advertiser having enough product to satisfy the initial surge in demand. The advertisements served an additional purpose of priming the market for over-the-counter sales by "getting the story across", as Clive Sinclair put it: "Not that big a proportion do buy on mail order, but they see the ads, and that helps to prepare them for buying when the item appears in the shops."{{sfn|Adamson|Kennedy|1986|p=22}}

Sinclair himself became a focal point for the marketing campaign, putting a human face on the business, while Sinclair Research was portrayed in the media as a plucky British challenger taking on the technical and marketing might of giant American and Japanese corporations. As David O'Reilly noted, "by astute use of public relations, particularly playing up his image of a Briton taking on the world, Sinclair has become the best-known name in micros."{{sfn|Adamson|Kennedy|1986|p=98}} The popular press soon latched onto the image. His "Uncle Clive" persona is said to have been created by the gossip columnist for ''[[Personal Computer World]]'',{{sfn|Adamson|Kennedy|1986|p=97}} while the media praised Sinclair as a visionary genius (or even, in the words of ''[[The Sun (United Kingdom)|The Sun]]'', "the most prodigious inventor since [[Leonardo da Vinci|Leonardo]].") As Ian Adamson and Richard Kennedy put it, Sinclair outgrew "the role of microcomputer manufacturer and accepted the mantle of pioneering [[boffin]] leading Britain into a technological utopia."{{sfn|Adamson|Kennedy|1986|p=114}}

Pricing was central to the marketing strategy, as it had been through Sinclair's career. The ZX81 had been designed to meet a £70 price point and was launched at a price of £69.95 (built) or £49.95 (kit). One Sinclair brochure presented a side-by-side comparison of the ZX81 with the four machines that Sinclair considered its main rivals – the [[Acorn Atom]], [[Apple II Plus]], [[Commodore PET]] and [[TRS-80]]. The comparison highlighted the vast differences in cost, from £630 in the case of the Apple II Plus to just £70 for the ZX81, though even by Sinclair's own comparison the Apple was by far the more capable machine.<ref name="SRbrochure">[[#SRbrochure|Sinclair Research sales brochure (1981)]]</ref>

According to Sinclair himself, the £69.95 price was chosen after applying the "[[Experience curve effects|experience curve]]" developed by the [[Boston Consulting Group]]. Sinclair's prior experience in the calculator market had highlighted the fact that a product will be more profitable selling at (for instance) twice the manufactured cost than at three times. He could have launched the ZX81 at a higher price, marketing it in a more traditional way as a premium product, but chose not to. In effect, he used the lower price to establish an unassailable lead before the competition moved in.<ref name="Lorenz">[[#FT150482|Lorenz (15 April 1982)]]</ref>

An essential part of Sinclair's marketing strategy was to use regular cost-cutting at strategic intervals to maintain market share. Ian Adamson and Richard Kennedy comment that Sinclair's approach was "to secure and extend [his] market lead and panic the competition. While most companies reduce prices when their products are in steep decline, Sinclair tends to discount shortly after sales have peaked. The advantage of his approach is that vacillating customers are drawn into the fold while the product's promotion retains a commercial urgency, and the costings of the competition are thrown into utter disarray."{{sfn|Adamson|Kennedy|1986|p=143}}

This tactic proved highly successful, with Sinclair announcing by March 1982 that it had sold 250,000 ZX81s worldwide. 50,000 computers were sold each month, 60% outside the UK, despite Sinclair and W. H. Smith being the only distributors.{{r|theage19820309}}{{r|Clarke}} Despite the launch of its successor, the [[ZX Spectrum]], Sinclair reportedly intended to increase ZX81 production to 150,000 a month.<ref name="libes198209">{{Cite magazine |last=Libes |first=Sol |date=September 1982 |title=Bytelines |url=https://archive.org/details/byte-magazine-1982-09/page/n488/mode/1up?view=theater |access-date=2024-12-30 |magazine=BYTE |pages=490–493}}</ref> When sales fell after the Spectrum's debut, Sinclair reduced the price of the pre-assembled version to £49.95 in May 1982. It was cut by another £10 the following April. Despite the increased competition from much more capable computers, the ZX81 was still shipping in excess of 30,000 units a month even as late as July 1983, more than two years after it had been launched.{{sfn|Adamson|Kennedy|1986|p=259}} By that time, according to Sinclair Research, over 1.5&nbsp;million ZX81s had been sold.<ref name="Sinclair-Research" />

=== Distribution ===
[[File:ZX81 kit.jpg|thumb|The kit version of the ZX81, sold by Sinclair through mail-order|alt=The component parts of the ZX81, including the case, keyboard and circuitry, resting on a blue sheet in front of the cardboard box in which it was shipped.]]
The ZX81's distribution arrangements were an essential part of its success and marked a watershed in the way that computers were sold in the UK. Sinclair had previously made its name as a mail-order retailer&nbsp;– the ZX81 was initially available only through mail-order&nbsp;– but the only truly effective way to reach the mass market was via high street stores. Fortunately for Sinclair, an opportunity to do just that was provided by W.H. Smith, a venerable book- and magazine-seller and stationery chain. The company had stagnated in the 1970s and was looking for ways to revitalise its image and expand its product range.{{sfn|Adamson|Kennedy|1986|p=109}}

Smith's had begun selling audio and photographic equipment and calculators at the end of the 1970s, with a modest degree of success. In 1980 its marketing development manager, John Rowland, hit upon the idea of creating "Computer Know-How" sections in major branches to sell computer books and magazines. Most of the items on display were imports from the United States but their relatively high cost reduced their attractiveness to the casual buyer.{{sfn|Adamson|Kennedy|1986|p=109}} The commercial success and mass market potential of the ZX80 caught Rowland's interest; he approached Sinclair, saw a prototype ZX81 and agreed to market the machine through Smith's on an exclusive basis for the first six months after launch.{{sfn|Adamson|Kennedy|1986|p=111}} As Rowland put it, "what we've done now is bring the computer-orientated publications together with an actual computer, to create the Computer Know-How section of the store", alongside computer software and blank cassette tapes. The ZX81 would be sold in 112 stores around the UK and would serve as the centrepiece of the "Computer Know-How" sections.<ref name="YC Nov 81">[[#YCNov81|Hartnell (November 1981)]]</ref>

Selling the ZX81 over the counter was seen as something of a gamble and Rowland's colleagues were initially unenthusiastic about the scheme. Branch buyers thought that the ZX81 was unlikely to sell more than 10–15 units per branch at launch.{{sfn|Adamson|Kennedy|1986|p=111}} Rowland himself thought that the ZX81 would sell about 10,000 units during the first five months of the retail agreement, equivalent to one month's mail order sales by Sinclair.<ref name="YC Nov 81" /> In the event, the ZX81 was a massive success for Smith's, it went on sale for {{£|long=no|99}} making it the first home PC in the UK to retail for under {{£|long=no|100}}. The "Computer Know-How" sections were swamped with eager customers, overwhelming the 300 staff who had been trained to demonstrate the machines; a ''Financial Times'' correspondent wrote of being "dazed and bewildered by the crowds of schoolchildren clustered round the ZX81 in your local branch of W.H. Smith."<ref name="Taylor" /> Within a year, Smith's had sold 350,000 ZX81s, making an estimated net profit of {{£|long=no|10 million}}. Sales of peripherals, software, books and magazines netted even more profit.{{sfn|Adamson|Kennedy|1986|p=112}}
<!-- Deleted image removed: [[File:ZX81 JP Mitsui advert.jpg|thumb|Advertisement in the Japanese computer magazine, ''[[ASCII (magazine)|ASCII]]'' Dec.1982. "Soon you'll be able to use it. It's for beginners."]] -->
[[File:Beocomp ZX81 with program listing.jpg|thumb|Beocomp ZX81]]
The British chain stores [[Boots UK|Boots]], [[John Menzies]] and [[Currys]] began selling the ZX81 as soon as Smith's exclusive distribution deal expired{{sfn|Adamson|Kennedy|1986|p=257}} and a number of companies secured overseas distribution rights for the ZX81, which was being sold in 18 countries by March 1982.<ref name="Needle">[[#Needle|Needle (15 March 1982)]]</ref> Sinclair launched the ZX81 in the United States in November 1981 at a price of {{US$|long=no|149.95}} assembled and {{US$|long=no|99.95}} in kit form, initially selling directly to the American market by mail order.<ref name="Wise">[[#Wise|Wise (2 November 1981)]]</ref> To be useful the computer needed an extra {{val|16|u=KB}} RAM pack which cost {{US$|long=no|49.95}}.  Sales reached 15,000 a month by January 1982, while [[American Express]] sold thousands more to its own customers. In February 1982 Timex obtained a licence from Sinclair to sell the ZX81 directly through thousands of retail outlets in the US, paying Sinclair Research a 5 per cent royalty on all Sinclair hardware and software sold by Timex.<ref name="FT13-02-82">[[#FT130282|Crisp (13 February 1982)]]</ref> The company was later to produce its own licensed clones and variants of the ZX81.{{sfn|Adamson|Kennedy|1986|pp=108–9}} By August 1982 Sinclair had lowered the American mail-order price of the assembled ZX81 to {{US$|long=no|99.95}} and kit to {{US$|long=no|79.95}}, and its advertisements stated that "more than 10,000 are sold every week".<ref>[[#BYTEadvert|''BYTE'' (August 1982)]]</ref> In December 1981 [[Mitsui]] obtained rights to distribute the ZX81 in Japan, selling it by mail order for {{¥|38700|link=yes}} (equivalent to £83 in 1982 prices), and had sold 5,000 units by July 1982. The Japanese market's favourable reaction to the ZX81 led Mitsui to begin selling the ZX81 over the counter in large bookshops from September 1982, with annual sales of 20,000 units predicted.<ref>[[#FT300982|Shibata (30 September 1982)]]</ref> In the Netherlands, the regular Sinclair ZX81 was for sale as well as a [[Bang & Olufsen]] branded version called ''Beocomp''.<ref>{{cite web |title=The Home Computer Museum |url=http://www.homecomputer.de/pages/f_info.html?Sinclair_ZX-81_Beocomp.html}}</ref>

The ZX81 was also sold for a while in [[duty-free shop]]s at UK airports. However, this fell foul of government export restrictions aimed at preventing the [[Soviet bloc]] countries from obtaining Western high technology goods. It was not uncommon for visitors from the [[Soviet Union]] and other eastern European countries to pick up gadgets in Western countries with the aim of [[Technology transfer|transferring their technology]] to their own states' industries. In 1983 the government ordered that the ZX81s were to be withdrawn from sale at airports.<ref>[[#DT300583|''Daily Telegraph'' (30 May 1983)]]</ref> There was no such restriction on sales to communist China and in November 1983 Sinclair Research announced that it had signed an agreement to export ZX81 kits to a factory in [[Guangzhou]], where they would be assembled for the Chinese market.<ref>[[#SU1183|''Sinclair User'' (Nov 1983)]]</ref>