Current sources and sinks  In electromagnetism, current sources and sinks are analysis formalisms which distinguish points, areas, or volumes through which electric current enters or exits a system. While current sources or sinks are abstract elements used for analysis, generally they have physical counterparts in real-world applications; e.g. the anode or cathode in a battery. In all cases, each of the opposing terms (source or sink) may refer to the same object, depending on the perspective of the observer and the sign convention being used; there is no intrinsic difference between a source and a sink. In some cases, the term current source refers to a boundary where charge flows from locations where it is not measured to locations where it is measured. In a similar fashion, a current sink may refer to the boundary where charge flows from locations where it is measured to locations where it is not measured. By analogy to the flow of water, a current source would be like a mountain spring - water flows from its source (a hidden location underground) to the surface where it is easily observed. Using the same analogy, a current sink would be like water flowing down a drain - water travels from where it is observed to where it is not observed. Shown at right is a general two-compartment model to help illustrate the definition of current sources or sinks. In this two-compartment model, the compartments are separated by a semi-conductive barrier (gray). An observer, symbolized by the eye, can "see" only one compartment at a time. Red arrows indicate the direction of flow of positive charges, while black arrows indicate the direction of flow of negative charges. The pink and green backgrounds are meant to symbolize different configurations, configuration 1 when charges are flowing in one direction and configuration 2 when they are flowing in the opposite direction. The difference between the left and right panels is simply the location of the "eye". A source or a sink is defined by which compartment is viewable by the observer. In biology, the schematic barrier in the figure could represent a cell membrane, and as a result, the two compartments could represent the inside and the outside of the cell. Generally speaking the point of observation would be outside the cell. Thus the cell would be termed a sink with respect to any flow of positive charges into it, and the cell would act as a source for any positive charges flowing out of it. Note that when considering the flow of negative charges, the definitions are reversed. Current sources and sinks have proven to be very valuable in the study of brain function.