This Hewlett-Packard advertisement, originally published in Scientific American, is used by permission. If errors crept in during the scanning process, please contact Dave Hicks

**It has 86 preprogrammed functions and operations, and 20 memory registers. You can program it to do repetitive calculations. You can time events with it. It costs $395***

Our new HP-55 not only gives you 38 more mathematical functions and operations on the keyboard than our HP-45. It will also remember an intricate calculation so that you can keep entering variables and see new answers until you're satisfied and turn it off.

You can program an equation with up to 49 steps in it--in addition to which your program has access to all 86 preprogrammed calculations and data manipulations, plus the 20-register memory. You can do direct and conditional-access branching (the calculator will automatically compare values and choose among alternatives). When you've entered your program you can step through it to verify and edit.

You can perform arithmetic into and out of 10 of the registers, store in all 20.

You can do circular trigonometry in degrees, radians, or grads, converting directly from degrees to radians, decimal degrees to D/M/S (and vice versa).

You can do true linear regressions. The special ลท key lets you fit curves.

You get instant true metric conversions without having to multiply by a constant.

You also have the HP four-register stack that lets you enter even the most complicated equation in an easy, natural way without having to keep track of parentheses and brackets. The result of each intermediate calculation is displayed as you perform it, giving you the continuous feedback you need to avoid ending up with a wrong answer.

And whether you need it or not, you get a 100-hour digital timer that displays and records hours, minutes, seconds, tenths, and hundredths-accurate to 0.01 percent. Results are stored in 10 memory registers, so you can clock 10 different events or "splits" from the same starting time-runners, reactions, rats, or whatever--and study the results when things calm down.

As noted above, this extraordinary little machine costs $395 *. It doesn't write on and read 100-step magnetic program cards like our incomparable HP-65, but it has more power on its keyboard. If you don't mind explaining a calculation to the HP-55 whenever you want it to start repeating at your command, you can save a lot of money. And so you won't have to think up your own explanations for many common types of problems, we offer two HP-55 applications handbooks--the Math Pac and Stat Pac--at $10* each.

People tend to keep their HP-65's locked in their desks. You can afford to treat the HP-55 more casually. Keep it in your pocket.

If you often find yourself repeating a tedious computation over and over, putting in different variables to see a range of answers, you're the person we designed the HP-55 for.

All you have to do is flip a switch to tell the calculator it's going to memorize a computation sequence.

Then you simply key in your calculation (we call it "keystroke programming") as if you were only going to do it once. Except that before and after you enter each variable you press [R/S] (for Run/Stop). That tells the calculator that whenever you run the program it must stop at that point, wait for a new number, and start computing again after the number is entered.

When you've finished the initial calculation, you press [R/S] to tell the HP-55 that the sequence is complete. Then you flip the switch to RUN and press BST , whereupon the machine will instantly calculate up to the first variable, stop and wait. When you've entered the new number, press [R/S] and the machine will zip through the next sequence, stop again and wait for another number. After the last variable is in, press [R/S] to see a new answer.

The HP-55 will remember your sequence as long as you don't switch it off. So you can iterate and reiterate until you run out of variables.

*Domestic USA prices only.

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