Whoever invented this method must be a genius. This was a portable calibrator for high-voltage partial discharge testing. Because of HV, it was battery powered and insulated from the mains. But sometimes the test signal needs to be phase-locked to the 50/60 Hz AC mains frequency, and the solution was:
> "with a photocell that synchronizes the instrument with the ambient lighting of the laboratory."
#electronics
![Figure 6.11 Portable calibrator
[shows a simple 100 mm x 100 mm test instruments with some pushbuttons, an on/off switch, and a photodiode at the top]
5. It can be synchronized with the test voltage; this is usually performed with a photocell that synchronizes the instrument with the ambient lighting of the laboratory.](https://misskey-taube.s3.eu-central-1.wasabisys.com/files/5e2ba358-bf27-41f1-8b95-8f69aeac5e44.png)