Bonus question with answer:
Q: How do thermometer manufacturers know how to blow glass so that the alcohol or mercury expands perfectly to tell you the temperature.
A: They don’t. They get it close, then put the thermometer in fluids at a two different known temperatures then mark the glass. Then they pick the correct pre-made stencil to fit those two markings and paint the rest.
(More or less)
I wonder if the more use an hour glass gets, the more out of time it gets.
considering the sand is always getting smaller and finer, the more you shake it up, or mix it, turn it over, the (ever so slightly) more bits of sand rub up against each other and smooth off the edges, making each part smaller.
also, the hole might get bigger too?
They don’t. They manufacture one and figure out how much sand it needs for they time they want, once that is settled they can mass produce exact copies of the hourglass and use the same amount of sand in each.
Minor variations will exist of course from minor variations in manufacturing or even the granules of sand.
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