Mig welding consumable - Contact tips:
Contact tips are designed to carry an electrical energy that originally came from the major power supply and move it to the welding cable. At this point in the electrical circuit the power is at a contact point. Since it is at the end of the gun or the tip, you put the two words jointly and you get contact tip. The mig wire is also called an electrode because it carries electrical energy.
There are different styles, shapes and sizes of contact tips for mig welding. The main part is that you match the cable size to the right hole size of the contact tip. So for .035" (0.9mm) cable you would use a .035" (0.9mm) contact tip. There will be a marking on the side of the contact tip that will say what size it is. For aluminum cable you require to use a slightly larger tip size to help prevent feeding issues. Often people use large tip holes sizes for flux cored cables as well.
A quick tip: I you are having problem feeding cable, maybe it is a bad batch of mig wire try using the next size up tip. This has frequently worked for people before.
The much power you run the equipment at, as in the higher the amperage the larger the physical size of the contact tip. Also the longer the tip the better the electrical conductivity. This is since there is more surface area for the cable to make an improved contact.