PREFACE

In this volume the authors have attempted to lay before the Motion-Picture Operators and Theatrical Employes generally, a reference and handbook making a specialty of electrical requirements about theaters.

A working knowledge of electricity in general is assumed, and therefore elementary ideas have been treated sparingly. A specialty, however, has been made of all matters peculiar to theaters, and it is thought that theater electricians will find in this volume everything that they need whether they be operating motion-picture machines or switchboards in first-class houses.

The two special chapters “Portable Stage Equipment” and “Theater Wiring” have been arranged so that they are particularly valuable for reference. They should be consulted before undertaking any electrical construction work, either for the stage or for the auditorium. These chapters embody all of the practical knowledge that has come to the notice of the authors in many years of actual experience with theatrical construction.

The aim of this volume has been to present in a simple and practical way the essential principles of Motion-Picture Work.

The Authors.

Table of Contents

MOTION PICTURE OPERATION
STAGE ELECTRICS AND
ILLUSIONS

CHAPTER I.
THE ELECTRICAL CIRCUIT AND ELECTRICAL HAZARDS.

Two and Three-Wire Systems.

Two and Three-Wire Systems.—If the theater electrician will take the trouble to trace the circuits in the building to their supply, he will find them entering the building either as two-wire or three-wire circuits.

A two-wire circuit is diagrammatically shown in Figure 1. The circuit, coming from 1, enters the building, passes through the fuses 2, and through switch 3 to the lights. A two-wire system will ordinarily be found operating at 110 volts, the current varying according to the number of lights turned on. In the drawing, for instance, only one light is shown with the switch closed, the other three switches being open. The current in the circuit is equal to that which passes through the single lamp. If another switch be closed, another light will burn and the current will be increased, so that the more lights be turned on, the greater will be the current.

The three-wire system, Figure 2, is almost universally used where the supply is from the outside and where any considerable number of lights are connected. The chief advantage of the three-wire system lies in its economy of copper. The middle or neutral wire ordinarily does not carry current, but it is a necessity whenever the number of lights burning on the two sides of the system are not equal.

With the neutral wire omitted, we have a straight two-wire system using double the voltage of the ordinary two-wire system and always operating two 110-volt lamps in series. Two lamps would always have to be turned on at the same time and if one of them should burn out, the other would be extinguished also.

A system using double voltage requires only half the current and consequently but half the copper. In order to obviate the necessity of always using two lamps together and at the same time economizing in copper, the neutral wire is provided. As long as the same number of lamps are burning on each side of the neutral wire, the same current always passes through two lamps in series and there is no current in the neutral. Should, however, the group on one side be turned out, the other would still continue to burn; but the path of the current to the dynamo, or bank of transformers, would be through the neutral wire.

The system is thus seen to possess all the advantages of the ordinary two-wire system since each lamp can be operated by itself and at the low voltage, while the actual supply voltage for the whole system is double that which is actually used at any lamp. We have thus two voltages at our command; 110 and 220 being the voltages in common use.

Electrical Hazards.

Electrical Hazards.—Since this work is not intended for mere beginners, we shall not enter into elementary considerations, but shall take up the matter of fire and life hazard, both of which are important items to which attention cannot be too often called.

The electrical current may cause fire by overheating the wires. This overheating may be due to the willful overloading of circuits; and to prevent this, no wire should ever be used to carry more current than is allowed by the table of carrying capacities given on page 238.

The overheating may also be due to an unknown load which is caused by a “ground” or a partial short circuit. “Ground” is the technical term used to designate the connection of a wire to any substance over which electricity may be carried to the wire of opposite polarity. A ground may be caused by a bare wire coming in contact with the iron framework of a building, wet wood, or moist substances of any kind. One such ground on a circuit can do no harm; but, if one ground exists, in the course of time another one may come on and, when the second one appears, if it is on the side of the circuit opposite to the first, there will be trouble at once.

If the two grounds are both “good”—that is, if they are of low resistance—we shall have a short circuit and probably blow a fuse; but if they are not “good,” we may have but a small current which may continue unnoticed for months. Such a current may eat away the copper of the positive pole and in time cause the wire to break, creating an electric arc and perhaps causing a fire. It may also cause wet wood through which it flows to become charred and finally ignited.

The ground is the bane of the electrical worker. If a system can be kept free of grounds, the chances of trouble are vastly reduced. The cause of most grounds is moisture. Nearly all substances except metals are fairly good insulators if dry; and nearly all of them are fairly good conductors if sufficiently wet.

Another very prolific source of fires is the electrical spark, large and small. The spark, due to the breaking of an incandescent lamp, often causes fires when it comes in contact with inflammable material or gases. The ordinary lamp cord also causes many fires because it is easily damaged and liable to short circuits which often result in arcing. Short circuiting two wires or breaking a wire carrying current may easily ignite inflammable material in the vicinity.

The best way to reduce the fire hazard to a minimum, is to install all electrical work carefully according to the rules laid down in the “National Electrical Code”.

The life hazard is one which concerns the operator personally and is especially great to those traveling with shows. Traveling men are often obliged to get along with all sorts of make shifts, especially in the smaller towns where one-night stands are the rule. Here it is often necessary to connect to trolley circuits or power lines of different voltages, frequencies, etc.

A person may suffer injury directly from a current of electricity by making himself part of the circuit. If the system on which he is working is alive and grounded, he may easily cause injury to himself by touching a live wire with his hands while standing upon anything that is grounded. By so doing he completes a path for the current through his body.

He may also become part of a circuit by holding a wire with both hands while someone is cutting it between his two hands. As long as a wire so held is intact, no shock can be received except to ground, but when the wire is cut or breaks, a very high voltage may be produced for an instant which will cause a current through the body of the man holding the wire. The extra high voltage is produced only if the wire is carrying current at the time it is being cut. Under these circumstances there will often be a strong flash, due to the momentary increase in voltage, produced by the breaking of the circuit, which may be excessive, especially if there is considerable inductance in the circuit.

The most frequent cause of injury is due to making contact with the two opposite polarities of a system. As a rule circuits, with which operators ordinarily have to work, are low voltage, i.e., not over 220 volts. But many deaths have been caused by this voltage, sometimes directly and at other times indirectly as, for instance, by causing a fall. People whose hearts are in any way defective should be careful about exposing themselves to shocks even at 110 volts.

It is true that many wiremen are in the habit of testing 220-volt circuits by allowing the current to pass through their bodies, but it will be noticed that they are very careful not to make a good contact. The current which passes through the body, when one touches two wires of opposite polarity very lightly with the finger tips, is but a fraction of what one would receive if he were unwittingly to grab two wires of opposite polarities with the hands, especially if the hands were moist.

Numerous cases are on record of persons having been killed by 110 volts under favorable circumstances; as, for instance, while in the bath receiving a shock from a so-called vibrator. The body partly immersed in water and perhaps a foot resting against a water pipe forms a conductor of very low resistance, and a comparatively strong current may pass through the body.

The most important precautions against injury while working on live circuits are:

(1) Insulate yourself from the ground.

(2) Handle only one side of a line at a time.

(3) If possible, work with only one hand at a time in contact with the wires.

(4) Use rubber gloves, or rubber boots where necessary, but bear in mind that they are of little value unless kept dry. Moisture will allow some current to pass over the surface of any substance no matter how good an insulator it may otherwise be.

(5) Always place yourself so that a slight shock which might cause you to lose your balance will not give you a bad fall.

(6) Remember that if once you make good contact with an alternating-current circuit, you cannot let go.

(7) Fix firmly in your mind the directions for resuscitation from electric shock, on pages 15-18.

When energy is obtained through transformers, there is another danger to be added to the above, viz., the possibility of the breaking down of the insulation between primary and secondary wires of the transformer. If this happens, we have suddenly and without warning, instead of the 110 or 220 volts supposed to exist between the wires forming the circuit, 2,000 or 3,000 volts. Such accidents are especially likely during thunder storms when lightning often breaks down transformers.

In order to reduce this danger to a minimum, the secondaries of transformers are grounded. It will be well for the electrician to assure himself that the secondaries of the transformers from which he is getting his supply are grounded. This can be tested by an incandescent lamp. Connect the lamp to ground with one wire and, with the other, try the two sides of the circuit. If the transformer secondaries are properly grounded, the lamp will burn at full candle power from one of the wires; this will show that the other wire is grounded.

A person working on such a circuit is of course more likely to receive a low voltage shock than if the secondaries were not grounded, but he is fairly well protected against the primary voltage or lightning.

RESUSCITATION FROM ELECTRIC SHOCK.

Rules recommended by commission on resuscitation from electric shock, representing The American Medical Association, The National Electric Light Association, The American Institute of Electrical Engineers: Dr. W. B. Cannon, chairman; professor of physiology, Harvard University. Dr. Yandell Henderson, professor of physiology, Yale University; Dr. S. J. Meltzer, head of department of physiology and pharmacology, Rockefeller Institute for Medical Research; Dr. Edw. Anthony Spitzka, director and professor of general anatomy, Daniel Baugh Institute of Anatomy, Jefferson Medical College; Dr. George W. Crile, professor of surgery, Western Reserve University; W. C. L. Eglin, past-president National Electric Light Association; Dr. A. E. Kennelly, professor of electrical engineering, Harvard University; Dr. Elihu Thomson, electrician, General Electric Company; W. D. Weaver, secretary, editor Electrical World. Issued and copyrighted by National Electric Light Association. Reprinted by permission. Follow these instructions even if victim appears dead.

I. IMMEDIATELY BREAK THE CIRCUIT.

With a single quick motion, free the victim from the current. Use any dry non-conductor (clothing, rope, board) to move either the victim or the wire. Beware of using metal or any moist material. While freeing the victim from the live conductor have every effort also made to shut off the current quickly.

II. INSTANTLY ATTEND TO THE VICTIM’S BREATHING.

1. As soon as the victim is clear of the conductor, rapidly feel with your finger in his mouth and throat and remove any foreign body (tobacco, false teeth, etc.) Then begin artificial respiration at once. Do not stop to loosen the victim’s clothing now; every moment of delay is serious. Proceed as follows:

a. Lay the subject on his belly, with arms extended as straightforward as possible and with face to one side, so that nose and mouth are free for breathing, see Figure on page 17. Let an assistant draw forward the subject’s tongue.

b. Kneel straddling the subject’s thighs and facing his head; rest the palms of your hands on the loins (on the muscles of the small of the back), with fingers spread over the lowest ribs, as in Figure on page 17.

c. With arms held straight, swing forward slowly so that the weight of your body is gradually, but not violently, brought to bear upon the subject, see Figure on page 18. This act should take from two to three seconds.

Immediately swing backward so as to remove the pressure, thus returning to the position shown in the Figure on page 17.

d. Repeat deliberately twelve to fifteen times a minute the swinging forward and back—a complete respiration in four or five seconds.

e. As soon as this artificial respiration has been started, and while it is being continued, an assistant should loosen any tight clothing about the subject’s neck, chest or waist.

2. Continue the artificial respiration (if necessary, at least an hour), without interruption, until natural breathing is restored, or until a physician arrives. If natural breathing stops after being restored, use artificial respiration again.

3. Do not give any liquid by mouth until the subject is fully conscious.

4. Give the subject fresh air, but keep him warm.

III. SEND FOR NEAREST DOCTOR AS SOON AS ACCIDENT IS DISCOVERED.

CHAPTER II.
THE ARC LAMP.

General Discussion of the Electrical Arc.

General Discussion of the Electrical Arc.—The name of the electrical arc lamp is derived from the arch-like appearance of the vapors which give out the light when the carbons are placed horizontally. The horizontal arc was the earliest form, hence the name which it carries to this day.

The arc proper is due to the vapors of volatilized carbon or other materials forming the electrodes, which may be consumed by the passage of an electrical current from one electrode to another through the intervening medium. In order that an arc may be formed, it is necessary first to bring the electrodes together. This, if the circuit is properly arranged, starts the current and when the circuit is partly interrupted, as by slowly separating the points of the electrodes, the current passes through the intervening space, with the result that a high degree of heat (about 3,500 centigrade) is produced. This results in volatilizing the carbon or any other material of which one or both electrodes may consist.

As long as the distance between the electrode points is small, the current will be quite strong and a hissing or frying sound will be given out. In order to keep the current within bounds during the time that the electrodes are together or while they are separated only a very short distance, some resistance, or reactance in the case of alternating-current arcs, is always connected in series in the circuit. If this were not done, there would be a short circuit at the time of starting or striking the arc.

The arc formed with very short separation of electrodes is generally spoken of as a low tension arc and requires very hard carbons and about 25 volts. This type of arc is very little used for illuminating purposes.

If the distance between the electrodes is increased gradually, the light becomes very unsteady and flickers considerably until at a certain point it begins to improve and give the long quiet arc. This condition will occur when, with direct current, the electrodes are about one-eighth of an inch apart. It will then be found that the voltage across the arc is from 45 to 50 volts, which is the best voltage to use with open arcs. If the separation be carried still further, the arc will grow longer and become flaming until finally it breaks entirely.

The resistance of the arc is closely proportional to the cross section of the electrodes and increases with the distance of the arc gap. It acts, however, very much as though there were a small counter e. m. f. set up within it.

The color of the light given off varies with the length of the arc somewhat, but depends mainly upon the material of which the electrodes consist. In the so-called flaming arcs, the peculiar color is obtained by certain chemicals imbedded in the material composing the electrodes. Whenever an arc is allowed to burn down until it reaches the electrode holders, a greenish light is given off which is due to the volatilization of the metal—usually brass—in these holders.

The light of a strong arc is extremely injurious to the eyes and should only be viewed through colored glass. Many very painful experiences have resulted from persons gazing upon arcs of 200 or 300 amperes, such as are used sometimes in cutting away metals of old buildings, etc.

The most powerful arcs known at the present time are those used in some steel mills for refining steel. These use upward of 10,000 amperes.

The length of the ordinary arc varies from one thirty-second of an inch to one inch. The light is not of much use and is rather unsteady until the electrodes have assumed a shape somewhat similar to that shown in Figure 3 for direct current, and Figure 4 for alternating current. With direct-current arcs, a crater is formed at the bottom of the positive electrode and, from this crater, about 80 per cent of the light is emitted. Where the light is wanted in a downward direction, the crater is always formed at the top and for this purpose the top electrode must be made positive; that is, the electricity must flow from the top electrode into the lower one. In some cases, where special illumination effects are desired, the bottom electrode is made the positive with the result that most of the light is thrown upward. In such cases strong shadows are thrown against the ceiling and the lamp is said to be burning “upside down.”

The positive electrode can always be distinguished from the negative (a) by the shadows cast; (b) by the form of the electrodes; and (c) by the fact that since it is heated to a greater degree, it will, when the lamp is turned off, remain hot for some time after the negative electrode has cooled off.

In case the arc is drawn out very long and operated in this way for a considerable time, the crater will almost wholly disappear and the electrodes will appear rounded off.

In an alternating-current circuit, the positive and negative poles reverse generally about 120 times per second and both electrodes in the alternating-current arc are positive and negative to the same degree. They are therefore very nearly alike, except that the heat rising from the lower one increases slightly the volatilization of the upper. The positive electrode in the direct-current arc is consumed approximately twice as fast as the negative electrode. The consumption of the two electrodes in an alternating-current arc is about equal and a crater much smaller than the kind formed in a direct-current arc is, therefore, formed on each electrode, instead of only on the positive electrode as in the case of the direct-current arc.

The general form of alternating-current arc carbons is given in Figure 4. The small elevations shown in the cuts are due to impurities and do not appear with first-class carbons.

When arc lamps are operated on alternating-current circuits, the best voltage for the arc is about 28; and consequently, for the same quantity of light, the current must be increased so that the amperage of alternating-current lamps is always much greater than that of direct-current lamps.

The alternating-current arc is much noisier than the direct-current arc, but with very high frequencies this noise ceases.

In general, arc lamps do not work very well on low frequencies. The time at which the current is practically zero is long enough to allow the vapor between the electrode points to cool off sufficiently to interfere with successful operation.

Any arc light is affected by draughts of air and can even be blown out. If this occurs often, there will be rapid feeding, a short arc, and great waste of electrode.

A magnet held close to an arc can be made to blow it out or force it to one side. This fact is made use of in some lightning arresters.

Generally speaking, arc lamps are of two kinds, open and enclosed. The enclosed arc operates at a much higher voltage and is but little used about theaters. The open arc is almost universally used for stage work and this is about the only place where it is still considered useful. This kind of arc lamp is, however, very hazardous in localities where inflammable material abounds and for this reason it is always enclosed with wire mesh when possible.

Lens lamps can be tightly enclosed since none of the light is wanted except that which passes through the lens in front.

The lamp houses should be of such dimensions that, with the highest amperage the lamp is capable of using, the outer walls will not become excessively hot.

Illustrations of standard lens and flood lamps, as made by the Chicago Stage Lighting Company, are shown in Figures 5 and 6.

Operation of Arc Lamps.

Operation of Arc Lamps.—From the standpoint of operation, arc lamps may be divided into two classes, viz.: hand-feed and automatic-feed. The hand-feed lamp is generally used in theaters and is practically the only kind admitted on the stage, or for stage illuminating purposes. Only a very few houses now use arc lamps for general illumination.

The operation of hand-feed lamps[1] is ordinarily quite simple and will be fully treated under the head of “Projection”, so that we may now consider only the automatic lamps. At the present time these are used mostly, if at all, for the illumination of the exterior of the theater.

The operator should first familiarize himself with the construction and principles upon which the mechanism of his lamp is based. For this purpose he should remove the outer jacket, thus exposing the working mechanism; turn on the current; and endeavor to learn the significance of each part. It is of course necessary that the operator understand the hazards due to manipulating live wires and that he should be very careful not to make short circuits or grounds which might destroy parts of the lamp.

Automatic-feed lamps are usually trimmed in the following manner: Bring the lamp within reach; remove the globe; take out the lower electrode; let down the upper electrode rod and thoroughly clean it with crocus cloth. This upper electrode rod is the principal thing that concerns the lamp trimmer; it must be perfectly straight and care must be exercised not to bend it accidentally; it must be clean so that the clutches may properly grip it; it must not be greasy. If it grows dirty or greasy, it will soon become pitted from the current that passes from the contacts to it.

The next operation is to remove the upper electrode and place it in the lower electrode holder. (The length of electrode necessary should be known. The lower one generally burns out first—it being shorter—and if the arc reaches the lower electrode holder, will begin to consume it; if the lower carbon is too long, the arc is liable to reach the upper electrode holder and destroy it.) The upper electrode may then be placed in position and aligned with the lower. To do this it is best to turn it about and try it until it aligns in all positions. The two electrodes should form a straight line, up and down, no matter which way the upper is turned.

In some forms of enclosed lamps, the clutch grips the electrode direct. In such a case all of the upper electrode must be carefully examined to see that it is straight and free from burs, and that it can pass freely into the opening at the top of the inner globe. The successful operation of enclosed arcs depends upon the confinement of the gas in the inner globe. This globe must, therefore, be kept as tight as possible without interfering with the operation of the electrodes which pass through it.

With enclosed arcs, the care of the inner globe is of great importance, because impurities are cast off which soon coat the inner globe and absorb much of the light.

The care of the outer globes in general is also an important matter. A dirty globe looks very unsightly and absorbs much light.

The following points should be carefully considered in handling and trimming lamps:

(1) Be sure that you understand your system and know whether it is a constant-current or a constant-potential system of distribution. With constant-current systems, the current is constant and the voltage over the arc is regulated; while with constant-potential systems, the voltage is constant and the current through the arc is regulated.

(2) With constant-current or series lamps, the line must never be opened, but must be shunted around the lamp if a lamp is to be cut out.

(3) With constant-potential lamps, the lamp must never be shunted but the circuit must be opened.

(4) In all cases each lamp should be controlled by a double pole switch.

(5) Constant-potential lamps cannot be operated without resistance in the circuit; this resistance may be in the lamp itself or outside.

(6) Never handle high tension lamps without insulating yourself from the ground; and handle live wires only with one hand at a time.

(7) Provide spark arresters for all open-arc lamps in the vicinity of inflammable material.

(8) Never leave a lamp without globes where the wind can strike it. It will be blown out or feed often, thus consuming the electrodes very fast and at the same time yielding a very poor light.

Green light emitted by the lamp will indicate that the electrode holders are burning. Strong shadows cast upwards indicate a lamp burning “upside down”. The positive electrode retains heat longer than the negative. The quality and size of electrodes has much to do with successful operation. Always use the kind of electrodes recommended by the maker of the lamp.

Direct-current arc lamps do not require much in the shape of reflectors as they naturally throw most of the light downward, when the upper electrode is positive. They should as a rule be suspended high.

Alternating-current arc lamps throw most of the light from the upper electrode slightly below the horizontal and that from the lower electrode somewhat above. If the light is wanted in a downward direction, suitable reflectors must be provided.

Testing of Arc Lamps.

Testing of Arc Lamps.—The constant-potential arc lamp is usually designed for a certain current and voltage. The enclosed arcs as a rule operate singly on 110 volts, while open arcs are run two in series on the same voltage. In order to test and see that the voltage and current are right, an ammeter and a voltmeter are needed. The current and voltage can both be adjusted by altering the resistance, which is always in series with such lamps. To get the correct voltage over the arc, be sure to connect the voltmeter to the two electrode holders so as to eliminate any other potential drops that may affect the reading.

Testing Carbons.

Testing Carbons.—The color of the light and the steadiness of it can of course only be determined by actual operation tests. The arc obtained by using large electrodes with low current density is liable to rotate around the electrodes, burn unsteadily, and flicker. This is due to the fact that the arc tends to establish itself at the point of least resistance. In order that the arc may burn uniformly, the current density must be great enough to force all of the electrode points into use.

As a rule the best electrode is the one that has the longest range from the low voltage point of hissing to the high voltage point of flaming. With such an electrode the greatest range in light can be obtained without either the hissing or the flaming.

The same qualities that give an electrode long range, as above, also indicate its purity and if we make a test for range, we shall therefore at the same time make a test for purity.

The test for range can be carried out by any ordinary hand-feed lamp. To make it, the electrodes are inserted and allowed to burn until their points have assumed the proper shape. The arc can then be shortened until the familiar hissing sound is heard. Note the voltage at which this occurs, being careful to have the voltmeter connected so as to get the voltage across the arc only. Now separate the electrodes slowly until they begin to flame and note this voltage. Ordinarily the hissing voltage will be about 42 and the flaming voltage about 62. The greater the difference between the two, the better the carbons are assumed to be. In making comparative tests on electrodes in this manner, care should be taken that all of the conditions of current and size of electrodes be the same.

The test for comparative life of electrodes is best made by arranging the different electrodes so that the same current will pass through each for the same length of time. If this is done, all that is necessary is to weigh the electrodes before and after burning. The approximate useful life of an electrode can be easily determined by burning it for a stated length of time, noting the length consumed and comparing it with the length available for burning.

CHAPTER III.
PROJECTION.

Setting and Adjustment of Carbons.

Setting and Adjustment of Carbons.—To project a picture upon a screen properly is an art and requires close study and some knowledge of all the factors involved. The most important factor is that of the light. Electric light is so universally used at the present time that it is hardly necessary to mention the other sources of illumination.