Why do many modern ships still carry a magnetic compass?
For most of history, sailors who lost sight of land had little more than the sun and stars to guide them. On a cloudy night in open sea, a captain might not know which way his ship was heading. Then came the compass.
The first compasses were made in China, where people noticed that a piece of lodestone, a naturally magnetic rock, would swing to point north-south if it were allowed to move freely. Early versions were spoons of lodestone turning on a polished plate. Over centuries the idea spread along trade routes, and by the twelfth century sailors in Europe were floating magnetised needles on water in a small bowl.
A compass works because the Earth itself behaves like an enormous magnet. The needle lines up with the Earth's magnetic field, so one end points towards the magnetic north pole. This is close to, but not exactly at, the geographic North Pole, and the difference changes slowly over time. Navigators call the gap between the two 'variation' and must allow for it when they plot a course.
Today, ships are guided by satellites, but many still carry a magnetic compass as a back-up. It needs no battery, no signal and no screen, and in a power failure it will go on working.
The first compasses were made in China, where people noticed that a piece of lodestone, a naturally magnetic rock, would swing to point north-south if it were allowed to move freely. Early versions were spoons of lodestone turning on a polished plate. Over centuries the idea spread along trade routes, and by the twelfth century sailors in Europe were floating magnetised needles on water in a small bowl.
A compass works because the Earth itself behaves like an enormous magnet. The needle lines up with the Earth's magnetic field, so one end points towards the magnetic north pole. This is close to, but not exactly at, the geographic North Pole, and the difference changes slowly over time. Navigators call the gap between the two 'variation' and must allow for it when they plot a course.
Today, ships are guided by satellites, but many still carry a magnetic compass as a back-up. It needs no battery, no signal and no screen, and in a power failure it will go on working.
Answer and explanation
Correct answer: D. It needs no power or signal, so it is a reliable back-up.
- Step 1: The final paragraph says it needs no battery, signal or screen. Step 2. It goes on working if the power fails. Answer: It needs no power or signal, so it is a reliable back-up. Why not the others: It is more accurate than satellites.: The passage does not say this. It is required by law in every country.: Law is not mentioned. It is cheaper than a satellite receiver and works better.: Cost is not discussed.
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