Get a Straight Answer

Items covered:

120.     Length of compass needle

Dear Jim
    ÊÊÊÊI see no reason why a long needle cannot keep the same magnetic polarity. No, the reason was probably that surveyors' magnetic needles were balanced on a single pivot, and the magnetic force in general has a downwards component, trying to give the needle a downward angle. A needle on a pivot may compensate for this by being slightly unbalanced, but that compensation only holds when the magnetic inclination angle is constant. ÊIn fact, it changes from place to place, and a long needle balanced at one place may start dragging at one end at another.

    ÊÊGauss overcame this by suspending compass magnets at two points near the center of mass, and some of his magnets were quite long. ÊMagnets used aboard ships are attached to a compass card floating in a fluid, and I guess that keeps them horizontal anywhere on Earth.

Response

Thank you very much for your response. Ê

    Actually the compass needles had a small twist of wire on them to counter-balance dip. ÊThis wire could be moved in (toward the pivot) or out as necessary like a weight on a beam balance. ÊThe typical surveyor's compass was about six inches in diameter with needles about five inches or a bit longer. ÊThey were graduated in half degree (thirty minute) increments and equipped with a vernier which made it theoretically possible to obtain an accuracy of one minute but as a practical matter they couldn't consistently achieve accuracies better than about six to ten minutes because of needle quiver and an inability to adjust the vernier without moving the compass. ÊThese limitations could have been partly overcome with longer needles which would have allowed a larger circumference that could have been subdivided into smaller increments. ÊAccording to contemporary text books, however, longer needles simply could not be magnetized as well as needles equal to or less than about six inches. ÊThey were made of soft iron and lost magnetism naturally so occasionally the compass man had to re-magnetize them by stroking them with bar magnets carried for the purpose.

    Incidentally, both the pivots (steel or iridium) needed to be sharpened occasionally and eventually replaced as did the agate or ruby caps from which the needles hung on the pivots because of wear. ÊTo replace and repair them in the wilderness required quite a bit of skill because it was necessary to adjust the pivots so the needle pivoted exactly in the center of the compass. ÊIt was difficult to sharpen a pin so its tip was exactly in the same place as the worn tip and no two caps or replacement pivots were identical. ÊA further problem with replacement needles was that the magnetic axis of the replacement needle was frequently different from the old needle. (the geometric and magnetic axes of needles are not the same.) ÊI mention all this to let you know that the compassmen were really technically skilled.

    The foregoing reminds me of a second question: ÊWhat causes needle quiver?

    With respect to nautical compasses, liquid is used to dampen the compass card (no pun intended) but this was not done until the latter part of the nineteenth century when manufacturers were able to overcome certain technical difficulties. Until then the compass cards were simply hung on a pivot like a 'land' compass. ÊEven with liquid present, the cards were hung on pivots. ÊThese pivots and caps wore out much more rapidly than those on 'land' compasses because the compasses were in constant motion about the cards so they required frequent maintenance or replacement. The needles of 'land' compasses, on the other hand were locked when not in use. ÊThe cards were magnetized by attaching a magnetized needle (usually several lined up in parallel) to the bottom of the card. ÊSimilar compasses were sometimes used for land navigation but they seem to have been exceptions.

Reply

    "So the molten metal is believed to be circulating. By moving through the existing magnetic field, it creates a system of electric currents, spread out through the core, somewhat like Faraday's disk dynamo, discussed earlier. Currents create a magnetic field--a distribution of magnetic forces--and the essence of the self-sustaining dynamo problem is to find solutions such that the resulting magnetic field is also the input field required for generating the current in the first place."

Ê Electrons are also small magnets, associated with a property known as "electron spin." The arrangement of electrons in an iron atom makes it quite magnetic. However the magnetic axes of atoms in unmagnetized iron point in random directions, and as many point in any direction as in the opposite one, so the effect cancels (as it does in most non-iron materials). ÊWhen you magnetize iron, the magnetic axes of atoms get lined up and their magnetic properties reinforce each other.

    Ê Ê Ê In soft iron, the alignment lasts only as long as the cause of magnetization persists, e.g. the electric current in a coil around a soft-iron electromagnet. Such iron is useful in cores of electric transformers for AC, which must reverse their magnetization 120 times each second, to track the ups and downs of AC.

    Ê Ê Ê Steel and some other materials, on the other hand, stay magnetic after being magnetized, either by an electric current or by a permanent magnet held close. ÊThat is useful for compass needles. The alignment is not of individual atoms but of "magnetic domains", relatively large chunks of iron crystals which are already magnetized naturally in the same direction. In unmagnetized steel, the magnetization directions of domains are oriented in random directions; running a current in a coil around them, or stroking the iron with a permanent magnet, lines them up Êin a new direction, as long as the magnetism sensed locally by each domain exceeds a certain minimum intensity (which depends on the material). SeeÊhttp://en.wikipedia.org/wiki/Magnetic_domain

Response

Thanks for getting back to me.Ê When a permanent magnet is moved next to iron, do the electrons orbiting the iron nucleus get attracted to one side of the iron nucleus, thus resulting in magnetic waves pointing in the same direction?

Reply

"Local anomalies originating in the upper mantle, crust, or surface, distort the WMM or IGRF predictions. Ferromagnetic ore deposits; geological features, particularly of volcanic origin, such as faults and lava beds; topographical features such as ridges, trenches, seamounts, and mountains; ground that has been hit by lightning and possibly harboring fulgurites; cultural features such as power lines, pipes, rails and buildings; personal items such as crampons, ice axe, stove, steel watch, hematite ring or even your belt buckle, frequently induce an error of three to four degrees."

Dear Arnie
        ÊÊ Ê ÊThe web-course whose home page you cite deals mainly with the Earth's magnetic field in space. ÊA similar (shorter) course dealing with the field inside Earth, its study and variations, is "The Great Magnet, the Earth,"Êhome page http://www.phy6.org/earthmag/demagint.htm . ÊYou may Êlook up section 15 there on magnetic reversals and at its end answered questions about it, but I would also encourage you to read through it, it's not that long.

    ÊÊ Ê ÊThe magnetic field from the Earth's core is gradually weakening by 5-7% per century, but the continuation of this trend is uncertain.