The Redstone Comparator, originally announced as the capacitor, is a block for use with redstone circuitry. It has two inputs: one from the back ('signal A') and one from the side ('signal B'). If signal B is stronger than signal A, there will be no output. Otherwise, it can be set to either repeat signal A at its current strength, or output the difference between both inputs in subtraction mode. Minecraft 1.5 Tanks to Internalpain666 for this contribution