Slide Rule Lecture

The slide rule was all we had in 1970 when I was taking an electronics engineering course. I doubt I could use one now other than multiplying and square and cube roots. I still have one around the house but have not used it in many years.. The CRC book was also a go to thing of the past.

On a sunny day (Tue, 21 Mar 2023 14:41:14 +0000) it happened Martin Brown <'''newspam'''@nonad.co.uk> wrote in <tvcfmd$551n$ snipped-for-privacy@dont-email.me:

Cool! spiral scale :-)

I had an Aristo Studio in my school days...

A long time ago, circa 1955, I proposed to Pickett that they replace the flat window with a circular one to magnify the readings and get more accurate settings. About a year later, I saw slide rules with circular windows on sale and I bought one. It worked so well I tossed my old ones in the garbage and used the new one.

I have not been able to find any example of a slide rule with a circular window in any of the historical sites.

I had a polycarbonate window in my Piper PA-46 Malibu, N4360V, and had no problem with crazing.

I also had a Cessna 152 with a poly window and no problem with crazing. It was for any of my employees who wanted to learn how to fly.

Poly windows have been used in aircraft windows since before WWII. As far as I can tell, there have never been any problems with canopies crazing due to UV exposure.

Polymethyl methacrylate (Plexiglas)

The material was developed in 1928 in several different laboratories by many chemists.

Historically, PMMA was an important improvement in the design of aircraft windows, making possible such iconic designs as the bombardier's transparent nose compartment in the Boeing B-17 Flying Fortress.

From early on Plexiglas found a market in aviation, as the alternatives were either flammable or too heavy.

[...]

Plexiglass is acrylic, not polycarbonate.

Malibu is pressurized, and it may need heftier material than PMMA for the windows. My old PA28RT201T Turbo arrow had PMMA (plexi) windows.

Plexiglass is most assuredly not acrylic. Acrylic shatters, which is why it is so hard to drill. It would be a disaster in aircraft. Even a small bird strike would disable the pilot.

Plexiglass is polycarbonate and has been used in aircraft windshields since before WWII. For example, in the DC-3, which is still flying today. It is much stronger than acrylic, and takes much higher velocities to cause damage. These velocities are not normally reached in civilian aircraft.

Military aircraft use bulletproof glass in the windshield, and plexiglass for the canopy. Civilian jets use a multilayer combination of glass and a shock absorbing layer. They are intensively tested before certification.

Please note, I have PLONKED you for a long time. The only reason I am replying to you is someone included your post in a new one. I will probably not see your reply and will not answer.

Just make it thicker.

Plexiglas is a trademarked name for polymethylmethacrylate, definitely an acrylic. Lexan is an early trademarked name for a family of polycarbonates, most commonly made from bisphenol A. A bit of googling or reading at Wikipedia (

and ) will confirm this.

Numerous references (plastic dealers, and the company which holds the Plexiglas® trademark) disagree with you.

Can you cite specific technical references which support this? All the references I have ever seen, state that plexiglass and polycarbonate are two different materials, with different properties. LEXAN and Makrolon are the trade namea I've usually seen associated with polycarbonate, although there are plenty of others.

Plexiglass was originally a brand name for a toughened cross linked Acrylic. The cheaper grades are more thermoplastic and less strong.

Today you can't be sure what material sold under "Plexiglass" brandname means any more it has in some places become "*any* water clear plastic".

ICI's Perspex and Du Pont's Lucite brands were the main players in water clear acrylic. ICI now absorbed into Lucite International.

It is a form of PMMA but more highly cross linked than the ordinary thermal plastic extruded sheets.

The work on using Polycarbonate for the F16 canopy is still classified in the UK but the US are a bit more cavalier these days. This URL of declassified military research details some of the then classified experiments on making Polycarbonate withstand the environmental rigours of flying high UV and weather. Some clever chemistry was involved.

I particularly like the front page "subject to export control laws and various threats of fines" section which hasn't been redacted. Fairly pointless if you put the thing onto a server and let Google index it!

The redacted bits such as they are haven't been done very well either!

The key difference between clear acrylic and polycarbonate is that at Mach 1+ a birdstrike often compromises acrylic but just bounces off polycarbonate - it is much more of an engineering plastic with more strength and toughness than acrylic for a given thickness and weight really matters in a jet fighter.

It took me a couple of minutes to get the hang of it but you can get nearly 5 sig fig multiplies once you figure out how to set up the two numbers to be multiplied. Two fine lines set the master to 1 and the other to x, then move the master to the other number and the slave moves with it. It really was quite something to behold.

The smaller very finely worked pocket watch one was even more impressive but I needed two pairs of glasses on to read it!

Mine was a random generic plastic one. In my school days I could pretty much do 3 sig fig mental arithmetic and just scribbled SR in the margins. I had to get a calculator at university - correct answers in crystallography were at least 5 sig figures and sometime more.

My first calculator was a TI SR-51A.

On a sunny day (Thu, 23 Mar 2023 11:17:18 +0000) it happened Martin Brown <'''newspam'''@nonad.co.uk> wrote in <tvhcg8$14a2p$ snipped-for-privacy@dont-email.me:

I do not remember the make, but bought my first calculator in London around .. now let me think... 1976? tried it and did some simple things

2 x 4 turned out to be different from 8.. So brought it back, 'Oh the man set, its made in the UK, here is one made in the US', that one worked! My first computah I bought was a Sinclair ZX80 in the very early eighties (had worked with a PDP10 or 11 before in 1978 or so, knew about Unix and some guys had a Motorola maxboard at work, and I had designed a hardware video digitizer back then). Had Brian Kernighan and Dennis M. Ritchie 'The C Programming Language' too.

At that time I had the TV repair shop, added a thermal printer and printed receipts and stuff... Wrote the code and made an RS232 interface for the printer. Then when ZX81 came out things really took of, added memory, I/O board, EPROM programmer board, modem board and finally wrote a CP/M clone for it so I could run programs from the CP/M user club they were on Viditel, an internet predecessor:

that time I was working with IBM stuff for a big company, designing ISA cards and embedded stuff. But the ZX81 stayed my home computer for a long time... added a IBM keyboard, and a 5 1/4 inch floppy drive :-) In 1998 I found a CD with SLS Linux that came with some magazine and from then on it was Linux only. No problem I had a nice Unix book too, just took off! Free gcc compiler!

I did have a PC with Microsoft win 3.1 or something and trumpet winsock for the internet at that time... And Free Agent for Usenet. No such thing for Linux existed, so I wrote NewsFleX (as Unix and C exercise):

using it, right now, ported to a Raspberry Pi4.

Linux has been polluted (to say the least) by RatHead's systemd, was looking for a way to get rid of it for the Raspberry Pi4 I have, There is a lot online.

What happened to that calculator and slide rule I do not know, I did donate a lot of stuff when moving house.. Still have boxes full of electronics stuff...

Thanks. I will appreciate your not answering.

(Would someone do me the favor of quoting this so he will see it. Thanks)

Oh, I missed that. It probably explains his misunderstanding of plexiglas. He needs more kindness than argument.