| 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
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
|| 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
A similar device was sold for $7.00 by Queen of Philadelphia
|| 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.
Return to Static Electricity
Return to Home Page