The changing frequency of the mains supply is not "Manipulated" in order to keep clocks accurate.
It's caused by an imbalance between supply and demand and the generating / distribution companies have a minute by minute battle to keep the frequency within it's declared value.
There is no way to store electricity in the grid, supply has to match demand. But adding or removing power from the grid is not instant. Turbines have to speed up or slow down, fuel rods have to be added or removed etc.
A good analogy is riding a bycycle. If you are on perfectly flat track you can keep a perfectly constant speed. If there is a hill up or down then the demand on your legs goes up or down, and so does your speed, unless you are really good at matching your effort with the gradient. As you go up a hill you will slow a little as you pedal harder to bump your speed up. and vice versa.
In the UK we have "TV Pickup" when soap operas finish we all go and put the kettle on (1.75 Million kettles in under 5 minutes that's 3 Gigawatts extra). Its such a big surge, and the frequency drops so fast we have to get more in from France!
our "National Balancing Engineer" braces himself, and the grid, for the end of Eastenders (TV soap opera) and sweats a bit when it's late and France lets us down. Watch him go quiet when the end theme tune (Drums. Dum Dum Dum diddiddy) starts.
Great project BTW.
And... What a good day to be posting about it. Heinrich Rudolf Hertz's 155th Birthday.