This device uniquely shows ignition spark presence and coil polarity while engine is cranking or running. The wrong coil polarity requires approximately 5000 more volts to jump the spark plug gap as compared to the correct polarity. This thumb sized tool is placed between the ignition coil and distributor. It shows by high intensity flashing lights that: spark is present, spark is not present, polarity is right, polarity is reversed. The Sparklite works with ALL types of spark ignition systems: points, magneto, transistor, high-energy, capacitor discharge, etc. It indicates properly with 6, 12, 24 volt, positive or negative battery systems. Instructions for use are included. This is a genuine NuRex product.