This paper reports an investigation of the performance of students at age 15 on written diagnostic questions involving the prediction of voltmeter readings in circuits consisting of two resistors in series.  The results show that these students have particular difficulty with qualitative questions.  Few perceive the two-resistor series circuit as a voltage divider and many display local, as opposed to global, reasoning in predicting circuit behaviour.  These are identified as instructional goals which need to be more strongly and explicitly emphasized.