I’ve often found myself working at a client site where Engineering test fixtures, or other setups are not metric, even though the technical specification may be in metric. In a few instances, technicians had placed lumber yard—yard sticks, marked only with inches, on the fixture as a “reference.” Without a metric ruler available, What’s a Metric Maven to do?
I had a visit from a metric advocate several months back, who helped my situation. We were having lunch at a local eatery when he looked down and said, “looks like that salt shaker is about 100 mm tall.” I wondered how he had determined that. He showed me his index finger and said “your index finger is about 100 mm in length.” He then placed it next to the salt shaker and and said “see, it’s just about the same.”
He continued “if you need to have the approximate length of a single millimeter, it’s very close to the thickness of a dime.”
I later went home and measured my index finger. I was quite surprised. It is very close to 100 mm, as the photo below shows. Women’s index fingers are shorter, generally about 80 mm, but often their middle finger is about 100 mm to the back end of the knuckle. The dime was indeed fairly close to a mm, at 1.34 mm thickness.
In the metric desert which is America, I often must resort to approximating distances with my index finger. I did this in the case of the yardstick test bed mentioned previously, and arrived at about 700 mm for a radio link range. It turns out that the distance across the knuckles of my hand from side to side is also 100 mm.
We now have a way to estimate metric lengths without a tool, but what about metric mass? It turns out that any store where you can purchase plain M&M candy, gives you access to an approximate metric mass standard. Each plain M&M weighs nearly one gram. I have taken eleven of them, placed them on a scale in the photo below. As you can see, they have a mass of ten grams.
Many Americans have told me they have no idea what a gram is, but when I tell them it’s about the weight of one plain M&M, there is instant recognition. If you read the nutritional label on a package of food, and it states that a serving contains 25 grams of protein, fat or sugar, that’s a mass of about 25 M&Ms. You can also estimate the mass of the serving size this way.
Should you have a small balance, or seesaw available, you can estimate metric volume. First you would place two identical small containers on either end of the seesaw, at equal distances from the center (fulcrum), so it remains balanced. You could then fill one container with say 50 M&Ms, and the other with enough water to just balance. The amount of water would be about 50 milliliters. Draw a line inside of the container and you have a 50 mL estimating cup.
If you have no M&Ms, or a balance available, but have a working knowledge of origami, you can create an approximate one liter volume by making an inflatable cube from a sheet of paper. If this cube is then filled with water, it becomes an approximate one kilogram mass estimate.
A decimeter is a metric unit not commonly in everyday use, but is equal to 100 mm. Fortunately, this length is used to define the liter and the kilogram. I say fortunately because we have already shown an index finger is about that length—100mm.
Unless you have a formula for the dimensions of an inflatable cube, given the initial paper size, one would have to experiment, to find the initial square paper size, that yields a cube with 100 mm edges. Conveniently the very hands you use to fold the paper, have a finger length which allows you to estimate 100 mm each time you make a cube. Here is a neat video showing a woman creating an inflatable paper cube.
The metric system is an amazing method of measurement. With only your finger, some plain M&Ms and a makeshift balance, or alternatively, paper folding, your finger, and some water, one can estimate all the basic metric units one generally ever needs in everyday life for length, mass, and volume. The metric system can be very MacGyver-like, but is also the most precise and accurate measurement system ever devised. It is a system for the common man and the engineer/scientist alike. It’s time we all used it.
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