The mains frequency
The stability of the mains frequency has been a subject of investigation by a few curious time-nutters. It should be obvious by now that the mains is not necessarily to be considered a reliable frequency reference. It all depends on the requirements you may have. To run your alarm clock, the level of accuracy should be quite sufficient. However, for anything more demanding, you would have to look for something else, such as a crystal oscillator (even the cheapest models would outperform the mains). Take a look at the frequency variations below, for instance. The frequency was measured with a Hewlett Packard 5334B with OCXO reference option, and the measurements were acquired with LabVIEW.
This is the frequency variation over 20k points with 10 sec. interval (ie. 200,000 seconds in total), starting at 16:14:30 on Saturday 2011-07-09.
The variations reach about 0.4 % peak. Do not expect old recordings with tape recorders or cutting lathes where the speed was governed by the mains frequency to be at correct pitch.
The variations reach about 0.4 % peak. Do not expect old recordings with tape recorders or cutting lathes where the speed was governed by the mains frequency to be at correct pitch.