The luminous channel of the spark discharge and the discharge current are simultaneously measured. It is concluded that the spark discharge in air attempts to establish a channel in which the current density is a constant value of the order of ${10}^{3}$ amperes/${\mathrm{cm}}^{2}$. During the formation of the channel, which takes place by an expansion process producing a pressure or sound wave, the current density is much greater. Throughout most of the formation time and the subsequent history of the channel, the light energy which is radiated is proportional mainly to the current within the channel. From potential measurements and the photographic records, the energy requirements for development are considered, and some possible relations of these requirements to the electrical circuits are discussed. The data are also discussed in relation to the progression of streamers. The pilot streamer theory of the lightning discharge is indicated to be unsatisfactory by comparison with measured discharge characteristics, and the associated roles assumed for diffusion and recombination processes in the spark discharge channel do not appear valid.