As the pantograph moves along under the contact wire, the carbon insert on top of the pantograph becomes worn with time. On straight track, the contact wire is zigzagged slightly to the left and right of the centre from each support to the next so that the insert wears evenly, thus preventing any notches. On curves, the "straight" wire between the supports causes the contact point to cross over the surface of the pantograph as the train travels around the curve. The movement of the contact wire across the head of the pantograph is called the "sweep".