Pictures show that streamer or sprite discharge channels emerging from the same electrode sometimes seem to reconnect or merge though their heads carry electric charge of the same polarity; one might therefore suspect that reconnections are an artefact of the two-dimensional projection in the pictures. Here we use stereo photography to investigate the full three-dimensional structure of such events. We analyse reconnection, possibly an electrostatic effect in which a late thin streamer reconnects to an earlier thick streamer channel, and merging, a suggested photoionization effect in which two simultaneously propagating streamer heads merge into one new streamer. We use four different anode geometries (one tip, two tips, two asymmetric protrusions in a plate and a wire), placed 40 mm above a flat cathode plate in ambient air. A positive high voltage pulse is applied to the anode, creating a positive corona discharge. This discharge is studied with a fast ICCD camera, in many cases combined with optics to enable stereoscopic imaging. We find that reconnections as defined above occur frequently. Merging on the other hand was only observed at a pressure of 25 mbar and a tip separation of 2 mm, i.e. for a reduced tip distance of p · d = 50 µm bar. In this case the full width at half maximum of the streamer channel is more than 10 times as large as the tip separation. At higher pressures or with a wire anode, merging was not observed.
The purpose of this study was to evaluate a technique of pushing a wheelchair at the level of the handle bars as a method for measuring rolling resistance of wheelchair-user systems under different field conditions. Under standardized conditions on a motor driven treadmill, rolling resistance was determined using a 2D strain gauge-based push technique at the level of the handle bars and a commonly used 1D strain gauge-based wheelchair drag test using an adapted push wheelchair and ISO dummy at several velocities and using different push handle heights. After verification of the method, rolling resistance of six different floor surfaces was measured with the experimental push wheelchair in a centre for rehabilitation. Using an analysis of variance for repeated measures, small but significant differences in rolling resistance were found between the drag and push tests on a motor driven treadmill. Belt velocity and push handle height significantly affected rolling resistance. In the field study in the rehabilitation centre, tiles and tarpaulin had the lowest rolling resistance, while high piled carpet had the highest values. It is concluded that the wheelchair pushing method described in this study is usable for the determination of (relative) differences in rolling resistance of different floor materials if performed under standardized conditions and procedures, such as a stable velocity (within a small range of variation), using an ISO-dummy and a constant pushing handle bar height.
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