The Magnetic Compass

Leaving aside who was the true inventor of the magnetic compass given the historical and political discrepancies that have characterized the debate for years to obtain the real paternity of the instrument that revolutionized the navigation of the high seas, its usefulness continues to importance on board even in our day.

icaptain, bussola magnetica, nord, sud, ovest, est, rosa dei venti, sospensione cardanica, cappelletto, mortaio, rosa graduata
Ancient on-board compass with cardanic suspension

The first texts describing it in nautical use in Europe date back to the 12th Century and see Arab and Amalfitani users as users.

The behavior of small magnetic needles was already known to the Chinese, these used multiple magnetized needles for shows: the needles were thrown like nuts and to the immense amazement of the spectators were always in the same direction.

One of the easiest experiments to recreate to understand the criterion of operation of a compass is precisely to take a needle, magnetize it by rubbing it on a magnet and make it float, perhaps with a leaf or a piece of cork in a glass or in a dish with water: however much we want to orient it when we leave it free the needle will always be in the same direction.

This direction is that of the force lines of the Earth’s Magnetic Field: Our Planet has a very natural magnetic field due to the composition of the Earth’s crust.

The direction that will give us a compass without the influence of other magnetic fields (which is completely theoretical) will then be a direction known as North Magnetic that varies from area to area and from year to year.

Let’s go back to the instrument: what can not be missing are the Magnetic needles placed along the N-S direction of a graduated rose on which in some cases it is reported the Rose of the Winds; these needles are the only sensitive instruments that a compass is equipped with and will orient themselves in the direction of the North Magnetic.

There can be a line of faith that will allow a smooth reading of the same.

Remember the experiment you introduced earlier? In order for everything to work, the Graduated Rose must be free to orient itself freely, which is why it is suspended on a device called Puntale or Pin which is in turn connected to the Mortar, that is, to the container in which the sensitive complex of the Tool.

The rose is supported by the tip or pin by means of a small indentation that takes the name of cappelletto.

icaptain, bussola magnetica, nord, sud, ovest, est, rosa dei venti, sospensione cardanica, cappelletto, mortaio, rosa graduata
Main parts of a magnetic compass

In the marine compasses, inside the Mortar we find a liquid based on water (90) and alcohol (10).

Overall, the liquid serves to dampen the oscillatory motions to which the Ship is sighed so that the rose can continue to work without major movements; in this case the large graduated roses also have a float that decreases its weight.

The solution also contains alcohol so as to avoid a possible freezing of the liquid at low temperatures.

The larger compasses are then fixed on board on a call of amagnetic material by means of a cardanic suspension that will ensure the perfect horizontality even in the case of sea format.

Finally, the real direction that the magnetic compass will provide us will be connected in addition to the direction of the Earth’s magnetic field to the onboard one and for this it will be properly compensated. This direction is called North Compass.