is a mechanical device that regulates the flow of fluids
) by opening, closing, or partially obstructing various passageways.
A valve doesn't necessarily mean a man-made device; there are several valves inside the human body, for instance in the heart or in veins.
Because their effect on a flow of electrons is analogous to the fluid devices, the electronic devices known in the US as "vacuum tubes" are called "thermionic valves" in Britain.
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