He tears open the back of the radio and does a bit of rewiring. Most of the rig is already set up the way he needs it; basically he’s looking for something that will convert electrical impulses into sound and pump that sound into the headphones, which is what a radio does. But the source of the signal is no longer a transmitter on a U-boat but rather the current flowing up one of Waterhouse’s wires, into the left razor blade, across the carbon bridge, into the right razor blade, and back down the other wire.