Cell constant = $\frac{l}{A}$ ($cm^{-1}$), where l = distance between electrodes and A = area. It is determined by measuring conductance of a solution of known conductivity (usually KCl). Cell constant = $kappa_{known}$ / $G_{measured}$. Once known, kappa of any solution = G * cell constant. Conductance is measured using a Wheatstone bridge with AC current (DC causes electrolysis and polarisation). The bridge is balanced to find R, then G = 1/R. Platinised platinum electrodes are used to increase surface area and reduce polarisation effects.