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A car's battery is designed to perform one task very well, and that is to power an electric starter motor whenever the ignition switch is turned on. Once a car starts, an electric generator called an alternator assumes responsibility for most of the car's electrical needs, including recharging the battery for future starts. If the battery's power level becomes too low to turn the starter motor, the driver may have to use a second battery to provide additional power. The cables used to connect the auxiliary battery to the dead battery are called jumper cables, because they allow electrical power to "jump" from one battery to another.
Jumper cables are insulated lengths of heavy gauge wire which have large spring-loaded alligator clips on either end. Two clips are usually marked "positive" with the use of red insulation and plus marks (+), while the other two clips have black insulation and "negative" marks (-). This is a very important designation, since the positive clamps on each end of the jumper cables should only be attached to the positive terminals of the batteries, and the negative clamps should only be attached to the negative post or grounded area of the engine block.
In order to use jumper cables safely, the two batteries need to be in close proximity to each other, which usually means maneuvering the second vehicle end-to-end or side-by-side with the vehicle containing the dead battery. Jumper cables can be 10 feet or more in length, but the clamps need to be seated securely on the battery posts in order to get the best connection. Once the batteries are positioned properly, the driver can begin to attach the jumper cables to both vehicles.