Who knew?! A legitimate application of mains hum!

[JohnD]

I meant that the synchonous motors of Nixon's tape recorders ran at a speed proportional to the AC frequency. It is almost, but not quite, a consistent speed. Therefore if the frequency slows, the recording speed also slows, and the Hz per inch of tape remains about the same. On playback, the variation will be masked.

Modern digital recorders don't work like that.

 

[YorkshireMidge]

Interesting video.

I remember making a simple touch switch many years back. The trigger input on the ubiquitous 555 timer chip is so sensitive, the mains hum picked up by your body when you're in a building where there's mains, is enough to trigger it (which could then drive a relay to give a clean output). i.e. you didn't need to press the contact plate - it was so sensitive, the tiniest almost imperceptible contact with the skin on your finger was enough. That was the only use I ever found for mains hum until now!