[This paper is part of the Focused Collection on Curriculum Development: Theory into Design.] Developing a solid understanding of simple electric circuits represents a major challenge to most students in middle school. In particular, students tend to reason exclusively with current and resistance when analyzing electric circuits as they view voltage as a property of the electric current and not an independent physical quantity. As a result, they often struggle to understand the important relationship between voltage and current in electric circuits. Following diSessa's interpretation of learning as the construction and reorganization of previously loosely connected elements of knowledge ("p-prims") into a coherent mental structure ("coordination class"), a new curriculum was developed that systematically builds on students' everyday experiences with air pressure (e.g., with air mattresses and bicycle tires). In order to make voltage rather than current the students' primary concept when analyzing electric circuits, voltage is introduced as an "electric pressure difference" across a resistor that is as much the cause for an electric current as air pressure differences are the cause for air flow. The objective of the curriculum is to provide a structure for students to develop a qualitative understanding of simple dc circuits that allows them to make intuitive inferences about the electric current based on voltage and resistance. With an effect size of d = 0.94 the new curriculum has proven to be more effective than traditional approaches for teaching electric circuits in a quasi-experimental field study with 790 students from Frankfurt am Main, Germany.