The Null Cell is a part of the Redpower mod and allows two Redstone signals to cross a single block without interfering with each other. It is very handy when you are crossing two Redstone wires in a compact circuit, machine ect.
A redstone signal carry through the device if one is using redstone dust on one side and red alloy wire on the opposite side (as of Tekkit 3.1.2).
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