Slide Rule of the Month: #1 Relay No. 105

When I was in ninth grade I was issued a slide rule.  It was made of bamboo with a gray plastic case, it was identical to the other 25 rules issued to my fellow students, and we all learned how to use them in math class.  Two years later, Texas Instruments issued its first pocket calculator, and the digital revolution in calculation, math classes, and science and engineering was underway.  Two years after that, the handhelds were becoming cheap and plentiful enough that desperate slide rules were amassing for secret meetings in dark urban dockside warehouses on all three coasts, planning to board the cargo vessels and dump millions of crates of calculators into the briny.  TI parties they were called.

Back when we had to learn to cipher with them, slide rules were sort of half tacky, half cool.  I recently came upon a collection of them, and realized that now that we don’t need them any more, they’re almost twice as cool as they used to be, and just a little less tacky—maybe about  ¼∞±∑ less, give or take.  I’m trying to use the K scale here to figure out what  almost double cool & that odd bit of tacky total up to, considering that tacky/cool equals unity, and whether such a stengelian sum can add up to more than 100%—or not.

Paratherm Corporation now owns that rule collection, and we are going to be featuring a vintage slide rule occasionally in the Unsubmerged Blog. 

Image of Keiki Slide Rule

Keiko Relay Bamboo Slide Rule, Edmund Scientific Private Brand

In these modern times of digital data, when the door modules in our cars may have more computing power than the second generation of Apple and Microsoft computers had, when we have pivoting spreadsheets and pirouetting cellphone aps for everything and everything, there is something we in the engineering fields miss sometimes (however fleetingly); an intangible eye-hand shorthand judgement that we wistfully long for, even reach back for, back to those times when we had eyeball graphs and slide charts and—yes—slide rules. I don’t know what we called it then, maybe mathematical intuition, maybe gut, maybe feel. Maybe gut feel.

Back then, one didn’t always have to have the exact precise perfect answer. And the slide rule was great for that. In using a slide rule, you sort of needed a picture in your head of what you were doing…

For example, “to find the force with which a spherical shell of copper, 1/100 of an inch thick, and 100 feet internal diameter, filled with inflammable gas 10 times lighter than common air, will ascend, or what weight it will keep in equilibrio; and to what height it will ascend…the density of air being .0012 …1
..and while you’re calculating it, your thoughts sort of merge with your fingers as your mind slides through the steps of the calculation while your fingers work the slide and the cursor, running a series of equations half in your head and half on the rule.

It was a beautiful thing, and so is this specimen.

Cursor and Case Close-up, Relay Slide Rule

Cursor Close-up, also Showing Relay Logo on Case

I reached more-or-less at random into the collection and pulled out what may be the perfect Slide Rule of the Month #1. It is neither esoteric, nor ultra cheap. It is very much like the bamboo rules we had in Middle School. When it was new, it probably cost something like $4.59. It is very well made, sturdy, with lots of useful reference and utility functions beyond its calculating scales. A beveled rule. Inches and centimeter scales top and bottom. And the nifty handy dandy celluloid back strip with conversions and physical properties. In print so fine that I’m not sure I could have read it when I was 14, and I certainly need my reading glasses now.

Image of Back of Slide Rule, Physical Properties & Conversions

Physical Properties & Conversions

 So there it is, SROTM #1.  Here are the details:

  • Manufacturer: San-Ai Keiki Co., Ltd., Japan,  for Edmund Scientific, Barrington NJ, USA
  • Model:  Relay No. 105
  • Size:  10-inch.  Length: 11.25 inches  Weight:  3.6 oz.
  • Scales: Front — K A   B CI C  D L  Back — S  ST T  
  • Material:  Bamboo and Celluloid
  1. From Mathematical Questions Posed in the Ladies’ Diary, Vol. III, 1817, by Thomas Leybourn 

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