Fan Railer wrote:
seems like you are clearly more well versed than I.
michaelk wrote:seems to me that it would help conduct- but i have to say it is beyond my expertise. (My guess would be it wouldn't help in the transfer from the steel rail to the shoe but it would get more current along the lendth of the steel rail).
Current speed is regulated by the drift velocity component of the conductor that they travel in. Since current is constant throughout the circuit, that means that it follows the lowest common drift velocity as regulated by the metal with the lowest drift velocity. If anything, the larger aluminum plate at the junction provides a bridge in the gap between the two separate pieces of third rail and ensures continuous current, but once again, it does NOT increase overall conductivity; only the range and standardization of conductivity along the line.
just a WAG- maybe the aluminum helps conduct higher current further distances and that segment is farther than normal between substations- but they can't just use aluminum alone as it wouldn't stand up to the physical wear of the shoe scraping across the top all the time so they still use steel rail for the wear surface?
One thing it makes me wonder though- wouldn't their be some crazy reaction between the 2 different metals with all that current running between them? One of the metals (I'd assume the aluminum) would erode over time.
Also- doesn't that reaction between the different metals cause a mess that inhibits current transfer if not specifically handled (hence aluminum and cooper wiring doesn't mix well to the point it can cause fires so you have to use that paste goo on aluminum wiring)?
so what is your hypothesis as to why they took the time to add the aluminum 'strapping' along the 3rd rail? Are there any reasons at all you can think of to add it?
I'm not positive about the smaller strip, to be honest.
But wiki seems to be:
One method for reducing current losses (and thus increase the spacing of feeder/sub stations, a major cost in third rail electrification) is to use a composite conductor rail of a hybrid aluminium/steel design. The aluminium is a better conductor of electricity, and a running face of stainless steel gives better wear.....A second method is an aluminium core, upon which two stainless steel sections are fitted as a cap and linear welded along the centre line of the rail.
PA's third rail design seems to be of the second method. So I guess your hypothesis in the beginning was correct, that it does increase range between substations while maintaining wear and tear rates. But then again, that does make sense... so conductivity, range wise, does increase, but the speed component should not.