It has always been my understanding that the contact tip is where the energy is transferred to the wire. I demonstrated this to myself once by forgetting to change tips when I changed from a larger to smaller wire size and my welds turned into crap. This seems like a pretty good description of the process:
The science that makes MIG welding possible. The welding circuit, power source, welding torch, earth return & the four types of MIG welding are all…
www.fosterindustrial.co.uk
Excerpt:
The torch liner protects the welding wire during welding and prevents the wire from kinking and misfeeding into the weld pool. Welding wire is ductile and can easily be misshaped by having the welding lead curled up or twisted.
The contact tip charges the wire as the wire passes through the hole in the end of the tip. The contact tip is made from copper because copper is a known good conductor of electricity. The contact tip, or more importantly the hole in the end needs to be the same size as the diameter of the welding wire. These come in sizes of diameter 0.6mm, 0.8mm, 1.0mm, 1.2mm, 1.6mm and 2.4mm.