Observations on both electromigration and the Soret effect in tungsten have been recorded using silhouette electron microscopy and electron microscan techniques. The surface structure developed when a tungsten wire is heated by a current density of 3.3×104A/cm2depends on whether the current is direct or alternating. An alternating current produces a smooth surface structure while a direct current produces a steplike structure due to electromigration of tungsten ions. The direction of the electromigration of tungsten ions is toward the cathode in agreement with the theories of Huntington and Fiks.Soret effect diffusion has been observed in tungsten wires when subjected to a temperature gradient of 5000°C/cm. Either electromigration or Soret effect diffusion can produce pronounced changes in the original geometry of the conductor resulting in large current density variations over a short wire segment.