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1945 Theatre Catalog, 4th Edition, Page 311 (287)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 311
Page 311

1945 Theatre Catalog, 4th Edition, Page 311

THE 30-AMPERE TUBE RECTIFIER is for theatres with lamps of less great electric demands. It is essential that the theatre have the proper currentchanging equipment. (Strong Electric Corporation photograph.)

and may be as high as 110 volts and requiring a ballast resistor in series with the arc suflicient to maintain a voltage drop across the ballast of at least 30 volts. This dc. power is most conveniently obtained through the use of the conventional 2-tube, single-phase rectifier, which has the necessary reactance to result in this desired voltage drop without the use of wasteful ballast resistors. In the past, when low-intensity lamps were the only source of screen lighting, satisfactory arc performance was attained by employing most any haywire hookup, wherein suffIcient ballast resistance was connected in series with the are so that it reduced the arc current to the desired number of amperes, without regard to the voltage of the dc. power supply.

High-Intensity Lamps

However, with the introduction of the more critical high intensity carbons, most stable operation is attained only through a rather exacting relationship between arc voltage, are amperage, line ballast and dc. power supply voltage.

In the case of the 1-kw. are, burning at 40 amperes with 28 volts, the d.c. power supply, as measured without the arc load, should not exzeed 35 voltse that is a drop of 6 volts in the line ballast-because a higher power supply voltage with more ballast, while obviously reducing the current to 40 amperes, would never result in a smooth burning arc, free from color change at the screen, and this higher voltage would require frequent adjustment by the projectionist of the lamp controls.

A high-intensity arc operating at 45 to 50 amperes with 35 volts across the arc performs best when the dc. power-supply potential is about 60 volts and there is 25 volts drop across the ballast resistor. A GEE-ampere are, burning at 40 volts, should have a d.c. power supply



of about 60 to 70 volts with a ballast of 30 volts.

Consequently, in selecting a power supply for lighting equipment due consideration should be given the recommendation of the lamp manufacturers for each type and capacity of arc lamp.

In the case of the moderate sized theatre, with its 18-foot screen and its l-kw. arc, the 4-tube, single phase rectifiers designed especially for use with this lamp results in most stable operation.

The larger theatres, employing 50 to 70 amperes, may use either motor-generators or the copper-oxide or six-tube rectifiers which have been designed especially to supply direct current to the large lamps. Because of the heavy electrical load, these larger rectifiers or generators must be supplied from 3phase power circuits, which are always available in these theatres.


Amperage and Light Theatremen quite generally have the

,mistaken impression that simply to in crease the amperage at the arc must necessarily result in more light at the screen, or that a large lamp may be burned at low current satisfactorily in an attempt to reduce operating costs. However, this is not actually true, because each arc amperage requires a matching optical system.

To be specific a low-tensity arc, burning 25 amperes, has an arc crater which is approximately 6 mm. in diameter. it follows that the optical system must reflect the arc crater as a 6-time magnified image of the crater, at the film aperture.

Principles of Reflectors

The magnification of an optical system employing an elliptical reflector system is a function of reflector focus and its working distance; that is, the size of the crater will be magnified as a spot at the aperture as many times as the distance from the arc crater to the center of the reflector is contained times into the distance from the center of the reflector to the film aperture.

In the instance of the low intensity arc, which requires a magnification of six times, the standard low-intensity reflector has a focus of 4 inches with a working distance of 24 inches, which is a ratio of 6 to 1 and, accordingly the system has a magnification of 6 times, so that the spot at the aperture is six times as large as the arc crater or just suflicient to cover the 36mm. aperture opening without undue loss of light.

Likewise other types and capacities of arcs have their characteristic peculiarities and dimensions that require specific consideration at the time the lamp and its reflector are designed uniformly to illuminate the fixed area of the film aperture, that is, a higher current lamp with a consequently larger light source, requires an optical system with a lesser magnification than does the small crater area of the l-kw. arc.

Reflector Diameter

There is, generally, confusion in the minds of projectionists as to the relative efficiency of lamps employing small diameter reflectors as compared to those

employing the large diameter reflectors. In explanation, the reflector diameter is designed to work with the projector lens system. In other words, the diameter of the reHector is calculated to project a 26-degree conical beam of light converging at the aperture.

Accordingly, when employing positive carbons 8 inches in length, the reflector must be set back 24 inches from the film aperture, so a reflector about 11 inches in diameter is necessary to fill this light angle of 26 degrees, whereas if a 14inch length positive carbon is employed the reflector must consequently be set back about 30 inches from the film aperture, and a reflector 14 inches in diameter is necessary to fill this light angle.


There are, of course, many other considerations in the design of projectionlighting equipment for theatres and of the optical systems and current-changing devices used in connection with them. But these few simple facts indicate the absolute necessity of operating any projection equipment under the exact conditions that have been the contributing factors in its design for this particular use.

THE SIX-TUBE RECTIFIER is suggested for the larger theatres, to supply the direct current for arc lamps. Because of the heavy load, a Suphase power circuit is needed. (Strong Electric Corporation photograph.)

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1945 Theatre Catalog, 4th Edition, Page 311