Roget's Spiral
   Another name for Roget's Spiral is the Contracting Helix, and this is how Davis lists it in the 1851 Manual of Magnetism, pp 233-234. He wrote: "The mutual attraction between different portions of the same current moving in the same manner, may be rendered evident by the instrument[s represented here]. A wire, coiled into a loose helix, is supported in a vertical position by a brass pillar connected with one of the screw-cups on the base board. The lower end of the helix dips into mercury contained in a glass cup with a metal bottom, by means of which the mercury is brought into connection with the other screw-cup. 

   When the battery is applied, the portions of the current traversing the

different turns of the helix of course flow parallel to each other, and in the same direction. Their mutual attractions cause the turns of the helix to approach each other, shortening it sufficiently to lift the end of the wire out of the mercury. This interrupts the current, and the helix is lengthened again by the elasticity of the wire composing it, producing continued vibration. A spark is seen in the glass cup at each rupture."

   The apparatus at the left is one which I have used for many years at Kenyon College, and on the right is a contracting helix at Washington and Jefferson College. These examples are unmarked, but have the brass ball feet of Davis apparatus. Davis priced the item at $3.50.

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