The Vernier Conductivity Probe measures the ability of a solution to conduct an electric current between two electrodes. In solution, the current flows by ion transport. Therefore, an increasing concentration of ions in the solution will result in higher conductivity values.
The Conductivity Probe is actually measuring conductance, defined as the reciprocal of resistance. When resistance is measured in ohms, conductance is measured using the SI unit, siemens (formerly known as a mho). Since the siemens is a very large unit, aqueous samples are commonly measured in microsiemens, or µS.
Even though the Conductivity Probe is measuring conductance, we are often interested in finding conductivity of a solution. Conductivity, C, is found using the following formula:
C = G•kc
where G is the conductance, and kc is the cell constant. The cell constant is determined for a probe using the following formula:
kc = d /A
where d is the distance between the two electrodes, and A is the area of the electrode surface.