@dc42 You understand correctly, yes my bed heater is spiral and DC-12V
The thermostat switching causes a change in current, on/off, which is dB/dt. This then induces a voltage in the strain conductor. That spiral heater has also been modelled, and analysed for the stimulus in my simulations. A labyrinth heater conductor would have less magnetic flux, but still potentially enough to trip the strain threshold. Gladly, amend this work to a main-line bed heater, etc.
I will email the document from my other workstation, when I log-in next day.