We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
•
Technical Section.
223
Data for Rinematograph Technicians.
THE COST OF ELECTRICITY.
An electrical unit is equal to 1 kilowatt-hour, or 1,000 watts for one hour. To ascertain the consumption in hours per unit of any piece of D.C. apparatus rated in watts, divide the rating into 1,000 : e.g., a 60-watt lamp will run for 1,000-:60= 16f hours for one unit.
The theoretical electrical horse-power is equal to 746 watts. For electric motors, a useful approximation is to reckon one unit per hour per rated horse-power.
In the case of A.C. supplies, maximum-demand charges are generally based, not upon the kilowatt, but upon the kilo-volt-ampere ; the relation between the two terms is known as power factor. Thus at a power factor of 0.8, a supply of 1 k.v.a. is equal in power output to only 800 watts. The loading of cables must also be based upon the k.v.a., and not the k.w. consumption.
Lighting and heating circuits (other than neons and other discharge lamps) have a power factor of unity ; the power factor of motors, arc rectifiers, etc., may vary from 0.6 to 0.9. It is advantageous to keep the power factor of such apparatus as high as possible.
CALCULATING ARC RESISTANCES.
The value of a resistance R to pass a given current C at voltage V is expressed, according to Ohm's law, by the formula —
-I
In calculating arc resistances, however, the value of the back voltage A of the arc must be considered. This may be reckoned as 45 volts for lowintensity arcs, and for high-intensity from 25 volts for a horizontal-negative mirror arc or 40 for an inclined-negative mirror arc, up to 65 or 70 volts for rotating-positive arc. The formula then becomes — ■
-d _ V-A
The earlier studs of a resistance should be so computed as to allow the arc to be struck on not more than one-third of its full current.
ORDINARY FILM CEMENT.
A myl Acetate 4 oz., Acetone 6 oz.
This may either be used as it is, in which case the cement will be a thin fluid with not much more body in it than water, or it may be thickened to any desired extent by dissolving it in celluloid chips. Celluloid film base from which the gelatine coating has been cleaned off will serve.
For non-flam, film glacial acetic acid may be added ; it is, however, preferable to use the cement recommended by the makers of the stock — e.g., for Dufaycolor use Ilford cement.