The introduction into a modern man-of-war of elaborate electrical plant has brought into prominence the necessity of having some kind of apparatus on board ship for testing the condition and efficiency of the plant from time to time. When it is remembered that a "plant" now-a-days includes circuits for search lights, incandescent lamps, for guns, torpedoes, gongs, bells, etc., the necessity for having some instrument wherewith to test these various circuits, or to localize faults in them, becomes apparent.
The leading of the wires on board a vessel is, of itself, a laborious undertaking, for so many precautions have to be observed. Thus, wires must be out of the way of an enemy's shot and shell as much as possible, they must be accessible, must be protected from chafe, salt water or undue strains, and the various circuits must be conveniently located for those who are to use them. As a rule, wires are led below the water line wherever possible, double circuits being used above the water line, and whatever apparatus is put on board ship should also be below the water line if possible.