You're correct- mercury delay lines read/write in a set order and you have to wait for your address to come back around in order to access it. Standing wave me

You're correct- mercury delay lines read/write in a set order and you have to wait for your address to come back around in order to access it. Standing wave memory would mostly defeat the point of having memory, since you'd need to address every single antinode individually. You might as well just have a bunch of latches. The real point of delay lines, shift registers, or core memory is to reduce the address space: you store bits in a way that is slower but simpler to access, which means you can store more things. Mercury standing waves would not make it any easier to store bits, so there's no advantage. Still, you could make a device like that. Only 2/3rds of the tube can store memory- the rest is a quarter-wave transformer, which basically makes the tube act as if it was open at both ends. You can construct arbitrary patterns with fourier decompositions: https://en.wikipedia.org/wiki/Periodic_summation https://en.wikipedia.org/wiki/Periodic_summation