How a balance notices the surrounding air
Two equal-mass objects can pull differently on a balance in air. Their volumes, the local atmosphere and the reference weight explain when that difference matters.
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Why a clock needs a frequency reference
Setting a clock and keeping it on time are different jobs. Its oscillator supplies the ticks, while comparisons with a frequency reference show whether those ticks make honest seconds.
What a thermometer really comes into contact with
A contact thermometer measures its sensing element. Its reading represents a pipe, liquid or patch of air only when placement, heat paths and settling time make that element a faithful stand-in.
The kilogram after the metal cylinder
The world's kilogram stopped being defined by one guarded cylinder in 2019. Its new constant is exact, but turning that definition into a working mass still requires balances, silicon crystals, comparisons and careful transfer.