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Basic SI units have definitions through experiments that seems to imply a pretty obvious setup.

Is there a standard experiment for calibrating Newtons?

The definition is the force needed to cause a 1m/s² acceleration of a 1 kg mass. I can imagine obtaining a 1 kg mass.

However, it seems error-prone to try and directly measure acceleration. If you mark out 2 consecutive meters (points A, B, and C) and then measure the time the object crosses each, then you can say you have 1 Newton if $\tfrac{1}{T(C)-T(B)}=1+\tfrac{1}{T(B)-T(A)}$.

But there are several objections to be raised about defining a Newton in terms of a mass, two distances and three times.

Back to the question: How do you (theoretically) calibrate Newtons from first definitions? Thanks

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Absolute accelerometers (or gravimeters) do measure the acceleration of a dropped weight. But instead of timing it between two points directly, the speed is measured by having it reflect a laser into an interferometer and timing the interference fringes. Commercial devices are available that can measure local acceleration to one part in $10^{-8}$.

Wikipedia article on Gravimeters

Once the local acceleration of gravity is known, force can be read from calibrated masses.

Conference paper on Force and Torque measurements

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