The tiny $99.95 Sinclair ZX81 mini home computer and its accompanying instructional materials are so well thought out that it could end up being the Model T of the home computer industry, drawing into the market tens of thousands of people who, until now, have felt so overwhelmed by price, technology and jargon that they were convinced a computer was beyond them.

In an industry where explosive growth seems to be the rule despite the recession, the Sinclair is the hottest thing going. Knowledgable sources estimate that somewhere between 150,000 and 200,000 of the 12-ounce computers have been sold by mail in the year since the model was introduced in the United States. A spokesperson for Sinclair will only say that 500,000 of the minis have been sold worldwide.

That means there are more ZX's in circulation worldwide right now than any other computer ever made.

It's no secret why. Price, pure and simple. At $99.95, the only thing that comes near it is Timex's version of the Sinclair ZX81 which is sold only in stores.

Yet, while price may be the reason it's been big so far, it won't be long before people will begin buying it for another reason:

It works and it is understandable.

That's because the Sinclair is specifically designed to get around the road blocks that have held many people back from buying a computer. It is the first computer solely designed to teach anyone who wants to learn, from kids to seniors, how to run and program a computer. By the time you've finished reading the instruction manual, you'll be able to write simple programs. For instance, you might concoct one that analyzes the performance of a particular stock over a period of time. Or you can program it to amortize any loan amount.

All you need is a little imagination.

As with anything that is designed for so specific a task, there have been compromises. For instance, the ZX81 will probably not be the answer to those who need a computer to handle small business work (although there is a VisiCalc-clone software package being offered). Nor will those looking for an inexpensive word processor be satisfied (because of the ZX81's keyboard design, there's no way you can type rapidly).

But if those aren't the things you're looking for, then you are in for a treat.

The trick is in the ZX81's keyboard design and in how it "cooperates" with the computer, and the instruction manual.

In most computers, the keyboard is laid out much like that of a typewriter, with upper and lower case characters that are accessed via the shift key. That's fine for a knowledgable computer operator. He knows all the command words. So, for instance, when he wants the computer to add two plus two (and he's using BASIC computer language), he types "PRINT 2+2". Sometimes the computer has been programmed so that all he has to do is type in PR and the computer immediately knows he means PRINT. But programming most computers still adds up to a lot of typing, on top of which the typist has to memorize all the necessary computer command words and syntax rules.

The ZX81, however, has a simplified keyboard designed to work hand-in-hand with the computer underneath. The result is that the ZX81 anticipates commands, thereby saving time and effort, and reducing the possibility of mistakes. ake the problem of two plus two. On the ZX81, instead of typing out "PRINT", you simply press the key labeled "P" followed by "2+2". That's because, above "P" on the ZX81's keyboard, the word "PRINT" also appears. Since commands are entered into computers in a standard sequence, the ZX81 "knows" that, in this case, you don't mean "P" but PRINT. It also knows that once you've pressed "P" the next series of keyboard entries will be the items it is supposed to calculate, so it automatically places a space after PRINT.

The advantages to the layman are important. For one, he avoids typing mistakes that can glitch a program (for instance, a period inserted instead of a comma can totally confuse a computer).

Another is that the layman doesn't have to memorize commands and sometimes confusing computer syntax. In most cases, the ZX81 won't let you make a mistake. And the manual is among the simplest and easiest to understand that I've encountered.

It's worth noting that the severest limitation of the ZX81 is that same keyboard. In addition to being uncomfortably small, it is also a "membrane" keyboard, which is simply a sheet of plastic on which the keys are drawn -- there are no keys, per se. Beneath the plastic lie the electronic contacts that cause the computer to respond. The problem is that sometimes you can't tell whether you've pressed the key hard enough to have made contact with the electronics below.

On the other hand, you won't have to worry about spilt milk seeping inside via the keyboard.

Something lost, something gained.

Setting up the computer is only slightly more diffcult than plugging in an electric typewriter. You simply attach two wires to the rear of a television. Then you plug in a transformer to a wall outlet and plug the other end into the ZX81.

You don't even have to worry about an on-off switch. Just remove the power jack when you want to turn it off.

A couple of tips for potential buyers: First, you don't have to wait for it to arrive via the mails. Buy the Timex version locally . If you shop around you'll probably find it discounted under $90.

Another thing to remember is that the mail-order Sinclair has about half the memory of the Timex version. While the amounts we're talking -- 1k versus 2k, if you're into jargon -- won't make too much difference to the neophyte who's just trying to learn, the different memory configurations that result can affect whether software (that is, commercially available programs) that work in one will work in the other. So be sure and ask the salesman whether any programs you end up buying work on your model.

As noted, you're also going to need a television. It makes no difference whether it is black-and-white or color since the ZX81 only generates black-and-white images. And, to save your programs, you will eventually need a casette tape recorder. But that's not necessary at the start.

Already there is a burgeoning industry for ZX81 peripherals--all those extra gadgets that can make life easier and more expensive. For instance, a number of companies are selling software that will transform the ZX81 into a game machine, a school tutor, or a small business or home finance aid. Prices on the software presently range from about $8 to $20.

There are also memory expansion packs (which you will discover are a must if you are going to be able to use any of that software) that can increase the ZX81's memory by 16k and on up. Starting price is around $50. Incidently, both the Sinclair and Timex have exactly the same memory capacity the minute you connect them to a 16k memory pack. The reason is that the memory pack bypasses the computer's internal memory.

You might also be interested in subscribing to one of the nationally circulated magazines and newsletters, like "Syntax ZX80" (RD 2, Box 457, Harvard, MA 01451), that are devoted to applications of the ZX81 and new software and peripherals.

And don't forget the ZX81 users groups. There are two in the Washington area -- Lanham-Sinclair Users Group, Cora C. Dickson, president (301) 577-6645, and Prince George's County Sinclair Users Group, Jim Wallace, president (301) 699-8712). Two more are being organized, one of which will be in Northern Virginia.

You owe all this to English inventor extraordinaire Clive Sinclair, who already has given the 20th century cheap calculators and other electronic gadgets. Right now, word has it that he's working on a flat television screen, but the ZX81 may turn out to be his crowning achievement. CAPTION: Picture, The revolutionary $99.95 Sinclair ZX81 minicomputer, attached to a small Sony television set. Photo by Ray Lustig -- The Washington Post; Picture 2, 16K Ram Pack accessory attaches to back of Sinclair ZX81 to expand minicomputer's memory.