For maximum power transfer, switch contacts should have zero resistance. Unfortunately, clean contact resistance is directly proportional to volume resistivity of the contact material and inversely proportional to the contact area. This resistance typically is on the order of a few milliohms for pure metal-to-metal contact. But certain phenomena that occur during switching can deteriorate the interface and greatly increase contact resistance. The degree to which these phenomena affect contact resistance depends on contact materials, operating environment, contact pressure, and the type and magnitude of current switched.