Has anyone here seen the Sky electricity usage meter (yes, Sky the UK satellite TV people)? I saw one tonight round a friends. There is a nice big LCD which tells you how much power you are using and all that, but the interesting bit is in the electricity cupboard, a mystery box which just clamps round the armoured cable going into the meter. It has no other feed, and no visible way in to change a battery so I assume it powers itself by induction and measures the consumption the same way, then sends this wirelessly to the LCD display. I wish I'd had a Jeenode with me to see if I could find the signal.
The curious thing is it's not round just the live, it's round the whole armoured cable so I don't quite understand how the fields of the live and neutral don't cancel each other out!