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A needle immersed in transformer oil is located just above a sheet of glass the opposite surface of which is conducting. By applying steps of high voltage, streamers are generated which propagate radially with numerous branches. A technique for guiding the discharges gives single quasirectilinear streamers, which allows more accurate investigation. The characteristics of these discharges (mean propagation velocity, final extension, electric current and charge) are determined and the measured charge is compared with the predictions of two simple models. This indicates that the streamer channels are not equipotential. A capacitive probe technique with associated optical fibres allows one to accurately determine the spatio-temporal variation of the potential along the streamer channels.