The thundercloud is regarded as a great influence machine, the ionization currents associated with it being the agents by which its electromotive force is developed and maintained. The moving ions which constitute these currents may be intercepted by solid or liquid cloud elements so that it becomes possible for them to be carried against the field and so increase an existing electromotive force. The early stages in its growth are due to the ionization current within the cloud initiated by the earth's fine-weather field. Later it is the external currents due to the thundercloud's own field which are effective. Lightning discharges may themselves contribute to the electromotive force of the thundercloud.