It is currently believed that the extension of an electrical “tree” in solid insulation depends on the establishment of a plasma within an existing tubule. Thus, voltage is conveyed from the electrode to regions deep in the solid, which breakdown locally where the stress concentration is such that the intrinsic strength is exceeded. Evidently the electric strength of the gas in the tubules is an important factor, but since they can be as small as 1 μm in diameter, the growth of avalanches is inhibited by the loss of electrons to the dielectric wall and by the field distortion caused by the resulting surface charge. When avalanches are inhibited, the electric strength of the gas is increased, so that the observation of complete penetration of a specimen by a tree channel not leading to breakdown is perhaps understandable. 1 On the other hand, it is reported that the electric strength of gas in sections of tree channels is about 40 kV cm −1 , i.e., only slightly in excess of the uniform field strength of unbounded air. Actual “tree” tubules are tortuous and the diameter varies along the length so that detailed quantitative investigations are difficult. However, it has proved possible to produce cylindrical tubules of air confined in PMMA and in glass of diameter down to 10 μm, which effectively demonstrate the effects of the wall.