A current collector, or "pantograph," is illustrated in the diagram. Note the spring that extends the collector, and the link at the bottom that causes the two sides to move together. The operating mechanism, that retracts the collector pneumatically, is not shown. Of course, there are many variations, but this is typical. There are two such mechanisms, one on each side, with transverse bracing. The structure is kept light, so that if it is swept away it does not do excessive damage and breaks away cleanly. It must exert a constant pressure about independent of the height of the wire, and must adapt to changes in height quickly without bouncing.