Now that I think about it more, the Texas Instrument calculator I bought back in the mid-70's was actually an SR-51:
I remember after our company purchased those first few 'office' TI calculators that you had to 'sign-out' for, I told my wife that as soon as the price dropped below $100, I was going to buy one for myself. Every Friday one of the local 'Stationary Stores' (when was last time you heard that term used?) would run an advert of their weekend sales and sure enough they finally listed a TI for $99.95. I told my wife that I wasn't going wait any longer and so I drove over there Saturday morning but when I got there they were all sold-out. However, the salesman showed me an SR-51 which they had just marked down to $149.95. Well at first I felt like this was some sort of 'bait & switch' situation (still not sure that it wasn't) but he was a good salesman because he asked me what I did for a living and when I said that I was an engineer he immediately pointed out that the 'SR' stood for 'Slide Rule' and that I had been using a slide rule at work that I really needed something like this and not some simply 'adding machine'. I was still a bit leery until he mentioned that it could do engineering unit conversions. Well I immediately scoffed at that since ANY calculator could do 'unit conversions', just look up the appropriate conversion factor and then multiple, or divide, your curent value to get the value in the units that you needed. But he NO, these were built into the SR-51. Sure enough, he flipped the unit over and on the back was a table of some 16 common conversions each with a code number. You would simply enter your current value then hit one of two button sequences (depending on which direction the conversion was going) and then 'code' number and you'd get the properly converted number, period. Well this sold me (as I said, he was a good salesman) and paid out the extra $50 (which back then was probably a full days pay) and I used it for many years, including when I took the second part of my PE exam in July 1976 (calculators were allowed as long as they were NOT programmable).
BTW, based on my research today verifying exactly which TI I purchased back there, I discovered that I should have hung onto it (can't remember what happened to it, I might have given it to one of my kids when they were school). Apparently the SR-51 is considered a 'rare' collectors item since it was replaced almost immediately with the SR-51A (which is probably why the store had cut the price since they originally sold for around $200) and there are collectors out there who will pay a pretty penny for one in good condition.
John R. Baker, P.E.
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Siemens PLM Software Inc.
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