Because conditions are constantly changing, there exists a need to measure the strength of the electric field constantly which translates into the need to alternately read the charged state of the sensor plate, discharge it, and read again, over and over. This is accomplished by repeatedly exposing the sensor plate to the external electric field to charge it (see Figure 3.), then shielding the plate to allow it to discharge (see Figure 4.).

Specially-designed charge amplifiers (see Figure 3. Charge Amplifier) within the field mill measure the amount of charge the charge plate receives from the outside electric field and converts this value to an analog voltage. Changes in this observed voltage level correspond to changes which take place in the atmosphere due to the presence of electrified storm clouds. As the observed values increase, so does the likelihood of lightning to be present in the area.