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 Summary-A
new motion picture arc lamp designed for use as a general lighting unit
is described, in which the new 8-mm., copper-coated, cored carbons are
used. Two special mechanisms feed the two carbons of the unit independently
of each other, the rate of feed of each being controlled by the voltage
drop in each arc. Each control circuit includes a voltage coil and a current
coil, acting in magnetic opposition, which arrangement avoids variations
of light intensity, flickering, and blinking. The lamps operate under
a voltage of 115, drawing a current of 40 amperes, d-c.
During
the past five years the development of carbon arc lighting equipment for
use in motion picture production was retarded by several factors. The
introduction of panchromatic film, and its almost universal acceptance
as negative raw stock, provided a photographic medium that was well adapted
to photographing with incandescent filament lamps. The introduction of
sound recording in connection with motion picture photography prohibited
the use of any type of lighting equipment that was not quiet in operation.
However, all through this period 150-ampere Sun arcs and Rotary Spots
have been used to a large extent in combination with incandescent lighting.
By making mechanical improvements in the mechanism of Sun Arcs and Rotary
Spots, quietness of operation was obtained which overcame the objections
of the sound technicians. The old type of broadside lighting units, used
extensively in the days of silent pictures, has been practically abandoned
in modern picture production, because its design inherently prevented
silent operation.
Early
this spring, one of the leading producers of colored motion pictures requested
Mole-Richardson, Inc., to investigate the possibility of developing a
motion picture arc lamp for use as a general lighting unit. This firm
was developing a new process of color photography, and it seemed that
arc illumination would provide the most satisfactory means of lighting
the sets to be photographed.
The
specifications were as follows:
(1)
The lamp should produce an illumination level of 200 foot-candies, as
measured at fifteen feet with a standard Weston photometer.
(2)
It must have a comparatively flat distribution curve over a projection
angle of sixty degrees or more, and the field of illumination should be
devoid of any hot spots, i. e., areas of illumination that are photographically
objectionable.
(3)
The feeding mechanism of the lamp should be so designed as to provide
a reasonably uniform level of light intensity during its period of operation,
and the spectrum of the light emitted should not show any alteration of
its characteristics during the period of operation.
(4)
It should be silent in operation, so that it may be satisfactorily operated
in conjunction with modern sound recording apparatus.
(5)
It should take such a form, and be so mounted, that it will be convenient
for placement, and be of such weight as to be easily handled on the set.
(6)
It should be economical in operation with regard to attendance, the consumption
of current, and carbon electrodes.
First
experiments were made with half-inch white flame carbons, which had previously
been used in practically all arc broadside lighting units. By improving
the reflecting surfaces, it was found possible to boost the light flux
of the old type of broadside units from 60 foot-candles, measured at fifteen
feet with a standard Weston photometer, to about 90 foot-candles.
We
quickly realized, however, that even though we overhauled and installed
new reflectors in our old side arcs, it would be impracticable to attain
the 200 foot-candle requirement desired by our client.
We
communicated with the National Carbon Company to ascertain what new developments
had been brought forth in arc carbons, which would be suitable for use
in equipment of the broadside type, and as a result obtained samples of
several types of carbons that were thought to fulfill the requirements.
After numerous experiments, we decided that probably the 8mm., special,
copper-coated, cored carbons that were recommended would best suit the
purpose.
An
old type broadside unit was adapted to operate with the carbons and was
supplied with chromium-plated metal reflectors. With the 8-mm. carbons
in both the upper and the lower carbon holders a marked improvement in
light intensity was attained. Utilizing practically the same current,
40 amperes, the light intensity was raised from 90 foot-candles, measured
at fifteen feet, to 120 foot-candles.
An
inherent fault of the old type of broadside arc lighting unit was its
inability to maintain a uniform level of illumination. When first energized,
the old type lamps would consume from 40 to 45 amperes, and produced their
maximum lighting intensity; by the time the feed mechanism came into operation,
the current in most cases dropped to approximately 32 amperes, and the
lighting level dropped about 40 per cent. The specifications set forth
demanded a much more accurate control of the lighting intensity.
Knowing
the limitations of the old style carbon control mechanism of the various
lamps that had been previously designed, it was decided to experiment
with a lamp in which each pair of carbon electrodes would be separately
controlled. An experimental model was built, and after a number of modifications
a mechanism was developed that reduced the fluctuations in light intensity
during the feeding cycle of the lamp to within 10 per cent.
In
previously designed broadside lamps it had been attempted to control the
feeding of the carbon by means of a single current coil in series with
the arcs, and by utilizing various means for equalizing the feeding of
the upper carbons toward the lower carbons. As far as we have been able
to observe, mechanisms operated on such a principle fail to provide good
operating conditions, due to the fact that the tolerances in the diameters
of the carbon electrodes must of necessity be rather large; and if it
happens that a carbon with a minus tolerance be placed in one side of
the twin arc mechanism, and a carbon with a plus tolerance be placed in
the other side, the carbon having the small diameter will inevitably feed
more rapidly than that having a larger diameter. It is most difficult
to devise a mechanism operating with a single control coil that would
overcome the difficulty without greatly complicating the structural characteristics
of the feeding device.
The
mechanism developed for the M-R Type 29 twin arc broadside controls each
pair of carbon electrodes, independently maintaining the voltage drop
across each pair of electrodes at 35 to 40 volts, and the feed of each
pair of electrodes is independent of the other and controlled by the voltage
drop in the arc that the mechanism controls.
Fig.
I is a schematic diagram showing the method by which this is accomplished.
Each carbon arc has its lower carbon electrode in a fixed position. The
upper carbon electrode is movable; and when no current flows, the lamp
is in contact with the lower carbon. When the lamp is connected to the
line, the circuit is closed with only the ballast resistance to impede
the flow of current.
The
current coils of each mechanism are in series with each other and with
the two arcs. The current from the positive side of the line passes through
the ballast resistance, 1, into the base of the lamp, through the switch
to the control coil of mechanism No. 1, and on to the upper carbon; thence
to the lower carbon, into the current coil of mechanism No. 2 through
the coil to the other upper carbon, then to the lower carbon, and back
to the line through the ballast resistance, 2. The energizing of the circuit
actuates the solenoid armatures, which, through their connecting linkages,
elevate the upper carbons in each are system, striking both arcs.
Fig 1. Schematic diagram of arc regulating mechanism.
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Above
each current coil, and surrounding each armature, is a coil wound with
fine wire and a large number of turns, connected across the arc controlled
by it. These coils are wound counter to their respective current coils,
and the instant the arc is struck a small current flows through each coil.
Since they are shunted across the arcs, the energy introduced into them
increases as the voltage drop of each arc increases, the magnetic flux
of each voltage coil opposing that of its corresponding current coil.
By properly proportioning the number of turns in the current and voltage
coils, and proportioning and spacing their respective armatures, it is
possible by this method to control the opening of the arc and to maintain
quite accurately a uniform voltage drop across the arcs. Ball-bearings
were introduced at the fulcrum of the upper carbon actuating levers, so
as to make the mechanism sensitive to the changes of voltage of the arc.
Simple, plate-type carbon clutches have proved entirely adequate.
Since
maximum efficiency with the carbon electrodes used was attained by using
a 5/8-inch arc gap, it was necessary to take precautions to prevent magnetic
"blowing" of the arcs. This was accomplished by connecting the current
coils of each mechanism so that they formed a closed magnetic circuit,
and by placing a steel magnetic baffle plate between the coils and the
arc.
Fig. 2 Broadside Twin Arc Lamp M-R type 29 |
The
entire mechanism is relatively simple, and may be economically manufactured,
because, except for connections in the wiring, each unit of the mechanism
is an exact duplicate of the other. To adjust each mechanism so that it
will operate in harmony with its adjacent unit, it was desirable that
a simple adjusting means be provided. This adjusting means is the movable
counterweight mounted on the arc actuating lever. As the lamps leave the
factory they are adjusted for operation on 115 volts, 40 amperes d-c-,
voltage readings being taken across each arc and the counterweights adjusted
for balanced operation.
Under practical and test conditions it has been found that with this mechanism
flickering has been totally eliminated. Even though the line voltage be
greatly disturbed, as it often is on motion picture stages when operating
under heavy loads, the mechanism is so responsive that such disturbances
are compensated without the "blinking" that was often experienced with
the old type twin arc lamps.
Fig 3. Twin Arc Scoop Lamp
M-R type 27
The
mechanism has been built into two types of lamp heads: the M-R type 29
Twin Broadside Arc and the M-R type 27 Twin Arc Scoop. The Broadside Lamp,
designed for floor use, is mounted on pedestal having two telescoping
sections, and may be elevated from height of four feet one inch, to eight
feet eight inches from the floor. The housing of the M-R type 29 has been
constructed of duralumin sheet metal and aluminum castings (Fig. 2). The
mechanism may be tilted from the vertical position thirty degrees forward
or backward without disturbing the operating characteristics. Chromium
plated reflectors, which have proved to be entirely satisfactory in this
type of equipment, increase the light flux of the lamp in excess of the
specification requirements.
The
scoop is illustrated in Fig. 3. Its housing, in addition to carrying the
mechanism, also carries the resistance units. To facilitate the dissipation
of the added heat of the resistance, the head has been amply ventilated
with louvers. The aperture of the lamp has been set at an angle to deflect
the light downward, as the scoop is primarily designed for overhead use.
To assist in carrying off the fumes from the arc coring, both types of
lamps are provided with a chimney midway between the twin arcs. This ventilation
contributes to the cleanliness of operation of the equipment, a large
portion of the white condensate from the arcs passing off through the
chimney.
Both
types of lamps are intended to be used with glass diffusers. A Prismatic
glass, sand-blasted on one side, has proved best for the purpose, its
high lead content inhibiting the transmission of ultraviolet radiation.
No complaints have been received from actors working under the lamps in
regard to injury of their eyes.
While
it is not anticipated that this new equipment will revolutionize motion
picture stage lighting, there are many types of photography and many special
effects for which this equipment is peculiarly adapted.
DISCUSSION
Mr.
Joy: Any one who has seen Mr. Mole's lamp in operation realizes that he
has made a very material contribution to the art of illuminating motion
picture sets. The feeding of the carbons is uniform and regular, as the
feeding solenoid of each are is controlled by the current and voltage
of that arc, resulting in a steady light from the unit. Tests have shown
that within an angle of 60 degrees in front of the lamp, the decrease
in light from the center to the outside is only about 15 per cent. Such
a small change over such a wide angle should be particularly advantageous
in photographic and motion picture work.
Member:
What is the bulk or weight of the equipment? To what extent does it add
to or detract from the regular incandescent equipment?
Mr
Mole: It would not add to the bulk or the number of units. Experience
has shown that the number of units used on the set depends entirely on
the set, regardless of whether arcs or incandescents are used. As many
units are used as the size of the set demands, so that the entire set
will be covered.
Mr.
Cour: What is the comparison in wattage? Mr Mole: That is very difficult
to answer. One cameraman on a 15 by 15 set would use 600 amperes and another
would use 1200 amperes, so there is no way of determining the saving.
More lumens per watt are radiated by an arc,than by incandescents, but
whether a man is working on a low level or high level, we don't know.
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