A study of the atmospheric surface electrical activity in the Maracay city (10°14'59.1"N 67°37'20.6"W 436masl) is performed, through the analysis of the Carnegie curve. We present the methodological construction of the Field Mill, for the measurement of the electric field atmospheric, and a counter of atmospheric ions, based on the capacitor Gerdien. We shown that the local heating due to convective movement during the morning could change the concentration of ions, and subsequently, produce a second local minimum in the curve of the local electric field, this second minimum is not a feature of the Carnegie curve. We conclude that the curve of local variation of the superficial atmospheric electricity activity, under conditions of clear skies and no clouds, for the tropical region, as in the Maracay city, is functionally similar to Carnegie curve.