In most theaters the height from stage to ceiling is about seventy feet or more. The height must be great enough to allow the scenery to be hoisted out of sight above the proscenium opening. There must also be sufficient space above the rigging loft to allow men to walk about straightening out ropes, etc. The total height should, therefore, be nearly two and one-half times that of the proscenium opening.

As a rule no openings of any kind are arranged in any of the enclosing walls of the stage, except those that are necessary as entrances. In houses which are likely to originate shows, a paint bridge is generally provided along the back wall.

Moving-Picture Theater.

Moving-Picture Theater.—Figure 126 is a plan of a typical small moving-picture theater. A spacious lobby is an important adjunct to any such theater and this space is often as valuable as the seating space inside. It affords shelter to those who cannot be seated at the moment. It is especially convenient in localities where race suicide is not yet very noticeable and where it forms a repository for many baby carriages.

A view showing the location of a picture-machine booth is given in Figure 127. This is the arrangement most in use although there are localities where it is required that the operating room be at the end of the theater opposite the main entrance, the object being to avoid the necessity of the audience passing under the fire to the outside in case a fire should occur in the operating room. This plan has so many disadvantages that it is never followed except where the law requires it. If a properly arranged operating room is provided, there is no necessity whatever for it.

The chief use of Figure 127 is to illustrate the importance of adjusting the height at which the picture is thrown upon the curtain to the pitch of the floor. The curved lines in the upper part of the picture indicate the floor conditions which would give each seat the same clearance over-the-one-in-front-of-it to a view at the bottom of the picture. The upper line shows the bottom of the picture, level with highest seat, the other with the same point somewhat below it.

Figures 128 and 129 show the plan and the elevation of a large operating room. Every operating room should be of ample size to allow the operator to walk on all sides of the machine. It should be strictly fireproof, well-ventilated, and provided with a door giving easy exit to a floor outside, level with the floor of the room. In altogether too many rooms the only means of getting in or out is by a ladder which leads directly into the room through a small trap door. Such an arrangement is extremely dangerous in case of fire. The fumes of burning film are poisonous if inhaled and spread very fast. If two men should happen to be in such a room at the time the film is ignited, it is likely that they would wedge each other into the narrow opening and both perish. The door to the operating room should open outward and be self-closing. All openings in the operating room must be fitted with fireproof shutters by which they can be instantly closed in case of fire. There should also be suitable ventilation to the outside to carry off all smoke from burning film.

The Screen.

The Screen.—The screen upon which the picture is projected should be a clear white but of a dull finish. All glare should be avoided and wherever there is a polished surface, this glare is always conspicuous.

There are a number of patent or special makes of screens on the market, but it is not the purpose at this time to discuss any of them. It will be sufficient to point out some of the simpler methods by which screens may be made.

A simple plaster wall is often used and is very serviceable, but it must not be polished and must be coated in some way to allow of washing off occasionally.

Perhaps, all things considered, a muslin screen is the best of all. Such a screen, if well stretched, gives a good flat surface entirely without glare and has the further advantage that it can easily be taken down and laundered as often as necessary. It is best to provide duplicate sets so that there may be one for use while the other is in the wash.

The only real objection to a muslin screen is the loss of light which it entails. It is not only a poor reflector of light, but it allows much of the light to pass through it. With many such screens, the picture is as well visible behind the screen as it is in front of it. This indicates a great loss of light and it is to prevent this loss of light that the other forms of screens have been devised. The most noticeable of these is the mirror screen, which is a mirror of ground glass. It is very economical in light but expensive in first cost and hard to keep clean. It must be washed quite frequently and careless men are apt to use too much water and get some of it on the back where it will soon begin to loosen the silver coating and ruin the mirror.

The screen should be surrounded so that no other light except that coming from the picture can reach the audience. To this end there should be a black frame around it and this black frame must also be a dull black and without glare. Velvet drapery is the very best thing that can be provided and should be brought in so close from all sides that no white rim around the picture will be visible, as such a white rim would detract greatly from the illumination of the picture.

The decorations around the proscenium should preferably be of a dark color and kept dull, especially in cities where it is necessary to maintain emergency illumination. If the front is light in color, there will be considerable reflection which will detract from the picture and require a higher illumination of it.

Wherever there is an opportunity to do so, the screen should be set well away from the nearest row of seats. This will not affect the view of those in the rear, and it will very much improve the view of those in front. It must of course be seen to that a clear view from both sides is possible.

CHAPTER XV.
OPERATING ROOM EQUIPMENT.

Construction and Ventilation.

Construction and Ventilation.—The operating room should be built of fireproof material, such as cement, brick, or tile. If it is of wood, it may be made nearly fireproof by lining the inside with heavy sheet iron. It will be well if asbestos is placed between the sheet iron and the wood which it covers. All woodwork, whether window frames, doors, or shelving should be thus lined. It may be taken as practically certain that sooner or later one or more reels of film will be burned in the room and every precaution should be taken to prevent the fire from spreading. If this is done there will probably be no serious results, for in the past there have been so many film fires which have been successfully confined to the operating room that audiences have become pretty well accustomed to them. If all operating rooms are strongly built and of ample dimensions, with all openings as small as practicable, it will help to inspire confidence in the audience and there will be but little likelihood of a panic.

The chief danger is to the operator. It is true that he is generally to blame, but it is not always possible even with the utmost care to prevent a fire. Above all the operator should be given every possible chance to get away in case of fire. An operator who feels that he can get away at the last moment is far more likely to stay and fight a fire than one who is penned up in a coop from which it is difficult to get out rapidly. There are in existence, at the present time, operating rooms where it is necessary to crawl on hands and knees some twelve or fifteen feet to get from the machine to the exit, and where the exit is by means of a trap door at the head of a ladder. Such operating rooms are an abomination and no self respecting operator would work in one of them.

The entrance and exit from the booth should be by means of a door at the right hand or crank side of the machine. The door should be self closing and should open into a space having a floor level with the floor of the operating room. The door should preferably be kept closed but if the provision illustrated in Figure 128 is made there can be no serious objection to leaving the door open for ventilation. A thin bar of iron is fastened to the door by a screw eye and is used to block the door open. A person rushing out will naturally knock this away and the door will close. The door could also be arranged to be held open by means of the strings, to be described later, which hold the shutters in place.

The ventilation of the operating room should be by means of a fireproof duct leading to the outer air. This flue should preferably extend above the highest point of the roof and should be of good size. The smoke from burning film is heavy and does not rise very fast but it is generated with extreme rapidity. Film burns at a temperature of 284 degrees Fahrenheit. If it is incased in a tank and fire communicates with it, the whole body of the film will be raised to this temperature in a very short time and will burn with the rapidity of an explosion. In the magazines, which are in general use in the newer houses, the combustion will be slower but nevertheless will take not more than about one minute.

It has often been suggested that a motor be provided in the ventilating flue, this motor to be kept running during the time the machine is in operation. This would no doubt help clear away all smoke very quickly but there is no assurance that it will be running at the time when it is most needed. A simple way to help out the ventilation without the expense and noise of a motor, would be to arrange the ventilating flue directly above the arc lamps so that heat rising from these would cause a draft. This can be further helped out by locating all resistances in this flue. They will then be out of the way in a safe place and assist in ventilating the room. The ventilating flue will need to be fitted with a damper for use in cold weather. This damper should be arranged to open by gravity and be held closed by a string connected to the shutter strings so that the damper may open when the shutters close.

The floor of the operating room is a very important part of it. It should, above all, be extremely rigid so that the machine may be fastened solid and thus obviate any vibration which will cause movement of the picture. All floors that are constructed of wood are somewhat elastic and aid this vibration. Movement of the picture is especially noticeable and annoying to those compelled to sit close to it. Floors must furthermore be lined with fireproof material and the only material used is sheet iron which, upon the floor, is very noisy and can hardly be nailed sufficiently to eliminate the noise. A metal floor lining has the further disadvantage of affording a good “ground,” so that any one standing upon it and touching a live part of the lamp is liable to receive a severe electrical shock. It will also soon wear through, make pockets to catch dirt, and allow a fire to communicate with the woodwork beneath it. The best floor material is cement, but this, when wet, also becomes a fairly good conductor and shocks may easily be obtained while standing upon it. There is no reason, however, why it should ever be wet in an operating room and it is therefore advisable to use cement.

The floor of the operating room should be kept clear as far as possible. Careless operators often allow their take-up magazines to become deranged and when they fail to work, allow the film to run on the floor. A thousand feet of film scattered loosely about occupies considerable space and is very likely to become badly tangled in case there is anything on the floor for it to tangle with. The writer has witnessed cases where it was necessary to take some of the film outside of the booth in order to get it straightened out. Any operator who has been in a similar predicament will appreciate the advantage of having every possible obstacle to a speedy re-adjustment removed. Above all, resistances and other sources of heat should be kept off the floor.

Every moving-picture machine requires at least two openings; one through which the picture is projected, which need not be very large; another through which the operator must view his picture, and this must be large enough to allow him to sit in comfort while watching the picture. In some operating rooms the opening through which the picture is projected is closed by a metal funnel which covers the opening and extends back enclosing the front of the projecting lens. This arrangement is very simple in the case of machines in which the revolving shutter is behind the lens. With many machines, however, this shutter works in front of the lens and the funnel would have to be extended so as to enclose the revolving shutter.

There is no reason why strong clear glass could not be used in the operator’s peep hole. This would of course interfere slightly with the clearness of his vision for focusing, but every operator should be provided with an opera glass for this purpose anyway, so that this objection would be easily overcome.

The same considerations which apply to motion-picture machines also applies to stereopticon lamps.

In the more pretentious houses a spot light is generally maintained. This is for the purpose of illuminating vaudeville performers. The opening, provided for this lamp, must be large enough to allow the light to follow an actor about the stage, and glass cannot well be used in it.

The best color for the operating room is green but it must be a dull dark green. Every vestige of light maintained in the room helps dull the vision of the operator to the light upon the screen. There is, furthermore, much light reflected from the film where the light strikes it and it is unavoidable that the operator should be bothered by this at times. All other light should be kept away from his eyes while the picture is running.

All of the openings in the room must be equipped with fire shutters. These are generally made of heavy iron and are arranged so that, if necessary, they can be instantly released and made to close all the openings, thus preventing the escape of fire and smoke into the auditorium.

The best arrangement of such a shutter is illustrated in Figure 130 at A. The shaded portion shows the opening; above is the iron slide, preferably one-sixteenth inch metal held up by a string. At the top is a step which will prevent the shutter from being pulled up out of the guides, for many operators would do this if the step were not there.

It is best to mount the shutter on a large piece of heavy metal which can then be bolted to the wall of the room. The hole in the wall can then be cut to the right proportions. When the machine is in place the shutter can be placed very accurately. In this way it will be possible to arrange for the smallest possible opening through which the machine can work.

Making up the shutter, as suggested above, is further of great advantage in old operating rooms of flimsy wood construction. If the guides in such places are fastened independently to the wall it is possible that they will not always remain in line with each other. A slight giving of the flimsy wall on one side may cause them to bind the shutter and prevent its free descent. If the metal on which the shutter guides are mounted is strong and heavy and the guides themselves substantially made, there will be but little chance to get them askew.

In that part of the guides in which the shutter runs there must be no screws, rivets, or anything else that could work loose and obstruct the shutters. Good methods of making up the guides are shown in Figure 130, at B and C. They should be loose enough to work without lubrication. At the bottom there should be a bumper to stop the fall at the proper place. This bumper should be padded with some fireproof or slow-burning material and should be narrow. If it is made wide the operator is apt to use it as a shelf for tools or other things and this would prevent it from closing properly in case of fire.

All of the shutters must be normally self-closing and must be held open by some arrangement which can readily be worked by the operator; or, in case he fails to close them, they should be arranged so that the flames will cause them to close automatically. This is often arranged for by simply attaching a light strong string to each shutter, running it through a hook eye directly above and attaching it to a master string which is led over every place at which there is thought to be a likelihood of fire. The idea is, that the first spread of flames shall strike this string and by burning it, release all of the shutters. In order to enable the operator to drop the shutter perhaps before the flames have spread at all, the string is carried to the door and fastened so that he, on leaving the room, may quickly release it. It is even better to carry it still farther so that one can release it from the outside.

Some people have recommended that these strings be soaked in oil to make them more inflammable and tarred rope has even been made mandatory for similar purposes in ordinances. This might make the string more readily inflammable, but it would at the same time make it last longer. A string or rope so treated would act like a lamp wick and the better soaked or tarred it might be, the longer would it last and thus fail to its purpose.

The principle of the arrangement of strings is shown in Figure 131. The string is fastened at the door in a manner allowing of easy and quick removal. Instead of the strings, light chains may be used provided that at a sufficient number of places fusible links are inserted. Such links are made up of an alloy which melts at a low temperature and allows the shutters to drop just as they would in the case of a string burning in two.

In many cases the strings are run over grooved pulleys. This is, however, inadvisable. Experience has proved that very often the string has come out of the groove and is wedged in between the shaft of the pulley and the side so that it will not allow the shutter to descend at all. A substantial hook eye is much better than any pulley. The strain being taken off the string when it separates, there is no friction and it slides through the eyes very readily. The door and the vent flue may all be brought under control of the same string so that all will operate together, the vent of course opening while the others close.

Where there is but one door there is little need of connecting it with the shutters, since, if it is provided with spring hinges it will naturally close after the operator has passed through it. In some operating rooms, however, there are two doors and both are often found open. In case of fire in such a booth it is extremely likely that one of the doors would be left open unless it is provided with some automatic manner of closing.

At D, Figure 130 is shown a shutter, provided with strong spring hinges, which closes over the opening from the outside. Such shutters should not be used where there is room for the drop shutters. Where it is necessary to have them, they should always be arranged on the outside of the booth; if on the inside there is too much chance of obstacles being in the way to prevent their closing.

It is very desirable that all of the shutters be closed after each show. If this is done there will be strong assurance that they will be in working order in case of need and this is about the only way in which they can be kept in order. The string arrangement above described is, however, not well suited to this purpose unless each shutter string be detachable from the main string. There are also various other ways of accomplishing this which will readily suggest themselves to the operator.

In every operating room there should be ample provision for keeping tools but it is advisable to have all tools, that are not kept in the lockers, on hooks instead of shelves. It is best to limit the amount of shelving to what is actually necessary. Shelving offers great temptation for the accumulation of rubbish. There must be a shelf upon which to locate the rewinding apparatus and upon which to place the machine while looking it over or making repairs, also a shelf to hold a small vise, etc. But this is all; any additional shelving will be inexpedient. A locker for tools can easily be arranged under the rewind shelf. A good safe place is an encouragement to an operator to keep a supply of good tools and nothing is more essential to the proper management of a machine than good tools.

If the operating room is large enough, it may also contain a locker for clothes but this should be fireproof. Preferably, however, the clothes locker should be outside of the room, since a film fire gives one but little time to pick up clothes or anything else.

There should, further, be a metal pail or receptacle of some kind for hot carbons taken from the lamps from time to time. A wise operator will also provide himself with a small receptacle in which he can keep the pieces of film which he finds it is necessary to cut out of films received from the exchanges. The inspection of films in the exchange is often very perfunctory and many lengths are left in that should be cut out. Cutting them out and saving them for a time will often help an operator to win out in disputes with his film exchange. There are operating rooms which have film boxes built into the wall in fireproof manner, some of them even surrounded by water. At the present time there are a number of good portable boxes on the market so that this is unnecessary.

Wiring of Operating Room.

Wiring of Operating Room.—Every operating room must have at least two circuits run into it; one for the arc lamp and another for one or more incandescent lights. In the larger cities an emergency lighting system is required for all parts of the house used by the audience. This system is always entirely independent of the main service and is supposed to furnish sufficient light for the audience in case the other lighting system should fail. It will be well if one light connected to the emergency system is arranged in the operating room. If this is done, there must be special provisions that no cords, fan motors, or portables be connected to it. A short circuit, or trouble of any kind on this light might put the whole emergency system out of operation. If such a light is installed, it should be at the ceiling and protected by a strong wire guard fastened to the outlet box. All wires carried into the operating room should be in conduit, with the exception of the asbestos covered wires used with arc lamps and resistances.

In addition to the incandescent light above mentioned, there should be several other lights so arranged that good light can be obtained in any part of the room. The color of walls being dark there is but little reflection and a single light will only illuminate a small space. One light should be arranged above the rewind reel and there is often another arranged under a piece of glass in a shelf above which films are patched. Only the best reinforced cord should be used and all work that is stationary should be in conduit. Several outlets should be arranged for portable cords so that one may have a light to bring down to the take-up reel or close to the floor if necessary. Every incandescent lamp in the operating room should be protected by a substantial wire guard fastened to the socket.

Fan motors should be provided with strong brackets located near the ceiling and close to the outlet from which the motor is to be run. Fan motors should never be allowed upon the floor.

Where rewinding is to be done by motor, it will be well to arrange a separate circuit for this motor independent of all lights. The commutator part of the motor should be enclosed so that there may be no possibility of a spark igniting the film.

Figure 132 is a sketch illustrating the arrangement of wiring by which two arc lamps may be connected to mains of sufficient carrying capacity for one only. F is the fuse box in which the arc lamp fuses are enclosed and may be large enough to accommodate a set of fuses for the incandescent-lamp circuit also. In many cases these are, however, taken from the house circuit since the meter rate for arc lamps is usually different from that for incandescent lamps. S is the main arc-lamp switch by which the whole installation in the room can be cut off. This is necessary especially if so-called compensarcs, economy coils, or transformers are used, for these always take some current if left alive even if the arc lamp be not burning. At C is the resistance or economy coil which serves for both lamps and at S is the throw-over switch. This switch is provided to make it impossible to burn the two arcs at the same time as this would overload the small capacity mains. The above is a cheap arrangement and is installed only where a set of mains exists which is too small to supply more than one arc at a time. It is not to be advised for a new installation because it is of great advantage to be able to burn both arcs at the same time. When changing rapidly from one machine to the other it is of advantage to have the second one on for a short time before the first is off, not only to save a little time, but also for bringing electrode points to the proper shape. Wherever it is possible to do so always provide mains of sufficient size to feed all of the arcs and give each lamp its own fuse, switch, and resistance or transformer.

Figure 133 shows a method by which it is possible to arrange for either one of two arcs to be run from a rectifier R, indicated by the square at the left, or transformer T, at the right. In case rectifiers, heavy transformers that cannot well be raised to the ceiling, or motor generators are to be installed, they should be located in a separate enclosure which must be fireproof.

In general, all of the wiring in the operating rooms must be installed in accordance with the rules given in the chapter on Theater and Stage Wiring. These are the rules of The National Board of Fire Underwriters and are familiarly known as the “National Electrical Code,” or “N.E.C.”

CHAPTER XVI.
CURRENT CONTROL FOR ARC LAMPS.

Voltages Required.

Voltages Required.—The commercial distribution of electrical energy is at voltages of 110 or 220, in most cases, and occasionally runs as high as 550. The direct-current arc requires for its best operation a voltage of from 45 to 50, while the alternating-current arc uses from 30 to 40. In order to secure satisfactory operation of arc lamps, it becomes necessary to provide some means of reducing the voltage at the arc to the proper amount.

Resistance Control.

Resistance Control.—The simplest method and the one universally applicable is that of inserting resistance in series with the arc. The drop in voltage is equal to the current multiplied by the resistance; hence, if we wish to decrease our voltage, say 65 volts, as would be the case in connection with a 110-volt circuit and a 45-volt arc, using 25 amperes, we should need a resistance of 2.6 ohms. Twenty-five times 2.6 equals 65 volts lost, which leaves 45 volts to operate the arc with. In connection with arc lamps, however, it is not only necessary to lower the voltage but some provision must be made so that the current, when the electrodes are brought together, will not become excessive. At the time the arc is struck, i.e., at the time the electrodes are brought in contact with each other, the current is limited only by the extra resistance in the circuit, for the electrodes then form a short circuit. In the above case, 110 volts and a rheostat with 2.6 ohms resistance we should obtain, during the time the electrodes are together, a current equal to 110 divided by 2.6 which equals about 43 amperes. If it were not for this resistance the current would rise to several times this value and blow out any fuse we might provide. It is not necessary that this resistance be in any particular place; if we have a very long run of small wire from the service to the arc, there may be sufficient resistance in this so that very little extra resistance is required. Somewhere, however, there must be some provision inserted in the circuit to prevent the current from becoming too great at the time the electrodes are brought together. In passing it may be noted at this point that an arc can be started without any resistance in the circuit by bridging the space between the electrodes with a small fuse wire which will melt the instant the current is turned on and establish the arc.

The resistance method is very wasteful of energy, as the following tabulation will show; but with direct currents it is the only method available, unless we are willing to provide a motor generator to give us the proper voltage. For alternating currents, resistances are not much used, except with traveling shows where the portability of the control as well as its fitness for all possible conditions is an important consideration.

TABLE IV.
SHOWING WASTE OF ENERGY WITH USE OF RESISTANCES FOR VARIOUS VOLTAGES.

The tabulation in Table IV shows that the higher our voltage the greater the loss of energy caused by the use of resistances. The figures apply, as given, only to cases where only a single arc lamp is used. Where several can be used in series the loss due to high voltage need not be greater than with lower voltage.

In Figure 134 we have shown diagrammatically the usual representation of a resistance. The more wire there is in circuit, the higher will be the resistance and the greater the drop caused by a given current. If we lengthen the arc, the current will be somewhat decreased and the drop in voltage over the resistance will be less, thus allowing a rise in the voltage at the terminals of the lamp. The energy lost in resistances takes the form of heat and all resistances, used for the control of arc lamps, give off much heat and must be located in safe places. The heating also makes them objectionable in small operating rooms in summer, but somewhat welcome in winter. The heat generated in a wire is proportional to the square of the current; hence if we double the current through a certain resistance we shall have four times the heat.

If several resistances are connected in series the total resistance will be equal to the sum of the individual resistances, and the current will be correspondingly decreased. If we wish to get more current than can be obtained with the use of one resistance we may connect up two or more in parallel. Two equal resistances connected in parallel will give approximately double the current that can be obtained through one of them.

Reactance Control.

Reactance Control.—A reactance, shown diagrammatically in Figure 135, may take the place of the resistance in alternating-current circuits and is preferable because it wastes comparatively little energy. It lowers the voltage over the arc but its operation depends upon a counter e.m.f. which opposes the impressed e.m.f. of the circuit and must be subtracted from the latter. The nature of reactances as well as of transformation, etc., has been fully treated in another work of the authors’, entitled, “Alternating Current Theory, Practice and Diagrams” and would carry us too far were it to be discussed in this work. Every reactive coil is made up of copper wire wound upon an iron core and contains both resistance and reactance. So far as the resistance in it is concerned, this causes a waste of energy, but it is always very small. There is also a waste of energy due to the hysteresis and eddy-current losses in the iron, but this is also small.

The reactance is proportional to the square of the number of turns of wire, if the iron core is fixed, and may be controlled either by adjusting the position of an iron core in a helix, or by adjusting the number of turns of wire around a fixed iron core. The light obtainable through a reactance is not of the best quality and reactances are not much used.

Transformation Control.

Transformation Control.—Another method of lowering the voltage is by means of the transformer. A diagram of an ordinary transformer winding is given in Figure 136. The fine-wire winding is the primary winding or coil, if the transformer is used to lower the voltage; and the other is known as the secondary winding or coil. The energy in both coils of the transformer, neglecting the iron and copper losses, is always exactly equal. The ratio of voltage between the primary and secondary terminals is in direct proportion to the number of turns of wire in each. If there are half as many turns in the secondary winding as in the primary, the voltage will be just one-half, but the current will be double. The transformer is self-regulating, within the limits of its capacity, and whatever energy is taken from the secondary, the primary will automatically supply.

A transformer must be specially built for the voltage and frequency, at which it is to be used; but many of them are provided with taps, such as shown in Figure 136, by which small adjustments of voltage or current can be made. A transformer must always be connected so that the switch, when open, will disconnect the primary wires. If these remain closed there will be a small current through the primary winding which will mean a considerable waste of energy.

Auto-transformer Control.

Auto-transformer Control.—The auto-transformer is a special type of transformer used to obtain reduced voltage and increased current. Its principle may be gathered from Figure 137. There is an iron core and two coils of wire as in ordinary transformers but the two coils are connected in series, as shown in the figure. It will also be noted that the arc is connected directly across one of the coils. The lower portion of the winding or coil is traversed by the alternating current from the mains at all times and this current also passes through the arc lamp when the circuit through it is closed. The current passing through the lower coil and the arc induces a current in the upper portion of the winding and these two currents then pass in parallel through the lamp.

If the reduced voltage were to be obtained from an ordinary transformer, the secondary coil would be called upon to carry the full current used by the lamp, while with this connection it carries much less. If the two coils are equal, the voltage will be reduced one-half, the current will be doubled, and only half of the current will pass through the secondary coil. The nearer equal the primary and secondary voltages are, the greater the saving in copper in the secondary coil. If it were intended to transform from 110 to 100 volts, the capacity of the secondary winding would need to be only one-eleventh of the total capacity. The auto-transformer is a very useful device but on account of the fact that the high voltage exists in all of its parts, it is not safe to use with the high commercial voltages outside.

If the auto-transformer is connected as shown in Figure 138, it can be used to raise the voltage; but in this case the current will be decreased. These auto-transformers, as well as the ordinary transformer, must always be connected to the source of energy by means of a switch so that they may be disconnected when not in use; otherwise there will be a small current in the primary circuit all the time which will show up quite strong on the watt hour meter. Transformers and auto-transformers are arranged to be portable. A general view is given in Figures 139 and 140; the former being the Edison and the latter the Fort Wayne.

Motor-Generator Control.

Motor-Generator Control.—The proper voltage for the operation of arc lamps can be obtained by the use of motor-generators. A motor-generator is a generator driven by a motor, the two armatures being placed upon one shaft or belted together. The motor may be driven by a current of any voltage desired. The diagram of such an outfit for direct current is shown in Figure 141. This type of machine is used, as a rule, only where the supply voltage is much higher than that used at the arc. Resistance must be used at each arc lamp.

Figure 142 shows the connections of an alternating-current to a direct-current motor-generator of the Fort Wayne Electric Company. The switch A is used to start it and is shown connected to a three-phase line. Aside from the field winding there are three wires leading to the generator. The wire B carries a compound winding inside of the generator which opposes the magnetization of the shunt winding. The wire C carries another compound winding which is arranged to strengthen the shunt field. D is a box containing two resistances, one for each arc lamp shown.

If only one lamp is to burn, the switch E is closed and the arc started in the usual way. When ready to change to the other arc lamp, switch E must be opened, the switch on the second arc lamp closed, and the arc struck. Then extinguish the first arc and close the switch E again. If both lamps are to be used continually, switch E must be left open.