Jim Paul wrote:

> You could add a diode across the batteries with a fuse so that is the
> batteries are inserted the wrong way, the fuse blows cutting off the
> power source.

  But then your appliance has "failed".  Not good publicity.  OK, if it
is used under supervision and can be repaired and put back into service.
Better to use a resistor (only need a few ohms) and diode.

  Reverse-inserted batteries promptly go flat.  "Darn batteries!"
Properly inserted ones last for "ages".  Appliance always works if
good batteries inserted correctly.  This is the "teaching" approach.
--
  Cheers,
        Paul B.