<PICT:1043828/C6-C7/1> A silver-silver chloride measuring electrode device 1, for comparing the potentials of metals in an electrolyte preferably in self-regulating corrosion protection installations for steel ships, comprises a housing 3 of insulating material enclosing a silver plate 4 covered by a coating 7 of CaCl2 which is adjoined by a thicker AgCl coating 8, the rest of the housing being filled with water and an insulating material such as PVC shavings 9 which connects with the outside water surrounding the device through a diaphragm 10 protected against damage e.g. by an insulated perforated plate 11. The device is preferably interchangeably secured on the ship's hull as shown, being situated in a water-tight box 16 to prevent water penetrating into the ship on possible damage to the device. For replacing a device under water, a sealing cap 18 may be provided. In operation, the voltage between cable 14 (positive lead connected to a copper bolt 5 contacting the plate 4) and cable 15 (negative lead to ship's hull) is measured. The diaphragm may consist of silk cord fabric, pig's bladder, wood or ceramic.