"If the air pressure in the organ pipe is high, it pushes the mercury down a little bit. If it’s low, it sucks the mercury up. I put an electrical contact into each U-tube—just a couple of wires separated by an air gap. If those wires are high and dry (like because high air pressure in the organ pipe is shoving the mercury down away from them), no current flows. But if they are immersed in the mercury (because low air pressure in the organ pipe is sucking the mercury up to cover them), then current flows between them, because mercury conducts electricity! So the U-tubes produce a set of binary digits that is like a picture of the standing wave—a graph of the harmonics that make up the musical note that is being played on the speaker. We feed that vector back to the oscillator circuit that is driving the speaker, so that the vector of bits keeps refreshing itself forever, unless the machine decides to write a new pattern of bits into it."