Potassium bubbles in tungsten wire

Tungsten filaments for incandescent lamps almost always contain potassium bubbles. These are required to produce the interlocking grain structure that gives the tungsten lamp filament a long life. In this article, we consider the formation of the bubbles from the ellipsoids of potassium produced by deformation, the growth of these bubbles as a result of internal pressure created by the potassium vapor within the bubbles, and the reasons why these bubbles can sometimes grow to very large sizes. In each case, we present a simple model to describe the process. We finally use nucleation theory to describe the overall process of bubble growth. Formerly Visiting Scientist, Institute for Technical Physics, Hungarian Academy of Science, Budapest, Hungary