Forces Between Parallel Wires

Apparatus to Demonstrate the Forces Between Two Parallel Wires

Made by the L.E. Knott Apparatus Company of Boston — first quarter of the 20th Century

This apparatus, in the in the Greenslade Collection is used to study the forces acting on two parallel, current-carrying straight wires. If the currents are parallel, the two wires are attracted to each other; if the currents are anti-parallel, they repel.  Fig 1 shows the apparatus set up to show Roget’s spiral, designed by Peter Mark Roget about 1835.  The end of the helical spring dips into a pool of mercury.  Current through the helix causes it to contract as the turns are attracted to each other, and contact with the mercury is broken.  As the coil relaxes, the current is reestablished, and the  process repeats itself.  In Fig. 2 two pairs of wires that run along side the vertical wooden rods carry the currents, which may be either parallel or anti-parallel.  Fig. 3 shows the wires replaced by iron wires, hanging from pins at their tops and dipping into mercury cups at the bottom.  Again, the currents can be either parallel or anti-parallel.

Although this apparatus bears the Knott label, it does not appear in the 1916, 1921 and 1922 catalogues in my possession.  If anyone has a Knott catalogue(s) in which this apparatus appears, I’d be grateful to hear from you.

Thomas B. Greenslade, Jr.


  (click for larger image)

Forces Between Two Parallel Wires

3 views of Apparatus to Demonstrate the Forces Between Two Parallel Wires by L.E. Knott Apparatus Co.


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