Thanks blewit. Perhaps you can identify this one:

This is part of the lighting system that is on my HPvelo. Powered by a hub dynamo.
How do I classify the wire that is used? Is it copper? Is it small gauge? How is it ... umm.. measured? ...
If I need to extend the length is it best to solder or can I simply twist and cover with electrical tape? .. the joint is going to be inside the frame so is protected from the elements.