How do companies that make measuring tools like rulers make sure the product is accurate? Is there a universally ruler that is used to check? How do they make sure the measurements are exact?
In: Engineering
If you have a laser, a detector and a good clock you can measure the distance of an object to a very precise degree. A meter is exactly defined as the distance light travels in 1/299,792,458 of a second.
There was at points in history actual physical objects which defined units like the meter, for example the International prototype metre bar but now they are defined in terms of universal constants.
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There are primary standards locked away from which secondary standards (copies) are made and certified. Those copies are sent around and used to make calibration standards. All standards are traced everytime they are copied, and become a little less accurate with each copy.
1. Company makes ruler.
2. Company checks ruler against their calibration standard and passes or fails ruler.
3. Company calibration standard is sent to a Metrologist every so often. Metrologist checks company standard against their standard.
4. The metrologist standardis checked against a secondary standard.
5. The secondary standard is traced to the primary.
There could be more copies than listed in my example above but that is the general idea. The unit of length is no longer based on a physical object but rather the speed of light in a vacuum. NIST is responsible for maintaining this system.
Edit: adding a great video that is easy to follow. https://youtu.be/EufnURd1U2s
Rulers, in particular, are “accurate enough” for their use. Many wooden rulers are stamped with the lines and numbers, so a few careful measurements are copied over and over. The piece of wood is “about a foot”. Plastic rulers, again are measured when their extruder template is made and the metal form is “accurate enough” when used to make the final product.
Tl;dr – most every day measuring devices are not exact, but they are good enough.
I’ve wondered this same thing except for time.
Almost a billion people in the world use their iPhone to tell the time (and reference it to set their other clocks). So where does Apple get their time from?
Is there a universal clock that all clocks are based on?
There **is** a standard bar that’s 1 meter long. It’s kept in Paris and high precision copies of it were distributed to government’s weights and measures departments around the world and from then copies were made and distributed internally to local offices and from there manufacturers bought copies for making and calibrating their own products.
The same goes for the kilogram.
These master copies are, presumably, extremely expensive as they are ultra high precision cut pieces of platinum, silver and palladium – intended to be as chemically inert as possible and fairly stable in length across a modest temperature range. These are a real “buy it for life” item.
Although the master copies are no longer technically the formal yardstick (literally), a while ago the meter was formally redefined as the distance light travels in a vacuum in a set time, because this is fixed and we can measure time more precisely than distance. The kilo has been/is in the process of being redefined also in terms of measurable fundamental units/properties, but that’s taken a bit longer to get to work/prove.
When a ruler is made, you check it against a ruler that you know is more accurate than the ruler you’re making.
In turn, when that ruler was made it was checked against another ruler that was known to be more accurate.
… repeat a bunch of times …
So, where does this end? Well, there are a handful of laboratories around the world that have the ability to make a very accurate ruler. This is done with a science experiment. The official definition of a meter is:
“The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.”
(Note that other units of length, such as inches, are now defined based on the length of a meter, so it always ends up with that definition of a meter).
But, to do that the laboratory needs to be able to time a fraction of a second very accurately. Just like rulers, clocks are compared against more and more accurate clocks. And, just like the meter, the official definition of a second is based on a science experiment:
The second is “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom” (at a temperature of 0 K)
Translating that into ELI5: If you take this material and do complicated stuff to it, it gives off microwaves. You can measure the frequency of those microwaves – i.e. how many waves there are per second. This is exactly 9,192,631,770, so if you get a different number then your clock is wrong and you need to adjust your clock until you get the right number.
The manufacturer will have a “standard” which is usually an object made of a stable metal that corresponds to the measurement they are trying to reproduce, and those standards are accurate copies of another standard, and so on and so forth, traced back to a single object that sets the standard for everyone. They then use those standards to check the accuracy of their product.
Clarifying question that goes back to OP’s question: I’m building a rocket ship. I need precise measurements. So I buy a precise measurement device of some sort.
How does the manufacturer know it’s *that* precise?
Official measurements are defined using actual physical items. Like there’s a one-kilogram metallic weight at some university (likely in the UK) that is the definition of a kilogram. I believe wavelengths of light are used to define distances. But those are really only useful in scientific situations (where they likely use lasers or other means to measure stuff anyway). The rulers and tape measures you use at home or school are just “close enough”. If you grab a tape measure from two different manufacturer, they will likely get out of sync the further along you go. After 25 feet, they might be off by 1/16 of an inch, for instance. Again, it’s close enough for daily use.
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