DaChesserCat's Journal: Conversion slide rules

I mention, in a previous journal entry, the fact that I want a wristwatch with a slide rule bezel. We do a lot of traveling, frequently to places where they use Metric, instead of the Imperial units with which I'm familiar. I have also traveled to places which had significantly different currencies. When I was in Norway, it was approx. 6.5 NOK = 1 USD. When I was in South Korea, it was about 800 SKW = 1 USD. When we went to Chile, it was about CLP 500 = 1 USD. Slide rules are very good at multiplication, which is all you really need to do to convert between currencies and between measurement systems.

In the absence of such a wristwatch, it might be helpful to print a set of concentric, circular slide rules on a piece of paper or cardstock and take that with me. I anticipate the following, on one side of the paper/card stock:

1. miles
2. meters/km
3. feet and pounds
4. grams/kg
5. ounces

Naturally, because kilometers, meters and centimeters all vary exactly by orders of magnitude, you could convert miles to meters or kilometers, just as easily. And you could convert feet to meters or centimeters, just as easily. The 1 or 10 mile mark would be around the 16 meter mark (1609 meters / mile). The 1 or 10 meter mark would be around the 30.5 foot (and pound) mark (30.48 feet / meter). The 2.2 or 22 pound (or foot) mark would be around 1 or 10 gram/kg mark (2.2 pounds / kg). The 28 gram/kg mark would be around the 1 or 10 ounce mark (1 ounce = 28.35 grams).

That would be a total of 5 concentric, circular slide rule scales. That's about as much as you could hope to put on one side of a piece of paper without getting too confusing.

They should probably be color-coded. Let's assume that those five scales were printed in:

1. red
2. green
3. blue
4. purple
5. black

(outermost to innermost).

• red -> green = miles -> meters/km
• green -> blue = meters/km -> feet
• blue -> purple = pounds -> grams/kg
• purple -> black = grams/kg -> ounces

Five scales, four useful conversions.

For the other side, starting from the outer-most scale and working in, I'd do:

1. gallons
2. liters and local currency
3. US Dollars

As such, this would need to be reprinted anytime I was going to a different country. The reason for this progression is simple. I need to be able to convert between liters and gallons. I also need to be able to convert between currencies. The three scales, in this order, would also allow me to convert local currency / liter -> local currency / gallon -> US Dollars / gallon. So, three scales, a total of three useful conversions.

When you determine where you're going, find the exchange rate and print these sets of scales on opposite sides of a piece of paper or card stock. Trim off the excess and throw the result in your bags. Then, when you get there, you have everything you need to convert between measurements and currencies, as well as do a more complicated conversion (USD / gallon to whatever / liter, and vice versa).

If you wanted to make a more permanent "device," you could make a double-sided circular slide rule. All of the scales on one side (length and weight) would be fixed. Only the inner-most scale on the other side (currency conversion) would need the ability to move. Someone with a laser cutter/engraver could probably fabricate the pieces, pretty easily.