Dip Circle in Glazed Case

Physics

Accession Number: 2017.ph.743

Instrument Name: Dip Circle in Glazed Case

Description:

The glazed case is mounted on a cast tripod with three-levels of screws. It has a rotating azimuth scale with six small handles (one of them is missing) and a brass locking screw with nickel finish. The needle is 10 cm long. Both the altitude circle and the azimuth circle are of 5 cm radius. The case has a diameter of 15 cm, and both front and back glazed windows are designed to be opened.

Alternative Name: Dipping Needles

Primary Materials: Metal, Glass

Markings:

On the altitude circle: W. & J. Becker LTD London & Birmingham No. 3601

Altitude circle measurement: 0 ~ 360 (=0).

Azimuth circle measurement (from top): 90 ~ 0 ~ 90 ~ 0

Dimensions (cm):

Height = 25cm; Glazed Case Diameter = 15cm ; Altitude Circle Diameter = 10cm

Function:

A compass showing the angle of the earth’s vertical magnetic field.

Condition:

Fair with one missing handle; the bronze coloring of the altitude circle has faded

Manufacturer: W. & J. George & Becker Ltd.

Additional Information and References:

In his own words in his Personal Narrative of Travels…. Humboldt carried “A dipping needle of tweleve inches, constructed on the principles of Borda and Le Noir. … An azimuth circle serves to find the plane of the magnetic meridian, either by correspondent dips, of by seeking the position in which the needle is vertical or observing the minimum of the dippings. The instrument is verified by observing on the east and west side, and changing the poles.” (Alexander von Humboldt. Personal narrative of travels to the equinoctial regions of the New continent, during the years 1799-1804, by A. von Humboldt and A. Bonpland. translated from the French by Helen Maria Williams, 1814, p 36.) Helferich notes in Humboldt’s Cosmos that Humboldt made 124 magnetic observations on his expedition.

“Immediately after leaving Europe, Humboldt began taking regular measurements of the earth’s magnetic field, using a kind of vertical compass called a dip needle. During the course of his journey, he made 124 magnetic observations ranging over 115 degrees of longitude and 64 degrees of latitude, also recording in every case the geodetic coordinates, height above sea level, and distance from any mountains or prominent rocks that might influence the results. As he traveled south toward the geographic equator, he noted with increasing excitement a steady decrease in the earth’s magnetic field. Even after Humboldt crossed the geographic equator in Ecuador, the magnetic dip—the angle with which the magnetic needle was attracted downward, toward the earth, continued to decline. But now, as the party traversed the Cajamarca Plateau, he finally registered a dip of zero: He had located the magnetic equator—the line where the vertical component of earth’s magnetic force is zero—at 7 degrees, 27 minutes south latitude and 81 degrees, 8 minutes west longitude. … Humboldt’s discovery of the magnetic equator on the desolate Cajamarca Plateau was a landmark achievement, not only proving that the earth’s magnetic field varies predictably with latitude but pinpointing the exact location where there is no vertical dip at all. ‘I have considered the law of the decrease of the magnetic forces from the pole to the equator as the most important result of my American journey,’ Humboldt wrote.”

Helferich, Gerard (2004). Humboldt’s Cosmos: Alexander von Humboldt and the Latin American journey that changed the way we see the world. New York : Gotham Books (Kindle Location 4263-4280). Tantor eBooks. Kindle Edition.

Source:
https://www2.humboldt.edu/scimus/AvH_HSU_Centenial%20Exhibit/Dipping_Circle/DipCircle.htm

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