Recipe is based on e.g. 5 eggs, but I want a smaller output so I want to use only 3 eggs[1]. I set the slide rule so that 3 is lined up under 5.
Then for any other ingredient in the recipe, say 7 tablespoons of milk, I look up 7 on the upper scale and read off 4.2 on the lower scale.
It doesn't matter which scale is upper or lower, and you don't have to keep track of whether you're multiplying or dividing, you just pick one to mean "recipe amount" and one to mean "actual amount". You anchor them against each other based on the critical ingredient, and then read off all other amounts. That's part of what makes it so easy to use even for children.
[1]: Or maybe I'm cooking to use up an ingredient about to expire, so I want to use all of my seven eggs, and so on.
Oh, that actually sounds really convenient. Next time I'm scaling a recipe by a factor other than 2, I'll get out the slide rule and enjoy the incredulous looks from everybody nearby. I can hardly wait!
I wish recipe sites just did that; enter a base for some product, get the rest based off percentages, rounded up to unit ingredients as best as possible
Most Pickett rules (once popular with secondary schools and still in abundance) are aluminum. Remove the slide, wash under warm water, lube with the nearest oil or fat, reassemble. Takes under a minute.
Usually I'd use Vaseline or a silicone lubricant, but expedient lubricants are fine if you'll wash it frequently and aren't trying to make multiple slide moves per minute.
The cursor requires a screwdriver if you really gunk it up (I'd be surprised), but it's still pretty easy. You don't need the cursor if you're just using the rule as a single-proportion lookup table, so you could take it off and put it away if it's a problem.
My only experience with a slide rule is a Hughes-Owens Versatrig, which is made from some bamboo-like material.
Knowing that metal slide rules were widely used makes me think that IKEA might come up with this “revolutionary” kitchen gadget in the next decade or two.
Then for any other ingredient in the recipe, say 7 tablespoons of milk, I look up 7 on the upper scale and read off 4.2 on the lower scale.
It doesn't matter which scale is upper or lower, and you don't have to keep track of whether you're multiplying or dividing, you just pick one to mean "recipe amount" and one to mean "actual amount". You anchor them against each other based on the critical ingredient, and then read off all other amounts. That's part of what makes it so easy to use even for children.
[1]: Or maybe I'm cooking to use up an ingredient about to expire, so I want to use all of my seven eggs, and so on.