Glass electrode
An electrode which, in the presence of a reference electrode, is commonly used to measure the pH of a solution.
It is made up of a glass ball (diameter less than 1 cm and thickness about a hundredth of mm) filled with a buffer solution of known pH (generally pH = 7) into which an internal electrode reference (in general AgCl / Ag in saturated KCl) is placed.
It is is an electrode whose potential is proportional to the pH of the solution into which it is dipped.