The Meter Reader: Early magnetic studies in the Lake Superior region

What is it about?

The magnetic method of mapping geology has a long and rich history in the Lake Superior region. The use of spatial variations in the normal geomagnetic field for identifying nearby magnetic rock formations and determining their depth began in North America in this region. Observations as early as the 1830s showed a connection between these variations and the location of intensely magnetic rocks. The strong interest in iron and copper ores associated with these magnetic rocks that were needed to support the growing industrial revolution and electrification of the U.S. led to magnetic surveying in areas of geological interest using mechanical magnetic needle instruments to detect unexposed ores. The principal instruments used in this surveying were a “dial compass” – a combination magnetic compass and sun dial – for measuring the changing orientation of the magnetic field and a “dip needle” for measuring the intensity of the magnetic field by observing the equilibrium angle of a vertical oscillating magnetic needle. The former was developed in the Lake Superior region and the latter, the dip needle, was brought to the Lake Superior region from the New Jersey Geological Survey shortly after the Civil War where it had been used for mapping iron ores since the early 1850s. These instruments were used in the region until the mid-20th century for ore prospecting and geological mapping, but were supplemented with modified dip needles, the Hotchkiss superdip and the Schmidt-type magnetometer, by 1920. These instruments were more sensitive and thus were useful in geological mapping of limited spatial magnetic variations. After World War II these instruments in turn were replaced by airborne magnetic surveying and electronic magnetometers.

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Why is it important?

Explains how ores and geology of the Lake Superior region were mapped in pre-WW II time.

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