A few years ago I sighted on eBay a set of three German circular slide rules of the Controller brand. There was a big one, 20 cm in diameter; a midsized one 11 cm across; and a small one at 7.5 cm. They all looked pretty much the same except for their sizes, and this reminded me of those three bears in the Goldilocks story — a thought that amused me enough to push me into buying them. So now I had a Father Controller, a Mother Controller, and a Baby Controller.
Actually this type of slide rule is fairly common, and is often seen on eBay. But then, one day, I saw an auction for what can only be described as Grandfather Controller: a truly large slide rule 30 cm across, and still identical in design to the Three others. This one is anything but common; in fact I’ve never seen anything like it, nor can I find any mention of it on the web. Naturally, I added it to my collection post haste, and now there were four!
Read the full article about this mysterious giant Controller on my HOC web site.
