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the arc either by increasing the current or voltage
at the arc is again illustrated in Figure XVI where
the visual light efficiency in lumens per watt is
plotted against the watts at the arc.
'
The curves in the above figures have all been
based on voltage and watts at the arc, A certain
amount of external resistance is required for "ballast"
with the type of side arc lamp used in the
moving picture studio. Therefore, to obtain the
overall efiftciency the line watts and line volts must
be used. This has been computed for the twin arc
lamp and the results are given in Figures XVII and
XVIII. Figure XVII shows for both the white
Flame and Panchromatic "O" Carbon Arcs the
effect on the light or lumens and Figure XVIII
shorvs for these same carbon arcs the effect on the
overall efficiency or lumens per line watt of burning
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them in a twin arc lamp on a 110 volt line under
various conditions of current and voltage across the
two arcs.
It is quite evident that a low arc voltage for a
given current and line wattage gives much less light
and a lower efficiency than a high arc voltage under
the same conditions. For example, take a current of
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40 amperes at the lamp and 4400 line watts with
panchromatic "O" carbons (See Figures XVII and
XVIII), if the voltage across the arcs themselves
was only 60 volts, that is 30 volts across each arc
(a condition giving a short arc length), the lumens
would be 138,000 and the lumens per line wat 31.3
but if the voltage across the arcs themselves was
100 volts, that is 50 volts across each arc (a condition
giving a long arc length) the lumens would be
292,000 or more than doubled for exactly the sa:me
line watts as aboae and the lumens per line watt or
overall efficiency would be 66.3 or more than
doubled.
It can also be seen that if the current is increased
and the voltage across the carbons kept the same
(that is the arc length the same) the efficiency of
the lamp and total light increase materially. For
example, for a current of 30 amperes and 80 volts
across the arcs with panchromatic "O" carbons (40
volts across each arc) the lumens are 138,000 and
the lumens per line watt 41.8 but {or a current of
50 amperes and B0 volts across the arcs, the lumens
have increased to 292,000 and the lumens per line
watt or efficiency increased to 53.1
These curves indicate very clearly the improvements
possible in the capacity and efficiency of the
side arc lamp by a proper regulation of the lamp
resistance and arc length. The limiting factors on
current are the capacity of the lamp wiring and
carbons. The limiting factor on arc voltage or arc
length is the steadiness of the flame. It might be
added that the higher the current up to the capacity
of the carbon, the steadier the flame for the same
arc length.
The National Panchromatic "O" Carbon Arc,
because of the greater amount of yellow, orange and
red light gives, as shown in the figures, more visu.al
light for the same watts than the white flame carbon
arc. The white flame carbon arc on the other hand,
because of the larger proportion of blue and violet
rays has a higher photographic efficiency for the
same watts than the panchromatic "O" carbon but,
as already indicated does not bring out the reds and
yellows as well as the panchromatic "O" carbons.
This difference in photographic power is comparatively
small, being approximately in the ratio of 5
to 4. In photographing yellow, orange or red
materials the photographic power of the panchromatic
"O" carbons for these colors is greater in the
ratio of approximately 6 to 5 than the photographic
power of the white flame carbon.
It is hoped that a study of the photographs and
curves in the paper will aid materially in applying
the White Flame and Panchromatic "O" Carbon
Arcs to the light problems of the moving picture
industry.
DISCUSSION ON
MR. DOWNS,PAPER
Mn. Balr,: In the discussion of Mr. Downs'
paper I would like to urge the men present to tend
toward a discussion of the practical use of the
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