After the ion collector, the air sample with ions entered the Faraday cup as shown in Fig.6 [16], where the ion current was measured with Keithley 6517A electrometer. In order to eliminate the leakage currents, the collecting electrode (filter holder) of the
Faraday cup was connected to the electrometer input that had been carefully shielded against externalfields with the low-noise triaxial connection cables, Keithley 237-ALG-2. The Keithley 237-ALG-2 has an insulation resistance of 1015 U, center conductor to shield, and contact resistance of