Roget's Spiral

Roget's Spiral

Roget's Spiral demonstrates that there is an attractive force between two parallel wires carrying electric current in the same direction. The wire is actually coiled in a helix and not in a spiral, prompting the alternative name of Contracting Helix. A pointed iron bob on the end of the helix dips into a pool of mercury, and the upper end of the helix and the mercury are connected to a source of EMF. The current through the helix causes it to contract, breaking the circuit and removing the force between the turns. The bob then falls into the mercury and the cycle starts once more.

The helix at the right is from Vassar College, and on the left from the Smithsonian Institution.

The apparatus was devised by Peter Mark Roget (1789-1868) who was a physician, a found of medical clinics, a tireless author of scientific articles and books, the inventor of the log-log slide rule, a chess expert, the secretary of the Royal Institution and, after 1840, the compiler of the Thesaurus bearing his name. About 1835 he published a description of the contracting helix.

The Vassar instrument at the left was made by the L. E. Knott Apparatus Company of Boston and cost $1.35 in 1916.

At the right is a Roget's Spiral from the early part of the twentieth century on display at the University of Cincinnati physics department.

On another page is the Roget's Spiral made by Daniel Davis that I use in demonstration lectures at Kenyon College.

This example is at the University of Texas in Austin. It is unmarked, but is probably not made by Queen, as was much of the early Texas apparatus. The date is ca. 1900.

REFERENCE: Thomas B. Greenslade, Jr., "Nineteenth Century Textbook Illustrations, LVIII: Roget's Spiral", Phys. Teach. 36, 38 (1998) Return to Electricity Home Page | Return to Home Page