Abstract: Apparatus for use with jumper cables, such as in jump starting an engine, which permits attachment of the cables to the terminals of a supply and a battery without regard to the polarities of the terminals, and thereafter aids the user in matching the polarities of the battery and supply. Two terminal blocks, secured to the jumber cables, have at least two spaced contacts apiece and are movable relative to one another between two alternative contact mating positions. The mating positions correspond to the two possible ways in which the battery and supply terminals can be interconnected, either positive-to-positive, negative-to-negative, or positive-to-negative, negative-to-positive. The blocks are biased to a third position in which the battery and supply are not interconnected and in which control circuitry determines which of the two alternative contact mating positions would interconnect the supply and battery with their polarities matched.