The Logomat Pfiffikus 2001
The slide rule in the photo next to the quarter is 48 mm in diameter and weighs 5 grams. A true featherweight!
Being so small is a limitation: the circumference of the disk is 150 mm, which is equivalent to a six inch pocket slide rule – useful for the occasional rapid calculation but not much more. However, the Pfiffikus 2001 has a scale wound in a 3 turn spiral, which totals some 350 mm in length, longer than a standard 10 inch straight slide rule. At 5 grams!
The device consists of four parts: A thin aluminum disc carrying the scales; a transparent disc with one radial hairline that overlays the scales disc; a plastic cursor with a second hairline; and a red pivot that the other three rotate around independently. Well – almost independently; the friction between the parts is carefully designed to control their relative movement. The design magic is this:
- The metal disc is a hair’s breadth larger than the transparent disc, so when you grasp the device with your fingers at opposite ends, the scales stay stationary while the transparent disc can move freely.
- The friction between the cursor and the transparent disc is much higher than the friction between the latter and the metal disc; thus, if you rotate the cursor it takes the transparent disc along for the ride. This means that once the two hairlines are set, you can move them as one without changing their angular separation – a key requirement in a circular slide rule.
- To change the angle between the hairlines you clamp the two disks together between thumb and forefinger and force the cursor to move alone.
All this results in a device that is surprisingly easy to adjust, rendering its operation a real pleasure. Not that there’s anything unusual about that operation, but I can’t think of a circular slide rule that does its thing so smoothly.
Here, for example, is how you’d calculate 26 x 3 on this slide rule:
Step 1: Set the hairline on the transparent disc to 1 on the scale, and the cursor to 26 (on the middle turn of the spiral).
Step 2: rotate the cursor and transparent disc, keeping their angular separation fixed, so the hairline on the disc points to 3 (also on the middle turn).
Result: Read the result under the cursor’s hairline: 78 (on the outer turn).
Step 1 (left) and step 2 of figuring 26 x 3
Note that you have to decide which turn of the spiral to use for the result – no big deal with only three turns to choose from. Now, if you had 20 turns, you’d need some help… see the Ross and Courvoisier spiral slide rules for solutions in that scenario.
The Pfiffikus is the smallest of a family of slide rules produced by the German company Logomat in the early 1970s. Although it is a functional slide rule, Logomat sold it as “swag” – promotional merchandise used to market a brand, which companies would buy imprinted with their name or logo and give out to their customers. My unit has the name BARCO-Venturi printed on the little vinyl sleeve containing the item and its folded instructions. Barco venturi was a brand name used by the Aeroquip Corporation for a line of measurement systems.
Exhibit provenance: eBay.
More info:
- More on this and other Logomat products can be found here.
- A scan of the instructions can be found on Miguel Ramirez’s site.