Axial Revolving Bar
   "In this instrument ... an iron bar is suspended without visible means of support by the action of a current flowing in the helix which surrounds its upper half, the bearings above and below serving only as guides. The bar is made to rotate by transmitting the battery current through either half. This mode of suspending the rotating bar was first suggested by Dr. Boynton. [One of the screw cups] on the stand connects with the brass pillar, and thence with one end of the coil. The other end of the coil dips into mercury, contained in a circular cistern of ivory. This cistern is supported below the helix by an arm attached to the pillar, and has an opening in its centre to allow the bar to pass through. A bent wire, projecting from the middle of the bar, conveys the current from the mercury through its upper half to the [upper bearing]. Some non-conductor ... insulates the [upper bearing] from the arm which supports it. ...The current travels in succession to the helix and the upper part of the bar. Since the bar becomes an electro-magnet by the influence of the coil, it rotates rapidly on its axis, [in a fashion analogous to the magnet revolving on its own axis.]

   In Silliman's Journal for March, 1847, will be found a figure and description of a similar instrument, contrived by Dr. Page." 

   (from the 1851 Manual of Magnetism, pp 175-175).   

This example is in the Harvard Collection of Historical Instruments.


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