The only obvious approach would be to either use a tap changer or convert the voltage later with some kind of variable transformer or power electronics.
2.
They can be restored before use by gradually applying the operating voltage, often performed on antique vacuum tube equipment over a period of thirty minutes by using a variable transformer to supply AC power.
3.
The " swinging link " is a form of variable transformer, that moves the input coil in and out of the space between turns in the main coil to change their mutual inductance.
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