Spotted Tube
   The Frontispiece to the 1844 edition of Henry Noad's "Lectures on Electricity" has the largest example of the spotted tube I have ever seen -- even if it is purely hypothetical. The words "NOAD'S LECTURES ON ELECTRICITY" are spelled out by sparks jumping the gaps between a series of tinfoil dots pasted on a pane of glass. The large EMF .produced by Armstrong's hydro-electrical machine in the center of the illustration was necessary for the spark to jump across the numerous gaps. In this form, the demonstration is often called a spangled pane. In another version, the dots are pasted in a helical fashion down the length of a glass tube, and is called a spotted tube. 

  This is the spotted tube I use for occasional demonstrations at Kenyon College. It is three feet high, and is listed in the 1860 catalogue of E. S. Ritchie of Boston at $3.50. The tinfoil dots are on the inside of the tube, which makes me suspect that there is actually an inner tube with the dots pasted on its outer surface.

   The picture of the discharge at the right was made by setting up my camera on a tripod in a dark room. The shutter of the camera was open, and my wife cranked a  Wimshurst machine until the potential across the line of dots built up to the point where the sparks jumped across the glass. 

   These two unmarked spotted tubes are at Oberlin College in Ohio (left) and Middlebury College in Vermont (right).
   This picture of a spotted tube was taken at Allegheny College in Meadville, Pennsylvania in the Fall of 1979. The late Richard Brown on the left had just retired, and James Lombardi on the right was the new member of the department. 

   Between them is a spotted tube that has the spots arranged in a sinusoidal fashion about the usual helical path. 

   This spotted tube, in the Garland Collection of Classical Physics Apparatus at Vanderbilt University, is normally hung from one of its hooked ends from  the prime conductor of an electrostatic machine. The other hook holds a chain going to ground. Here it is placed on a stand usually used to support  Geissler tubes.
   These two small spotted tubes are at St. Mary's College in Notre Dame, Indiana. The smaller tube has the spots on the outside, where they are subject to damage. 

   If one demonstration is good, two are better! This philosophy is illustrated in the apparatus at the left below (Transylvania University) and the right below (The National Museum of American History at the Smithsonian Institution). A series of spotted tubes are driven by discharges from a spinning electric whirl. In the center is a trio of spotted tubes at Washington and Jefferson College in Pennsylvania. The tubes come from two different pieces of apparatus and are temporarily mounted on a wooden base.
   The tinfoil dots did not have to be arranged on a cylinder. In this example, on display at the Museum of the United States Military Academy at West Point, New York, the dots were arranged on a spherical glass vessel. 

   A similar device was sold for $7.00 by Queen of Philadelphia in 1889.

   This example of a spotted pane is in the collection of Middlebury College in Vermont, and is probably home-built. Similar designs were shown in any introductory physics texts published in the second half of the nineteenth century. 

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