Still a few more problems to solve

TEXAS INSTRUMENTS T1-92 CALCULATOR Pounds 199.99 including VAT

The Texas Instruments TI-92 is a lightweight (620g) powerful calculator. The word "calculator" hardly does it justice, as it can plot graphs, solve equations, do statistics, and do geometry. These features may sound run of the mill, but the TI-92 is sophisticated, and some people will find it fascinating to play with. It can solve equations using algebra, so for example it will solve the quadratic equation ax2+bx+c=0 and show you the roots exactly like this: In fact the TI-92 can do useful mathematics up to undergraduate standard, and could be found worthwhile for anyone struggling to do, say, homework involving integrating by parts. Moreover, it is programmable, so it can be extended to help in specialised tasks, such as classroom teaching, or solving repetitive problems.

Pictures bring ideas to life and make them easier to understand. The TI-92 can draw graphs easily. You can either use menus or the built-in command language using the TI-92's full but smallish keyboard. If, for example, you type Graph x2, it plots a neat parabola; you can then use various buttons to move around or expand the graph to see different bits of it. The TI-92 can draw graphs that, unlike a parabola, would defy drawing by hand. It can draw three-dimensional surfaces, and shade them so they look very effective. Graphs can also be drawn straight from data, and the statistics functions can be applied to the same data.

There is a word processor feature that, if you have a printer or computer attached, allows essays to be written that include mathematics - you do the writing, set the problems, and the TI-92 does the maths. The geometry is perhaps the most surprising feature of the TI-92. Anything you can do with a ruler and compass, you can now do better with a sort of Nintendo button. You can bisect angles, draw tangents, and generally have a huge amount of fun. The machine can rotate and reflect shapes or "learn" so-called macros to trisect angles. You can even animate constructions.

The screen on the TI-92 is a bit small for the huge range of features it provides. The geometry, especially, is a bit fiddly - but even so, it is far easier to do than find a sharp pencil and a compass that does not slip! You can split the screen into two parts. Of course, this makes things even smaller, but it allows you to experiment with, say, formulas on one side and see their graphs on the other side. You can write down equations, get the TI-92 to solve them, fiddle a bit, have another go, or go back to some earlier work and revise it. Although the TI-92 has a battery conservation feature that switches it off when it has not been used for a minute or two, it is nice that when you switch it back on again it is doing exactly what it was before it went off. Nothing is lost when it is switched off and on.

Finally - although this is hardly the right word when reviewing a gadget with hundreds of functions - the TI-92 can be used as an ordinary calculator, though of course it is far more useful than that. It can do complex number calculations properly, and handles errors more helpfully than most hand-held calculators. The ability to edit calculations is indispensable once you start exploiting its capabilities to solve things you would never dream of doing by hand.

Technically, the TI-92's features can be summarised thus: 128k ram; programs up to 70k. Its display is monochrome 240x128 pixels. The standard package comes with a cable to link two TI-92s together, and there is an optional connection for Mac or PC use. It can be interfaced for collecting laboratory data, and used with an external screen for classroom viewing. The computer algebra it uses is based on the well-known Derive system; its geometry is based on Cabri Geometry.

That this all fits in a light-weight, small package is amazing. I dropped my review model onto a British Rail Standard Platform; it was undamaged and still running when I carried on with my sums on the train.

But when you shut the 518-page manual it suddenly becomes a complete mystery. The TI-92 is unusable without the manual. You cannot find out how to use its features. It is even a mystery for most people just how to open the case. I checked with 20 people: they all had problems.

Even with the manual, it has features that cannot be found and there are really quirky features. There is a key called ESC, but whenever you can press it, the calculator says "ESC=cancel". So why was not the key called cancel? There are two 2nd buttons, and three "enter" buttons. To stop a calculation in progress, you do not press QUIT, ESC or OFF, but ON! The editor in the geometry mode is utterly different from the other editors: for example, you say 2nd-ENTER instead of the up arrow key to get back the last calculation. Even after several weeks' study for this review, I must admit there are fundamental things I still do not understand. Many of my questions simply are not answered anywhere. The TI-92's rough edges are burdens on the user, and make it unnecessarily hard to use.

If you want to do anything interesting, you have to remember exactly how to use the functions you need, whether tExpand, cFactor, PxlCrcl or a name you cannot remember. If you are not careful, it is easy to get caught out by the TI-92's many little rules - for example, on how to find complex roots. There are many traps for the unwary. Should you use xmax or maxx to do statistics? And there are several hundred other names ready to catch you out, sx, c1, q3, tc, as well as all the names you used last week but have forgotten about. I also found one or two bugs.

Though you can define new functions to do new things, it is not possible to extend the calculator sensibly. The built-in derivative function, one thing you might want to extend, does not know about functions of functions, and you cannot program it to.

The TI-92 plus its manual is heavier and bulkier than a PC laptop, which must therefore be its ultimate comparison: standard packages like MathCAD, Derive and Mathematica are more powerful and easier to use. For many people their extra power, the wide availability of helpful information, and their integration with everything else done on PCs, will be decisive factors. Texas Instruments have shown they can create a feature-laden, technical gadget. All they need to do now is make it usable. (A good start would be a HELP button, or, better still, to simplify the TI-92 so help was not essential.) Until the TI-92 is improved, the risks of making undetected mistakes, or just getting stuck, are unacceptable.

Harold Thimbleby is professor of computing research at Middlesex University.