Abstract. Positive and negative streamers are studied in ambient air at 1 bar; they emerge from a needle electrode placed 40 mm above a planar electrode. The amplitudes of the applied voltage pulses range from 5 to 96 kV; most pulses have rise times of 30 ns or shorter. Diameters, velocities and energies of the streamers are measured. Two regimes are identified; a low voltage regime where only positive streamers appear and a high voltage regime where both positive and negative streamers exist. Below 5 kV, no streamers emerge. In the range from 5 to 40 kV, positive streamers form, while the negative discharges only form a glowing cloud at the electrode tip, but no streamers.

The diameter and branching structure of positive streamers in ambient air are investigated with a fast iCCD camera. We use different pulsed power circuits and find that they generate different spatial streamer structures. The electrodes have a point-plane geometry and a distance of 40 or 80 mm, and the peak voltages over the discharge gap are up to 60 kV. Depending on circuit and peak voltage, we observe streamers with diameters varying gradually between 0.2 and 2.5 mm. The streamer velocity increases with the diameter, ranging from 0.07 to 1.5 mm ns −1 , while the current density within the streamers stays almost constant. The thicker streamers extend much further before they branch than the thinner ones. The pulsed power supplies are a switched capacitor supply with an internal resistance of 1 k and a transmission line transformer supply with an impedance of 200 ; additional resistors change the impedance as well as the voltage rise time in the case of the capacitor supply. We observe that short rise times and low impedance create thick streamers close to the pointed electrode, while a longer rise time as well as a higher impedance create thinner streamers at the same peak voltage over the discharge.

Precise measurements of the electrical resistivity of pure copper samples have disclosed two discontinuities in the temperature dependence of the resistivity. They occur at approximately 6.7 K and near 12.1 K. The second is associated with impurity effects whereas the first appears to be a characteristic of the metal itself. It is similar to the discontinuity previously observed in the temperature dependence of the electrical resistivity of Ag by Kos and independently by Ehrlich and Schriempf and attributed by the former to one-step umklapp processes.

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Chem. Commun., 14, 471 (1973).Preliminary measurements at room temperature in the energy range of 0.6 to 0.06 eV indicate a definite Drude flattening in reflectance for E \\b. The reflec-