The CS110 Electric Field Sensor, and field mills in general, are referred to as induction probes because the applied electric field induces charge onto sense electrodes. The amount of charge induced by a given field depends on the voltage at which the sense electrode is at.
Normally, it is most convenient to ground the instrument, making the sense electrode at earth ground potential when measuring the vertical component of atmospheric electric field at the surface of the earth. This is because the instrument then appears to be an extension of the earth ground. For example, in a flush-mounted upward-facing configuration with the instrument earth grounded, the imaginary electric field lines will terminate on the instrument as if it were an extension of the earth ground. Negligible field distortion occurs at the instrument aperture, as if the instrument were not present. This configuration mimics the parallel plate factory calibration done on the instrument and is why a flush-mounted upward-facing configuration is used for site correction of inverted and elevated configurations.
The instrument can be connected to other voltage potentials besides ground, or left electrically floating, although the measured results will differ because of the change in voltage between the instrument and the source of charge generating the electric field of interest. The closer the instrument voltage potential is to the voltage of a source of charge generating an electric field to be measured, the less there is an induced charge on the sense electrode. For example, if an induction probe was placed next to, and facing, a large conductive sheet that was at some voltage with respect to earth ground, and a voltage of the instrument was varied by means of a connection to the CS110 ground lug, it would be possible to adjust the instrument voltage until zero charge was induced on the sense electrode from the nearby sheet. For this to occur, the voltage applied to the induction probe would need to equal the voltage applied to the large conductive sheet mentioned. This approach can be used to determine the voltage of the sheet and is referred to as a non-contacting voltmeter. The CS110, or any induction probe for that matter, can operate as a non-contacting voltmeter with the addition of the adjustable supply.