NavList:

There are certainly funny things going on around 132kv overhead power cables.  I know this because every time we walk under one my wife’s hair stands on end (or perhaps it’s jumping up and down at 50-60 cycles, but that’s outside the limits of the human eye to notice).  Funnily enough, my hair’s unaffected.  Perhaps it’s given up the ghost after all those years under a cap or flying helmet.  If you want to amaze your grandchildren for a change, rather than them continuing to amaze you, take a four foot neon light tube with you and point it towards the cables.  You’ll be acknowledged as a true space magician.

I don’t think you can come up with an answer to the difference in effects on compasses without knowing the particular magnetic field transducer in Paul’s electronic compass.  Such transducers must have come on a lot over the last 30 years from electromagnetic flux valves to tiny chips that are designed to work in three axes.  I can vaguely remember flux valves involve picking up a near saturated ac magnetic field which has been modified by superimposing the Earth’s field.  Under HT cables is probably not the best place to attempt to use such a transducer. 

The latest tiny three axis chips look like magic, and I was prepared to leave understanding them to younger generations.  This discussion’s tempted me to take another look.  More soon maybe!
DaveP
One time Navigational Guidance Lecturer, RAF School of Navigation