|
When people
think of instruments that help with direction finding, the first
one that springs to mind is probably the magnetic compass. It
is the oldest instrument for navigation and has been a vital
tool for navigators at sea for centuries. The compass allows
ships to steer a selected course. By taking bearings of visible
objects with a compass, the navigator is also able to fix a
ship's position on a chart.
Where
was the compass first used?
The origin
of the compass is shrouded in mystery. Certainly the Greeks
knew about the attractive properties of magnetism in ancient
times. Similarly, the Chinese were probably aware that an iron
bar stroked with a lodestone acquired a directional north-south
property as long as 2000 years ago. However, the precise date
at which this knowledge was used to create the first magnetic
compass is unknown. By the 10th century, the idea had been brought
to Europe, probably from China, by Arab traders. Magnetic compasses
of a very simple kind were certainly in use in the Mediterranean
as early as the 12th century. However, early compasses were
not very reliable. Although the magnetic compass was in general
use in the Middle Ages, little was known about precisely how
it worked.
How
does a magnetic compass work?
A magnetic
compass works because the Earth is like a giant magnet, surrounded
by a huge magnetic field. The Earth has two magnetic poles which
lie near the North and South poles. The magnetic field of the
Earth causes a magnetized 'needle' of iron or steel to swing
into a north-south position if it is hung from a thread, or
if it is stuck through a straw or piece of wood floating in
a bowl of water.
How
were needles magnetized?
Needles were
magnetized by stroking them with a lodestone, a lump of magnetic
rock called magnetite. The needle did not keep its magnetism
permanently, so a lodestone was carried on the ship so that
the needle could be stroked whenever the magnetism wore off.
How
accurate is the magnetic compass?
As long ago
as the 15th century, mariners noticed that the needle of a magnetic
compass does not point accurately to Earth's true north. Columbus,
for instance was aware of this on his voyages across the Atlantic
in the 1490s. Instead, the needle makes an angle with true north,
and that angle varies from place to place on the Earth's surface.
This means that there is a different magnetic variation for
different places on Earth. These variations were investigated
on a famous 17th century voyage by the great scientist and astronomer
Edmund Halley. It was thought at this time that the longitude
of a ship could be found by the compass variation, but this
proved to be untrue.
How
were the simplest compasses improved?
A great improvement
came when the needle was mounted under a card on a sharp pin,
and placed in a little turned wooden or ivory box.
How
were these cards marked out?
At first,
compass cards were marked out not in degrees, but in points.
There were 32 points, matching the directions of winds which
sailors would be familiar with at sea. The four main points
– North, South, East and West – are called the cardinal
points.
Old compass
cards are very ornamental, often covered with decoration and
painted figures. All cards have the North point decorated with
what is often called a fleur de lys, like the old royal
symbol of France. In fact, the sign comes from a very decorated
'T' for Tramontana, the Latin word for the North wind.
There
is a lot of movement on board a ship at sea. What happens to
the compass then?
To stop the
needle and card from swinging wildly on board ship, even early
compasses were gimbal mounted in a square box by an attachment
with swivelling rings. This means that the compass is hung in
a way that makes it unaffected by the movement of the ship on
the sea. The remains of one such compass, housed in a special
stand called a binnacle, was found in the wreck of King Henry
VIII's flagship, the Mary Rose, which sank in 1546.
At that time, the compass would have been lit at night by a
candle.
By the 19th
century, the ships compass had become the familiar large, gimbal
mounted instrument, enclosed in a binnacle with its own light.
Do
iron ships pose particular problems for magnetic compasses?
Yes. The magnetic
field of the iron body of the ship itself affects the reading
on the compass.
When iron
and steel ships became common, many scientists studied the problem.
One of the earliest was the Astronomer Royal, Sir G.B. Airy,
who in 1838 used the iron steamer Rainbow for his experiments.
Airy thought of a method of neutralizing a ship's magnetism
by placing magnets and pieces of unmagnetized iron near the
compass.
Which
other scientists successfully improved the compass?
Another problem
was solved by a Scottish scientist of the 19th century, Sir
William Thomson, who later became Lord Kelvin. He introduced
a compass design with the needle system slung on fine silk threads
through a very light skeleton card. The card was made of fine
rice paper so that there was very little friction on the pivot.
Jewels, such as agate and ruby, were used to reduce friction
on the pivot itself.
It was also
realised that compass movement could be dampened by filling
the bowl with liquid. Alcohol is ideal for this since it only
freezes at a very low temperature. Liquid compasses, because
of their greater steadiness, are used in most ships, especially
small boats and lifecraft.
How
was the problem of magnetic variation solved?
Variations
do not worry navigators now because of the introduction of the
gyroscopic compass. It was invented in 1908. This uses a spinning
gyroscope which keeps the compass pointing not to the magnetic
north, but to Earth's true North. A rapidly spinning gyroscope
is at the heart of the gyrocompass. Once the gyroscope is set
spinning, it remains pointing in the same direction, regardless
of the ship's heaving motion.
Today, a ship
anywhere in the world can check its exact position by means
of a signal from a satellite in orbit. However, all navigators
still have a compass on board as well. Tracy Edwards, who captained
the yacht Maiden in the 1989-90 Whitbread Round-the-World
Yacht Race, used Navsat (satellite navigation) and found it
had so many technical problems that she often used a magnetic
compass instead.
|
About this site
