The amount of the deflection will depend on the depth of water and the strength of the magnetic force generated by the magnetic ores. However, the magnetic force will seldom be strong enough to deflect the compass needle in depths greater than about 1500 m. Similarly, a ship would have to be within 8 cables of a nearby land mass containing magnetic ores for a deflection of the needle to occur. Deflections may also be due to wrecks lying on the bottom in moderate depths, but investigations have proved that, while deflections of unpredictable amount may be expected when very close to such wrecks, it is unlikely that deflections in excess of 7 will be experienced, nor should the disturbance be felt beyond a distance of 250 m. Greater deflections may be experienced when in close quarters with a ship carrying a large cargo such as iron ore, which readily reacts to induced magnetism.
Power cables carrying direct current can cause deflection of the compass needle.
The amount of the deflection depends on the magnitude of the electric current and the angle the cable makes with the magnetic meridian. Small vessels with an auto-pilot dependent upon a magnetic sensor may experience steering difficulties if crossing such a cable. The Effect of Magnetic and Ionospheric Storms on the Compass Needle Disturbances on the sun may cause disturbances of the magnetic compass needle and interference with radio communications. At the time of an intense solar flare or eruption, a flash of ultra-violet light and a stream of charged particles are emitted from the sun. Short radio waves which travel round the Earth by being reflected from a higher layer of the upper atmosphere cannot penetrate this barrier of ionisation and a radio 'fade-out' is experienced.