Why does this age-old device still exist?
By Jim Sparks
Just to put everything in perspective, Albert Einstein commented that when he was still a child his father gave him a magnetic compass. He spent hours pondering how the small pointer always oriented itself to a specific direction. It was this allure of the compass that first attracted Einstein to the wonders of the natural world.
Most of today's most sophisticated commercial aircraft still include in the otherwise all electronic flight deck, a small permanent magnetic (or whiskey) compass. Every now and then a maintenance requirement surfaces where this device must be tested and if needed an adjustment made. Why does this age-old device still exist in our current world of technological advancement? More importantly why do we have to adjust it and what can introduce errors requiring these adjustments?
Magnetism and troubleshooting
The early explorers knew of the lodestone and ancient sea captains considered this chunk of rock to be a prized possession. They observed that the orientation of a suspended lodestone was always relative to a specific direction when it came to a stop. They also realized that these directional properties could be transferred to various metal pointers mounted on bearings. This was accomplished by mechanical contact or simply having the loadstone in close proximity of the pointer.
One of the key elements in troubleshooting compass systems is a basic understanding of magnetism. There are only three other elements that can become magnetized at other than extreme ambient temperatures: iron, cobalt, and nickel. It is the unique property of their atoms and more specifically their electron arrangement that allows polarization. Unfortunately, in all cases these materials will lose their magnetic properties shortly after the source of magnetism is removed.
There are, however, several alloys that will retain polarity and can be classified permanent magnets. Alnico is one of these alloys and is widely used in the construction of permanent magnetic compasses. This material is a result of joining iron with aluminum nickel and cobalt, and magnets manufactured from it are 50 times stronger than its predecessors.
Polarity and magnetic strength
Each magnet has both a North Pole and a South Pole. Like poles tend to repel while opposite poles will attract. An invisible force exists between the North and South Pole of any magnet and this flux field intensity is a direct result of the strength of the magnet.
Magnetic field strength is based on a mathematical definition that takes into account the amount of force the field has on an electrical current flow. The unit of measure of magnetic force is the Tesla (T), which is calculated by multiplying the current flow through a wire by the wire length in a magnetic field providing the field is perpendicular to the conductor. This is then multiplied by the field strength determined in Newtons per ampere.
A Newton is a measurable force calculated in a fashion similar to foot pounds or inch pounds. In fact one Newton is equal to 100,000 Dynes and one Dyne is the force required to move one gram one centimeter.
The strongest magnetic field ever created is around 40 Tesla. This means a Tesla is a very large unit of measure. In fact the Earth's magnetic field is only rated at .06 millitesla. Unfortunately this means aircraft magnetic sensors will react to the strongest magnetic field available and this is not always the Earth.
The Earth is a magnet
Yes, the Earth is a permanent magnet. Many physicists theorize that the core of the Earth has a solid mass of iron and nickel surrounded by hot molten metal. This causes a reaction, which produces electrical current flows that in turn cause polarity. It is also theorized that the Earth's magnetic field will reverse itself every several hundred thousand years. You may have heard the terms Magnetic North and True North. True North applies to the Earth's rotational axis where as Magnetic North actually lies along the East Coast of Greenland at about 70 degrees north latitude.
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