These may be local or worldwide.
LOCAL: such current can be natural or caused by human activities. The
Latter(stray,industrial,or cultural current) may be caused by electric trains,rural water pumps,and pipelines. Natural local currents represent the phenomena of spontaneous potentials or self-potentials. Some deposits in the earth, such as certain metallic sulfides and graphite, constitute buried natural electric cells because of their high electrical conductivity and also because of oxidation and reduction processes associated with groundwater. Thus,hidden ore body,such as a copper ore deposit, can be discovered by measuring the electric field at the surface of the earth, which may be as large as I  V over a distance of 300 ft(100m). Two other sources of spontaneous potentials are ground-photographs of lightning flashes obtained with a transmission diffraction granting in front of the camera lens.Most of the light passes straight through the grating and is imaged by the lens on the film. The remaining light is deviated by the grating and split into characteristic spectral colors,which compose the "signature" of the atoms and ions withing the hot lighting channel.