Standard Deviation Machine
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Recently while doing research on an old teaching machine, I came across a patent for an instrument with scales and wheels that was designed to calculate the standard deviation (or root-mean-square deviation). It actually calculates the square root of the sum of the squares of the deviations, and then provides a table for calculating the standard deviation based on dividing by the square root of the number of inputs.
The operation required the calculation of the mean, as described in the patent, and adjusting the scale of the values to allow them to be marked on the scales.. not an easy task, but still, all things considered, and incredible piece of engineering.You can see the original diagrams and patent write up here.