I've been wondering for a while what the ~50kHz signal is often appears on the oscilloscope. There is an antenna-like electrode involved, so I know how I'm receiving it, the question is where does it come from? I suspected the CRT monitors nearby so they're already powered off. This is a long-standing mystery to me, I've encountered it in different experiments over the years - often it involves a long unshielded oscilloscope probe acting as an antenna. Your thoughts?