code: def _em_conversion(orig_units, conv_data, to_units=None, unit_system=None):
      
      conv_unit, canonical_unit, scale = conv_data
      if conv_unit is None:
          conv_unit = canonical_unit
      new_expr = scale * canonical_unit.expr
      if unit_system is not None:
          # we don't know the to_units, so we get it directly from the
          # conv_data
          to_units = Unit(conv_unit.expr, registry=orig_units.registry)
      new_units = Unit(new_expr, registry=orig_units.registry)
      conv = new_units.get_conversion_factor(to_units)
      return to_units, conv
docs: Convert between E&M & MKS base units.
  
      If orig_units is a CGS (or MKS) E&M unit, conv_data contains the
      corresponding MKS (or CGS) unit and scale factor converting between them.
      This must be done by replacing the expression of the original unit
      with the new one in the unit expression and multiplying by the scale
      factor.