Magnetic Models Of The Earth

   The magnetic model of the earth at the left below was designed by Peter Barlow (1776-1862), who is otherwise known for Barlow's mathematical tables, the Barlow lens (a negative lens used to increase the power of telescope eyepieces), the tensile-strength testing machine and Barlow's Wheel. Barlow presented a similar model of the earth to the Royal Institution of Great Britain in London in 1824. Current passing through the coils sets up a dipole magnetic field similar to the magnetic field of the earth. This can be detected and observed with (missing) small bar magnets pivoted on the L-shaped wires in the upper corners. This apparatus is at Transylvania University in Lexington, Kentucky, and there is a similar piece of apparatus in the collection of the Irish National University, Galway in Ireland, both made by Elliott Brothers of London.

   The very different magnetic model of the earth at the right is marked "D.C. Murdock/West Boylston/Mass.", and is in the collection at St. Mary's College in Notre Dame, Indiana.  I suspect that only the globe was made by Murdock, for the complete apparatus, with the wooden bar passing through the globe from pole to pole, is listed in the 1851 catalogue of Daniel Davis, Jr. of Boston at a cost of $4.00. The illustration shows a current-carrying coil of wire around the equator and a compass needle being used to sample the magnetic field. Davis' discussion of the demonstration is included with the pages on his apparatus.

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