IEEE Power & Energy Magazine - July/August 2018 - 10

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When Electrodes Erode
Your January/February 2018 issue has
a fine article about arc lights (R. D.
Barnett, "Arc lighting systems," IEEE
Power & Energy Magazine, vol. 16, no. 1,
pp. 56-64, 2018). I learned that in a dc
arc, the positive electrode erodes in a
concave shape, thus casting light downward along the axis of the carbon rods.
the article did not say how the electrodes erode when excited by ac. Do
both electrodes develop hollow depressions? Are there two light peaks
along each electrode? I seem to remember that movie projectors (probably ac)
mounted the rods or thogonal to the
line of sight to the screen, implying a
nondirectional arc.
Myron Kayton

Digital Object Identifier 10.1109/MPE.2018.2819027
Date of publication: 18 June 2018

ieee power & energy magazine

Author's Reply
In the ICS text, Electric Lighting and
Railways (Scranton, Pennsylvania,
1901, pp. 18:6-10), the unknown author
states that for the open
(no outer globe) "alternating current arc,
both carbons become
pointed or have very
small craters, so that
the light is thrown upwards much more than
with the direct-current
lamp." Presumably, the
lamps that produce craters differ from those that produce points
in some unspecified way.
With a closed arc lamp, the light
distribution is similar to the open ac
lamp-light is also thrown upward but,
in this case (closed), both carbons have
flat ends. the author says that this is
a result of the arc's tendency to "shift
around over the ends," but he offers no
mechanism to explain the phenomenon.
In closed lamps, the erosion of the
carbons is greatly reduced because of
the partial exclusion of air, and perhaps
this affects the shape in some way. With
respect to the question regarding the two
light peaks, I believe the reader is implying that, because of the shifting polarity,

there could be two alternating sources
(top then bottom) of light. this may be
the case but for such a brief interval that
it would not be discernable to the human eye. It should be
emphasized that the ac
arc lamps were not in
widespread use. Projecting light up was not
of much use in street
lighting, the realm of
most arc lights, so not
much information was
provided for students
who would not frequently encounter ac arc lights.
About the projector question,
pp. 18:89-90 from the same book deal
with arc lamps used for "photo-engraving work, blueprinting, searchlights" and for "projection work." In the
latter type, dc arc lamps were used because, in ac lamps, the arc hummed
loudly and moved around more than
for a dc arc. Focusing mirrors were used
with projector arcs, and the feed mechanism was more precise to keep the arc at
the focal point of the mirrors. I hope this
answers your questions. Let me know if
you need anything more.
Bob Barnett

Your January/
February 2018
issue has a fine
article about
arc lights.

p&e

july/august 2018

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