Very little is known about how X-rays are produced from long laboratory sparks. Up to now, nobody has studied separately the influence of positive and negative electrodes. Thus, the goal of this letter is to demonstrate the influence of the positive and the negative electrode in the production of runaway electrons in laboratory sparks. Results show that, emissions are affected by the distribution of the equipotential lines around the cathode. This mechanism could also be related to negative stepped leaders during their path to the ground and perhaps could also be present in TGF's development.