While Mr. Penning no doubt appreciates having this gauge named after him, one can get a better idea of how this gauge operates from its original name - the cold cathode ionization gauge. Limiting the discussion to the type we use here, the cathode is the outer cylinder of the gauge tube. It is cold; that is, it is at ambient temperature. The anode consists of a wire loop (tungsten, I believe) mounted in the center of the tube and mounted to an electrical feed-through. A potential of up to about 3KV is applied to the anode, which induces field emission of electrons from the cathode. The electrons ionize gases inside the vacuum chamber and the charged products are collected as a current. This small current is a function of the chamber pressure (yippee) and the species of gas present. There is a magnet mounted on the outside of the gauge tube. Its function is to increase the path length traversed by the electrons thus amplifying their effectiveness. And that's pretty much it for how they work.