= Guard electrode of a capacitor
A <guard electrode of a capacitor> surrounds an electrically isolated sensing <electrode> and is held at the same <electric potential>. The small insulating slot separates their readout paths. The guard extends the nearly planar <equipotential>, moving the main outer edge away from the sensing region; it does not abolish the <fringing field of a capacitor> everywhere. In a capacitance fixture, fringe <electric current> is routed through the guard instead of the sensing readout. Keysight describes this three-<electrode> method in <Materials Measurement: Dielectric Materials> (Solution, 16451B fixture).<Materials Measurement: Dielectric Materials>[Solution: guard absorbs current through fringe capacitance]{ref}
Measuring the sensing and guard <electric currents> together would include the diverted response again. Grounding an arbitrary surrounding <electrode> also does not reproduce a guard unless its <electric potential> matches the sensing <electrode>. Finite guard width, the insulating slot, plate misalignment and specimen air gaps limit the approximation. Use <capacitance measurement calibration> and the geometry appropriate to the sensing region when inferring <permittivity>. The source also warns that guarding changes the <electromagnetic field> unfavourably at high <frequency>, so the low-<frequency> electrostatic picture has a restricted range.<Materials Measurement: Dielectric Materials>[Solution: air gaps, high-frequency guard limitations, fixture compensation]{ref}
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