Positive or negative earth.

[Bsa guy1]

Hi is a c15 + or - earth.
Thanks

[SIE7RA]

They are positive + earth

[chaz]

unless a previous owner has changed to negative- Earth
same applies with 6 or 12 volt systems, updated modern restorations may have changed due to accessability of parts

[AdrianS]

Before you connect a battery, make sure there is an in-line fuse present. If you connect the battery the wrong way the fuse will blow! DAMHIK