The flow of charge carriers, such as electrons, protons, ions or holes (places with a lack of negative charge that can be considered as carriers of positive charge). Most often, the flow of electrons in a solid conductor is considered. However, for historical reasons, the direction of current flow in a circuit is considered to be the direction of positive charge carriers, or the opposite direction to the electron flow. This is called conventional current. In the case of direct current, the charge carriers move in one direction only; in the case of alternating current, the direction of their movement is periodically reversed.