An alternative solution is to use studs. All the systems have a switch in the stud and a means to switch on the stud only while it is covered by the moving vehicle. As at least one stud must be covered by the collector at all times a long collector is used. The length has to be slightly greater than the maximum distance between any two studs. This collector is known as a skate or ski collector. This type of electrical power collector needs to move in the vertical plane to allow for natural differences in the height of the power supply studs. It is used on some full size tramway systems where there is a need for overhead wires not to be used, usually in areas of scenic value.