hugh flung_dung,
How do you get DC power to the bulb inside the compass, vary the brightness and switch it on an off while having no significant effect of the compass reading?
If I remember correctly the right hand rule applied to DC. However, if one has the positive and negative cables side by side the magnetic effect is according to the UK CAA "usually insignificant"
The compass deviation card should say if the swing was done with the radio equipment on or off.
If the engineer checked that the radios, pitot heat etc had no significant effect on the compass as required by the airworthiness procedures and then did the compass swing with everything off as allowed (recorded on the dev.card). Would you say then that all flights should be completed with the radios off?
Why do the radios get special treatment? - Simple - many sources of alternating voltages and currents as various frequencies. Some of which - DME, Transponder, WX Radar can be high powered.
But please do not turn on the wx radar during the compass swing while the engineer is out front unless you want to endanger them.
Regards,
DFC