John Williamson presented the following explanation :
Harry Bloomfield wrote:
Remember the old type charge / discharge ammeters, where used a couple of
turns of cable which used to sometime be fitted to vehicles ? I have one
fitted to the tractor mower which is not marked + and - and I'm sure it
reads the wrong way. The needle goes left when on charge, I seem to
remember the norm is to move to the right.

Can anyone remember, before I take steps to correct it?

My Lard Roller ammeter is marked - to the left, + to the right, as have all
the others I've used. Has your tractor mower been converted from +ve to -ve
earth at some point?

No, it is not that old - I suspect the coil windings (2 turns) were
simply fitted the wrong way and it has bothered me since I got it.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk