I worked a lot with torque wrenches in the Army, and always wondered how the “first” torque wrench was calibrated without another one to verify that it was accurate? Was there another tool to verify the calibration was correct and if so how did that one get calibrated. In my head it keeps asking “well how did the next one get calibrated?” Every time I think about the first precision tools of any type, not just for torque.
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A calibration is made against a “standard” – a standard is an item designed and verified to be accurate by comparison to a higher standardx and ultimately up to a primary standard.
Historically, primary standards were physical objects. For example the length of 1 metre used to be the length of a stick kept at a Paris laboratory. Periodically the top labs in each country would send their sticks to Paris to be checked against the primary.
These days, primary stands tend to be scientific experiments. For example, a metre is now defined as the distance travelled by light in a specific period of time. A laboratory can do the experiment to time a beam of light against an atomic clock and that gives the distance. If the experiment is done properly, the length will be exact to the accuracy of the experiment.
This experiment can be difficult to do, so only a few laboratories in each country will have the equipment do it. They will then have sticks which act as secondary standards which can be used for calibrating other sticks, and so on.
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