Magnetic Survey and the Magnetometer. [Volume: 8, Issue: 5, 1889, pp. 262-266]

262

UNITED STATES ASSOCIATION OF

[VOL. 8,

Magnetic Survey and the Magnetometer.
By ERNEST SJOSDEDT, M.E., Katahdin Iron Works, Maine.
[Tnanslated from Prof. Rob. Thaln's paper in Jernkontorets Annaler for 1879,
and presented at the Tenth Annual Meeting of the United States Association of
Charcoal Iron Workers, Duluth, September 23d.]
AMONG the instruments constructed for locating a magnetic oredeposit, the common miner's compass is an old acquaintance, and,
in a somewhat modified and improved form, it has been used by
Brooks, of Wisconsin, Wrede, of Sweden, and others, for an approximate determination of an ore-vein; but not until Prof. Thalen,
of Upsala, had constructed his magnetometer and showed us how to
use it in the field has it been possible to determine more accurately,
and, according to strict mathematical formulas, where most of the
ore is located, its depth of centre, its extent, etc.
With the common dipping needle, reliable results can only be
obtained when in the hands of an experienced miner, who is well
acquainted witl the peculiarities of the instrument, and versed in
the geology and the mineralogy of the ore-field and its surroundings. As commonly used by " professional" ore prospectors, little
reliance can be put in the results obtained, at the best, and at times
it may be used fraudulently, as is told of some " experts" who (according to the position of the instrument) could show a "big dip,"
or a considerably smaller one, at one and the same ore-deposit, depending upon if, at the time, he represented the purchasers or sellers.
Conscientiously and intelligently used, however, a miner's compass is very useful for discovering magnetic ore-deposits, and with
the aid of one carefully graded, but not too sensitive, or one so constructed that the needle has a motion in azimuth, as well as in altitude, magnetic reconnaissance maps can be made which will show
the extent of the deposit, the direction, and sometimes also the dip,
of the magnetic ore-vein.
A few remarks regarding the interpretation of the working of
the miner's compass may here be in place. It is easy to understand, for instance, that, as a general thing, there need not be any
ore directly below the point where the needle stands perpendicular,
although it is generally considered that the ore is concentrated here.