The frequency of line voltage is almost perfectly controlled: it has to be so as to keep all the generators linked up on the grid in sync. Each day, around midnight, there is a brief time when frequency is adjusted so as to get the number of complete cycles in the day correct. This is done because many timers (clocks) rely on the power frequency for accuracy.
Ordinary fluorescent lights try to minimize flicker, and won't work as well as a simple neon bulb, as in the circuit testers mentioned above.