ARGO-YBJ measurements on the EAS-footprint stretching as an effect of the geomagnetic field

The ARGO-YBJ experiment proved the effect of the geomagnetic field (GeoMF) on the development of extensive air showers (EAS) and the dependence of the trigger rate on the coupling angle between GeoMF and EAS axis. For showers with the core on the detector the trigger rate decreases with the EAS-GeoMF coupling. When the core is very far and the detector is triggered by the EAS tail the trigger rate is enhanced by the GeoMF effect. As a consequence in both cases the azimuthal distribution is not fiat. Here we discuss in detail the GeoMF effect by selecting showers as a function of their distance from the array center and analysing the stretching of the charged particles due to the Lorentz force in the lateral distribution. The expected correlation is fully confirmed