This Core Resource focuses on the flow of charge in series and parallel circuits. Two indirect detectors are used in forming a model for what is happening in the wire: the relative brightness of identical bulbs, and the deflection of a magnetic compass placed under the wires of the circuit. From a series of lab observations, students are able to determine that current (flow of charge) is equal everywhere in a series circuit, (electrical devices do not use up electric charge, charge is conserved) and in parallel circuits, the total flow of charge into a junction equals the total flow of charge out of the junction. Qualitatively, students also see that the flow of charge is inversely related to the resistance of the circuit provided by the bulbs and that flow of charge increases as the resistance decreases by adding additional parallel branches.