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This simple version of a micrometer caliper will allow you measure things that are too small to measure with commonly available means. Although its accuracy is less than perfect, its capability is somewhat amazing, especially considering the cost of the materials.
(Note: As an alternative to calibrating by loosening the wing nuts to move the dial, you could also just initially make sure the zero on the dial is somewhere near the bottom, and then adjust the indicator by moving it or bending it until it lines up with the zero mark.)
Tighten the screw with the clothespin until the clothespin slips and the end of the screw is up against the fender washer. The zero mark on the dial should be lined up with the reference indicator. Loosen the screw a little less than one whole turn (until the zero on the dial is almost to the reference mark again) and insert a piece of paper between the end of the screw and the washer. Tighten the screw again, once more using the clothespin until it slips and the end of the screw is up against the paper. Then read the dial.
Each mark on the dial represents about one one-hundredth of a millimeter, so the mark that lines up with the reference indicator indicates the thickness of the paper in hundredths of a millimeter. Ordinary copy paper usually has a thickness of between ten to fifteen hundredths of a millimeter (10 to 15 on the dial).
Try measuring other objects. Some things to consider are an index card, a paper clip, a piece of thin spaghetti, regular or heavy duty aluminum foil, nylon fishing line, wires of different gauges, plastic bag material, or even a human hair.
Periodically check to make sure the reference indicator still reads zero when there is no object in place. If it doesn't, then recalibrate the dial.
If the object you’re measuring is larger than one millimeter, you’ll have to keep track of how many turns you have to loosen the screw (and therefore the dial) before measuring the object, and then add the appropriate amount to the dial reading. Each complete turn of the screw moves the end of the screw one millimeter.
You can increase the reliability of a measurement by doing multiple trials and finding their average, rather than relying on a single trial.
The 24 in the specification of the screw (#10-24) indicates that there are 24 threads per inch. This is known as the "pitch" of the screw. It means that it takes 24 complete turns of the screw to move it one inch.
One inch is equal to 2.54 centimeters, or 25.4 millimeters. So there are 24 threads in 25.4 millimeters. This is close to 1 thread per millimeter. And though it is off by 1 millimeter in about 25 millimeters, that works out to just 4 millimeters in 100, or 4%, which is not bad considering the cost of this instrument!
Since there is one thread per millimeter, then one complete turn of the screw moves the end of the screw 1 millimeter. The dial divides one turn of the screw into 100 parts, each of which represents 1/100 of a turn. Since a whole turn is a whole millimeter, each division on the dial represents one hundredth of a millimeter, or 0.01 millimeter.
The micron is a unit of measurement still in common use in some areas of science and technology, but it is no longer an official SI unit. How does a micron relate to a meter, a millimeter and a micrometer (the SI unit, not the instrument you built)? How many microns are represented by one division on the dial?
Familiarize yourself with the following terms: sensitivity, precision, accuracy, deviation. See if you can apply them to your micrometer caliper.
Making and/or using this micrometer caliper can provide unique hands-on insight and experience in the measurement of very small things, but with limitations on precision and accuracy affecting its use as an everyday working tool. Pre-digital mechanical micrometer calipers operated on the same principle, though with greater precision and accuracy, but were relatively expensive. Now, however, there are inexpensive digital calipers with exceptional capabilities that provide a useful working tool for many aspects of science education.
The inspiration for this micrometer came from the original PSSC Physics materials created by MIT’s Physical Science Study Committee in the late 1950s and early 1960s.
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Attribution: Exploratorium Teacher Institute