9 BOATING EQUIPMENT, SCIENTIFIC, NAUTICAL AND ELECTRICAL APPARATUS AND INSTRUMENTS, MEASURING, SIGNALLING AND CHECKING APPARATUS AND INSTRUMENTS-NAMELY, CURRENT AND POTENTIAL METERS, POWER SUPPLIES, REFERENCE ELECTRODES, CATHODES, SACRIFICIAL ANODES, THROUGH HULL FITTINGS, CASES AND WIRING FOR NAUTICAL CATHODIC PROTECTIVE DEVICES AND SYSTEMS