In particular with the advent of HVDC(high voltage direct current). Using mercury(metal), the resistive losses can be greatly reduced. There has even been some experimental research into neon plasma based electrical contacts. They have been used to supply the large currents required by particle accelerators. What is interesting is that AFAIK it is the only type of electric generator that can produce pure, ripple-free DC current(and very large currents as well). One could build a high current output rotary converter using an AC motor coupled to a homopolar generator.