In the developed world, electricity is delivered via AC - alternating current. One of the specifications of that electric current is the frequency at which it oscillates. Most of the world is tied into electric grids running at either 50 or 60 Hz.

In the early part of the 20th century, electric clocks driven by synchronous motors were developed and became popular. Because of that, an impetus existed to insure that the long term stability of the grid frequency was maintained. As technology moved into the electronics age, the idea of synchronizing a clock to the electric grid's frequency was often designed into clocks even if they no longer used synchronous motors. There are three distinct electric grids in the continental United States: the East, the West, and Texas (it really is like a whole 'nother country). The grid frequency is not synchronized between the three grids, but it is independently maintained and synchronized within each region.

But how good is it?

There are nominally 60 cycles per second, but there are variations due to the loads imposed on the grid. Since the grid is so large, instantaneous control of the frequency is not feasible. But it can be generally directed, and measured. For periods of time when the frequency drifts low, it can be countered with a period of higher frequency.

If it were perfectly accurate, there would be 5,184,000 cycles per day on average. We can certainly count them, but we need an unquestioned yardstick against which to compare.

GPS is an excellent option. Its stability and accuracy makes it a phenomenal value for stable and accurate time comparisons. The AdaFruit Ultimate GPS module, for example, quotes a 10 ns accuracy of the PPS output. Moreover, since that output is continuously disciplined by the GPS system, it should have (for our purposes), perfect long-term stability.

So if we count the number of cycles of the AC frequency that occur between two PPS leading edges, we should get 60 every time.

Of course, we won't. The AC frequency is not very well disciplined on a second-by-second basis. So we're sure to see 61 or 59 every once in a while. It's not outside the realm of possibility, in fact, to get 62 or 58.

But the hypothesis of the experiment is that over a long enough term, every period of 61 cycles should be balanced by a period of 59.