The anode is zinc, and is actually referred to as "sacrificial zinc" in this application. It is required on SOME heaters, but not on an Atwood. This application is very common on boats that sit in salt water. There is a form of electrolysis that attacks metal surfaces due to a difference in grounds. The zinc is put in place as a sacrificial element so that the electrolytic process attacks the zinc and not the more important pieces of metal like a propeller, or propeller shaft, or engine components. Boats will have several large pieces of zinc attached to the boat in various places.
The same concept applies to motorhomes and campers that are plugged into "shore" power, and may have a difference in grounds.
Back to the question...Atwood does not require a zinc anode, and further, if someone installs one, like they did in MY Atwood, it tends to cause problems. Mine got fused to the internal threads of the heater, and was all but impossible to remove.
Jim
