An ignition spark tester is an instrument that measures the ignition voltage without the need to remove the spark plug. Ignition spark testers come in a variety of forms to suit different types of ignition system, including conventional and electronic ignition systems, and coil-on plugs (which do not have plug wires, so that inductive test equipment cannot be used). On a typical ignition spark tester the calibration screw is adjusted to the proper gap size, the spark wire is put on the end of tester, the tester is grounded with clamp on engine, and the engine is started to check the spark.