Transponders

[whites75]

Sorry the class clown here again (ahem)....until a few days ago I wondered how the hell do you tell the finishing order in races, then, if I am thinking right I find that cars have a transponder in them to relay the lap count and timings tp the computer controller/

Does everyone generally buy their own, does one transponder work on all tracks or do you need a few different types to work on various tracks?

What sort of price are they please