Note: A typical construction is a tube of an inert material (e.g. agar agar) filled with a solution containing an electrolyte with approximately equal ion mobilities of the cation and the anion (e.g., $\mathrm{KNO}{\phantom{A}}_3$, $\mathrm{KCl}$), with the ends of the tube immersed in the electrolyte solution of the half cells.