with complicated frames to support the heavy cast-steel collector plate, while double-deck cars usually have heavier collectors with less complicated frames. To maintain good electrical contact, the bow collector must exert quite strong pressure on the wire above, and so a complex system of springs or weights are put into use to ensure good electrical contact. The wire itself has to be tensioned and kept as flat as possible, a requirement shared for both bow collectors and pantographs. The steel rails on the tracks act as the electrical return. Operation Properly the bow collector should be mounted in such a way so that the top edge of the collector plate would rise several inches above the wire when the collector frame is standing straight up. Thus the collector usually leans opposite to the direction of travel; when the time comes to travel in the opposite direction, the collector must be swung over. To allow this to happen, the overhead wire must be raised by several inches at places where the bows are swung over, such as terminals and turn-outs. This operation is usually achieved by ropes and pulleys. The collector is folded down to a horizontal position when the car is not in use. Some early cars had no means to swing the bows over. It was thought that this would happen automatically when the tramcar started travelling the other way, but collectors such as these were a failure. Most Soviet trams (of which some are still in use in the USSR's successor states) had no means to swing the bows over. These trams were "single-ended", and not designed to travel both ways. Some trams, such as the KTV-55-2 tram, had two bow collectors to handle both directions. Advantages and modern usage The bow collector has fewer moving parts than the trolley pole, but is heavier and sometimes more complicated to construct. The construction of overhead wires for bow collectors is simpler than trolley pole wiring. As bow collectors do not have revolving mountings, the collector cannot jump off the wire or follow the wrong one at intersections, as trolley poles sometimes do. Thus overhead 'frogs' and guides for trolley poles are not necessary with bow collectors. Bow collectors are, however, much noisier than trolley poles. The overhead wires for bow collectors are stretched tighter than for trolley poles, and straight sections are 'staggered', that is, the wire does not