A needle immersed in transformer oil is located just above a sheet of
glass of which the opposite surface is conducting. By applying HV step
s, streamers are generated which propagate radially with numerous bran
ches. The characteristics of these discharges (mean propagation veloci
ty, final extension, electric current and charge) are determined and t
he measured charge is compared with the predictions of two simple mode
ls. A technique for guiding the discharges gives single quasi-rectilin
ear streamers, which allows a more accurate investigation. The mean el
ectric current and the final charge indicate that the streamers channe
ls are, to a first approximation, at the potential of the needle. The
simple picture of a finite potential gradient along the channels is di
scussed with reference to the intermittent character of the discharge
phenomena.