From a study of previous experimental work on spark discharges, it is evident that an investigation of the impulse breakdown of uniform-field gaps under various conditions of irradiation would provide data for assessing the superiority anticipated for this form of gap, (a) for studying discharge phenomena whose effects would be masked by the dispersion of the results with other forms of electrode, and (b) for impulse-voltage measurement. Further, any information obtained, relating to the formative time-lag of spark breakdown in atmospheric gaps of length >1 cm., at very low over voltages, would extend spark theory. With non-standard impulse waveforms of either polarity applied to a 2 cm. uniform-field gap in atmosphere, breakdown characteristics have been recorded for three conditions of irradiation, viz. (i) irradiation by ultra-violet light, (ii) irradiation from radium source, and (iii) no deliberate irradiation. Transition curves, showing the voltage range (V) for a transition from 0 to 100 percent frequency of sparkover, and curves of "time-lag vs. percentage overvoltage" are drawn for all conditions. Comparative data for a 2 cm. gap between 6.25 cm. diameter spheres confirm the advantages to be expected from the uniform-field gap. The impulse waveform restricts the maximum values of formative time-lag (Tf) that can be observed, owing to the fall fall of voltage on the wave tail. For this reason, formative time-lag data have also been obtained for air in uniform fields with static applied voltages up to 110 kV.