Lightning current waves are resulted by electric discharge from the clouds to the ground These waves are stated as a wave-front time and wave-tail time. The comparation between the wave-tail time and the wave front time defined as a constant "k". A mathematical approach to determine the impulse wave which base on the constant "k" has been published however these were limited by the big constant "h" or the wave which has a long tail. This paper will explain how to determine the constant "lc" of impulse wave which has a short and long tail. The constant "k" resulted from the experiment will be applied to determine the impulse wave. The comparation from the experiment and mathematical approach resulting very satisfactory outcome.
