This paper reports two studies into the understanding of the concept of potential difference in the current-potential difference-resistance context among 8th-12th graders (Study 1), and the efficiency of a learning device derived from Social Judgment Theory (Study 2). These two studies showed that: (a) when asked to infer potential difference from resistance and current information, most participants only relied on current and ignored or greatly underestimated the importance of resistance information; (b) exposure to an electricity course did not much alter the way participants infer potential difference from resistance and current information; (c) a simple learning device considerably improved the way participants infer potential difference from resistance and current information; and (d) this observed improvement promised to be durable, at least in the short term. (Contains 7 figures and 1 table.)