NOTE 4—A suitable apparatus could consist of a continuous-reading electronic voltmeter designed to operate on an input of less than 5 × 10 − 12 A, when an electrode system having 1000-MV resistance is connected across the meter terminals and provided with a metal shield connected to the ground, as well as a satisfactory terminal to connect the shielded connection wire from the glass electrode to the meter without interference from any external electrostatic field.