The standard self inductor 1000 micro-Henry shown above was made in 1930s by Y.E.W. in Japan. We have been using it as a reference standard for the calibration of L/C/R test equipments for more than 50 years.
　
In those days, the primary standards of the frequency, voltage and impedances (L/C/R) were derived from the period of the earth's rotation, the cadmium battery, the standard 1 ohm resistor, etc which were averaged internationally. The standard inductor shown above had been calibrated by those classic standards.
　
Now, the primary physical standards are given theoretically by the cesium atomic clock, Josephson's effect, quantum hole effect, cross capacitor etc. in extremely high accuracy. No need to take an average internationally.
　
Still, our 1 milli-H inductor keeps 1 milli-Henry (between 1.000 and 1.001 mH) regardless of the fantastic evolution of the primary standards. Old stubborn checks annually the novice national standards!
　
All of our L/C/R test equipments examine this 1 milli-Henry inductor before they go to the customer. Suffice to say, the answers are always 1 milli-Henry as they have been for more than 50 years.