Precursor-Effects in T-tubes were investigated by means of 3 cm-microwaves and photomultipliers **. The gas-filling of the tubes was predominantly air. In general, it is possible to differentiate between six precursor-phenomena which can be subdivided into groups of two. Each group consists of an “optical” precursor observed as a luminosity of the gas and of an “ionizing” precursor observed by the appearance of free electrons in the gas. The classification of these phenomena is as follows:Both precursors are probably caused by the uv- and vacuum-uv-radiation of electron avalanches or streamers during the breakdown phase. The experiments indicate that part of this radiation propagates by diffusion (velocity of diffusion about 108 cm/s).These precursors are caused by uv- and vacuum-uv-radiation of the discharge plasma the formation of which follows the ringing of the discharge current. Their behaviour differs from that of the 1st and 2nd precursor.5th precursor: (ionizing) detected by microwaves6th precursor: (optical) according to literature.These last two precursors are caused by the radiation of the plasma propagating in the tube. In order to clarify the discussion it is appropriate to distinguish these precursors from the third and fourth one.Observed electron densities are between 0.5 x 1012 and several 1012 electrons per cm3, effective collision frequencies are accordingly between 35 and 10 Gc/s.
