Two long magnets 19 and 20 (Fig. 1) are hinged to each side of the ship, projecting below the waterline. At the outer end of each magnet is a watertight glass cap, one carrying contact 27, the other contact 28. These contacts are wired to an alarm bell, as Fig. 4 shows.

Fig. 4 also shows "a dynamo" 16 in circuit (at 17 and 18) to energize the wire coils of the magnets.

A submarine sneaks along one side of the ship and is drawn up to the magnets. On contact with the submarine hull, glass caps 29 smash, contacts 27 and 28 close the alarm circuit through the hull, electrocuting or stupefying the sub's crew with the current.