[G4_Pete]

Thanks, I have now looked at the coils in new light. It is only in these old instruments that you can see the first principles in action. It also neatly shows what happens if the current and voltage are out of phase, that is only the phase overlaps will have force to deflect the coil and move the pointer.
That also explains the lower limit of 25 Hz which below I am guessing there will be significant "wobble" on the pointer.
Pete