def _em_conversion(orig_units, conv_data, to_units=None, unit_system=None):
    """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.
    """
    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
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.