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electrical charge distributed through a three-dimensional region. In an electron tube, for example, a negative charge results because electrons that are emitted from the cathode do not travel instantaneously to the plate (anode) but require a finite time for the trip. These electrons form a cloud around the cathode, the cloud being continually depleted by electrons going to the plate and replenished by electrons emitted from the cathode. It is this cloud of electrons that produces the negative space charge.