When my late father first moved to Menorca in the early 1970s, electricity was still somewhat...quaint and unusual.
In his first flat, half of the flat was wired for 127V and the rest was on 230V. Which could be interesting as the sockets weren't labelled!
A friend had rather a posh hi-fi system and was concerned about Menorcan voltage stability upsetting it. So he bought himself a very expensive auto-transformer. The output voltage was monitored and a motor would move the secondary winding tap as required to maintain the correct voltage if the input value fluctuated. None of today's solid state business, it hummed away in a stout, well-insulated metal cabinet.
But after a few weeks, he found that the input voltage had stayed rock-steady at 230V throughout!