My turn to chip in: I once made an operation sensor for a high power 
heater (~15kW). I was not allowed to connect to the circuit in any way. 
Including not open boxes to get at the cables etc. So I used a 
differential thermistor sensor with an opamp. One thermistor was on the 
board and the other was pvc-taped to the high gauge conductor that fed 
the heater control unit (3 phase). The heater cable would run hot. 
Voila. Inertia was half a minute give or take a little, the cable was 
2/3 inches external diameter PVC 5 wires (3ph + gnd). It would heat up 
2-3 degrees above ambient with the heater on. That gives more than 2% R 
change with any thermistor and is easy to detect.

Another thing I tried was to use a cassette tape playback head and 
preamplifier. The head would be brought in contact with the cable. The 
output ac would be huge when the load was on. The tape head picks up the 
stray field near the cable. The direction and orientation of the head 
were important but not that important. The output ratio with/without 
load was 1:100 or larger.

Peter
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