FIGURE 142.
As long as current is used through wire B, there is no loss of energy in any resistance and should the current in the arc rise, as when the electrodes are brought together, the increased current in the series winding, cut into this wire, would weaken the field and thus keep the current down. When current is used through the wire C, the series field winding strengthens the field and builds up the voltage sufficiently so that the lamps may be operated through the resistances. The field strength may be further regulated by the rheostat R.
Another connection of the Fort Wayne motor-generator is shown in Figure 143. In this case the lamps may be operated either from the compensarc C or the generator. By throwing either one of the switches connected to the arc lamps up, the corresponding arc lamp is connected to the compensarc. By throwing the switch down it is fed from the generator. The lamp, by which the picture is being projected, should be fed from the generator and when nearly ready to change, the other may be started on the compensarc. This lamp will burn with a short arc and when it is connected in parallel with the one on the generator, it will immediately extinguish the latter. No attempt must be made to burn both arcs from either the compensarc or the generator. This generator is also wound to protect itself against an overload.
FIGURE 143.
Where these connections are to be installed, it will be best to consult the local inspection departments concerning the necessary fusing for the compensarc and the generator. In some localities the possibility of throwing both arcs on either compensarc or generator, where these have capacity for but one arc at a time, will be considered very objectionable.
FIGURE 144.
Another combination of motor and generator sometimes used is shown in Figure 144. By tracing out the circuits it will be seen that the armatures of both are in series and that the electrodes, when they come together, form a shunt about B. With the electrodes separated, if current is turned on, it must pass through both armatures in series. Thus the counter e.m.f. of both armatures opposes that of the line and they operate at a certain speed. Each motor has a natural tendency to send current in opposition to that impressed upon it by the line. If then the electrodes are brought together, they at once form a short circuit around the armature of B. The current in B reverses and it then begins to act as a generator and sends current through the arc lamp. The current which passes through the armature of A also passes through the arc lamp. A is then a motor and operates B as a generator.
The voltage at the arc is less than the line voltage by as much as the counter e.m.f. of motor A amounts to, neglecting the drop in voltage due to resistance. No resistance is needed if the winding is properly arranged and there is not the less in heat which goes with the use of resistances. This arrangement can be used with direct-current circuits only. It is not suitable where the supply voltage is very much higher than the voltage used at the arc. A field rheostat is provided to adjust the field strength of B. A is equipped with the ordinary motor-starting rheostat only.
Rotary Converter Control.—This is a machine used only where the supply is alternating current. The voltage delivered to the converter must be the same as that desired at the direct-current terminals. This machine has an armature essentially similar to that of a direct-current dynamo. Alternating current is supplied to it at one set of terminals and direct current is taken from the others. This armature acts as motor and generator at the same time. Whatever voltage regulating is necessary with this machine must be done on the alternating-current side. Changing the field strength does not materially affect the voltage so that no means for regulating the field strength is provided.
The polarity of the direct-current terminals depends upon the position the armature happens to be in when the alternating current is applied to it and is very apt to come in wrong when the machine is started. It is therefore necessary to have a polarity indicating voltmeter in the circuit and to watch it when starting the machine. If the polarity is wrong, the switch must be opened and in a moment thrown in again; and if still wrong, this process must be repeated until the polarity comes right. Each arc lamp fed from a converter must be equipped with resistance.
FIGURE 145.
The Martin rotary converter is especially designed for motion-picture work and may be provided with the proper connections for either single-phase, two-phase, or three-phase work. There is a stator ring which entirely surrounds the armature. This ring is made up of laminated disks with squirrel-cage bars and slots alternating. The squirrel-cage bars are joined at the end to a copper bar and it is by the aid of this squirrel-cage that the motor may be started and brought into step. The squirrel-cage also prevents “hunting” which is one of the common troubles experienced with synchronous motors or converters. Into the slots are wound special compensating coils to balance the armature reaction and keep the neutral point in constant position from no load to full load. This prevents sparking at the brushes. On the outside of this damper ring or squirrel-cage winding is the regular shunt-field winding used with direct-current motors or generators.
Figure 145 is a diagram showing the connections of the Martin Rotary Converter as installed by the Northwestern Electric Company of Chicago. This switchboard is equipped to operate two moving-picture arcs, two dissolving stereopticon lamps, and one spot light. Each lamp is provided with a throw-over switch so that current may be used, either from the alternating-current mains direct or from the direct-current side of the converter.
Figure 146 is another panel board for moving-picture work made up by the same company. In this case resistances are provided for use when the arc lamps are operated from the converter. In case it is desired to run from the alternating-current mains, transformers or compensarcs are used. The emergency feature of these panel boards is highly to be recommended. It must be borne in mind that one may suddenly be forced to deal with an operator who has never seen a converter and knows nothing of its operation; and there is also always the possibility of some trouble with the machine.
FIGURE 146.
A Martin rotary converter to be operated from a single-phase line is shown in Figure 147. This machine is started through the commutator side. In order to start this machine it is necessary first to close the main switch. Next throw the switch 2 to the right and leave it there for about five seconds. It may then be thrown over to the running position at the left and allowed to remain in this position. If the polarity is not correct, the switch must be opened again for an instant and closed again; and this process must be repeated until the polarity comes in right. To stop the converter, first open the main switch and then the throw-over switch. The manner in which the above machines are preferably set up is shown in Figure 148.
FIGURE 147.
FIGURE 148.
The Mercury-Arc Rectifier Control.—The mercury-arc rectifier has three essential parts: the rectifier tube, the main reactance, and the panel. The rectifier tube, Figure 149, is a glass vessel from which the air has been exhausted and in which there are two graphite electrodes, A and A´, and one mercury electrode B. From the two upper electrodes current can pass in the direction of the mercury only. They are always positive and the term anode is usually applied to them. B is always negative and the term cathode applies to it. Each anode is connected to a separate side of an alternating-current circuit and is thus subject alternately to positive or negative potential.
When current has once been started, the tube is filled with ionized mercury vapor through which the electricity can flow, from whichever of the two anodes is positive, toward the cathode B. Under no conditions, however, can electricity flow from the mercury in the tube toward the anodes. The action of the tube is started by tilting it sufficiently, so that the mercury in the bottom of it connects the starting anode C to A. This starts the current and when the tube is returned to its upright position, the mercury bridge from C to A is interrupted; but the current then continues from one or the other of the anodes.
Should the current be interrupted, even for an instant, the tube would cease to work until it had been tilted again. In order to provide that the current, which is alternating and comes to zero twice in every cycle, may never cease in the tube, it is necessary to provide some reactance. Such a reactance causes the current to lag behind the e.m.f. and in consequence lap over the time when it would otherwise fall to zero. While the current from the rectifier is always in the same direction, positive from B to the lamp, it is also a pulsating current changing in value to some extent.
In Figure 149 a complete diagram of the connections of the General Electric Company Mercury Arc Rectifier for moving-picture arcs is given. This type of rectifier is entirely automatic and is much used. The front and back connections are shown in Figure 150. The following instructions are taken from a publication of the General Electric Company:
The leads marked A C should be connected to the lower side of a double-pole switch located near the moving-picture machine. The upper studs of the switch should be connected to the A C source of supply.
FIGURE 149.
The leads marked + and - should be connected, respectively, to the positive (upper) and negative (lower) electrodes of the moving-picture lamp.
If the A C supply voltage is 110 volts; then connect the flexible lead marked Z to stud marked 12; and flexible lead marked Y to stud marked 6.
If the A C supply voltage is 220 volts; then connect lead Z to stud 7, and lead Y to stud 1.
Note:—Do not disturb the other connections that are made on studs 1, 6, 7, and 12, but only place leads Y and Z as directed.
FIGURE 150.
The tube holder should be reversed so that the clip and support will be turned away from the panel instead of towards the panel, as it is when shipped.
Remove the tube from its box, being very careful not to handle it roughly and not to strain the seals in any way whatever. Care must also be taken to prevent the mercury from suddenly flowing into any of the arms; otherwise the resultant pounding might damage them.
Examine the tube for vacuum by noting the sound the mercury makes when allowed to roll gently about in the large chamber. If it makes a clear, metallic click, the vacuum is good; but, if the sound is dull and the mercury sluggish in moving, the vacuum is either partially or wholly destroyed. If the vacuum is poor, the life of the tube may be short or it may not start at all. To insure careful handling and safe delivery, Mercury-Arc Rectifier tubes are always shipped by express in the special box as they come from the factory.
Place the tube in the holder by inserting the small part of the tube just above the anode arms in the upper clip; then gently lower it until it rests firmly on the lower support. Connect the tube and beaded leads according to the above diagram.
Adjustment of current (number of amperes) at the arc is obtained by connecting leads marked X to studs marked 11, 9, 7, 5, 3, or 1 of the regulating reactance. Stud 1 gives the maximum and stud 11 the minimum number of amperes. In starting up the first time it is best to start with lead X on stud 11 and move toward the maximum position by steps until the desired current is obtained, as indicated by the ammeter. For this adjustment it is advisable to connect an ammeter in series with the arc in the moving-picture machine.
With the above instructions carried out, all that is necessary to start is to close the switch in the A C line; then bring the electrodes of the arc together. The automatic shaking device should then rock the tube until the arc in the tube starts; as soon as the arc in the tube starts separate the electrodes.
The best and whitest light can be obtained when a 5⁄8-inch cored-carbon electrode is used above and a 1⁄2-inch solid-carbon electrode below, care being taken not to get solid carbons too hard. The average current in the arc should not exceed 30 amperes and it will be found that excellent pictures can be obtained with 25 amperes or even less and the cost of energy, carbons, and condensers will be less.
Operation of Generators.—The generator should be located in a clean dry place. If it is belt driven, the belt should be run horizontal if possible and so that the slack side will be on top. This increases the arc of contact with the pulleys and allows the belt to run with less tightening. The frame should be provided with a slide for the purpose of adjusting and tightening. The proportion between the largest and the smallest pulleys used close together should not be greater than about 6 to 1.
To start a dynamo it is best first to disconnect it from the switchboard. Start it running and adjust the voltage by the field rheostat. When the voltage has arisen to its proper value and everything is running smoothly, the main switch may be closed. If there is much of a load, it will probably be found that the voltage has fallen off a little and it will be necessary to re-adjust it. Next, look carefully after the brushes and set them at the points where there will be the least sparking. A good modern generator should not spark at all. All of the bearings must be carefully looked after and watched for heating. If they are not properly oiled or in good condition, they may heat considerably. The armature should run with considerable end play as this helps to distribute the oil over the bearings and even up the wear on the commutator surface.
Small generators sometimes lose their residual magnetism and it is then impossible to start them generating. In such a case the fields may be connected to a live-lighting circuit; or a small exciting current may be obtained from a battery. One should know which is the positive pole of the field and apply the battery or line current accordingly. A test for polarity may be made by placing the ends of wires of opposite polarity in a vessel with water and bringing them within an inch or so of each other. Under these conditions bubbles will be given off at the negative pole. The polarity of the generator will vary with the polarity of the field, the direction of rotation, and the connection of the brushes. By reversing any one of these we may reverse the polarity of the current delivered. Shunt dynamos cannot well be operated in parallel; where it is desired to operate several dynamos together, compound-wound machines are used.
Operation of Motors.—The speed of a direct-current motor is always such that the counter e.m.f. of the motor becomes nearly equal to the impressed e.m.f. of the line. In order to speed up a motor it is necessary to weaken the fields; and conversely, to slow it down we must strengthen the fields. The above methods are necessary if the motor is to run at a nearly fixed speed with a variable load. The speed can also be controlled by a variable resistance placed in the armature circuit. This method, however, does not result in a steady speed with a variable load. It has very little effect if the load is light, and very much if it is heavy.
In order to start a motor it is necessary to have some resistance in the armature circuit. In very small motors the armatures are usually wound with sufficient resistance so that no external resistance is required. The larger motors are, however, equipped with starting boxes which limit the current through the armature until it has attained sufficient speed so that its counter e.m.f. will keep the current in check.
These starting boxes are usually wound with fine wire and cannot stand the starting current very long. The handle must be moved over steadily and slowly and not allowed to remain on an intermediate position unless it is known that the box is meant to be used as a speed controller as well as a starting box. The direction of rotation of a direct-current motor can be reversed by reversing either the field or the armature current. If both are reversed, it will continue to run in the same direction.
Alternating-Current Motors.—Synchronous alternating-current motors are not used in theater work but the rotary converter is frequently used and may be considered as such. This machine must run at a certain speed which depends upon the frequency of the current supplied and the number of poles on the machine.
Rotary converters are of different types and may be started either from the direct-current side or from the alternating-current side. Some of them are provided with connections so that the alternating current may be applied on the direct-current side of the armature. For starting and operating these converters, the instruction of the maker should be consulted.
For motor-generators where alternating current is used, the induction motor is generally employed. It may be either single-phase, two-phase, or three-phase. All of these are essentially constant-speed motors. Simple repulsion motors and single-phase induction motors that start as repulsion motors are reversed by shifting the brushes.
Alternating-current series motors are reversed in the same manner as direct-current motors by reversing either field or armature. If both are reversed, the motor will continue to run in the same direction.
A three-phase induction motor will reverse its direction of rotation if any two of the line wires connected to the primary winding be reversed. If all three wires are changed in order, the direction of rotation will remain unchanged. Synchronous motors when started as induction motors are also governed in this way.
These motors of the larger size are generally started through an auto-transformer. For the smaller ones, it is customary to provide a throw-over switch, one side only of the switch being fused. The motor at starting takes very strong currents which would blow the running fuse. To reverse a two-phase induction motor the two wires of one phase must be reversed.
Be sure that all belts are sufficiently tight.
See that all bearings are well oiled.
Let all shafts have sufficient end play.
Use no oil cans of iron around dynamos or motors.
Keep files and other iron or steel away.
See that all connections are good and tight.
Allow no metal dust or gritty substances to accumulate at the insulation of exposed parts.
See that the brushes fit properly and do not cut or scratch.
Use no emery paper on commutators.
Lubricate the commutators very sparingly and wipe off as much of the lubricant as you can.
Keep everything about the machines clean and allow no oil drippings to accumulate.
Place the starting box for a motor so you can see the motor start from the box.
Always place a switch which will disconnect all of the wires close to the motor.
If possible arrange motors so they may start without load.
Allow no motor or generator to be placed in the operating room of a moving-picture theater.
The wireman should not fail to consult local rules or inspection departments as to whether any rules conflict with those given below. He must be warned to consult local authorities or rules, too, because safety rules are liable to change.
The purpose of this chapter is to furnish a ready reference work concerning questions of electrical construction in theaters which come up daily in all progressive play houses. To this end the subjects have been arranged in alphabetical order and the practical considerations, as well as extracts from the National Electrical Code governing construction, have been given together. The aim has been to enable the workman to find all information concerning construction work grouped together, so as to obviate the necessity of looking through various parts of the book for the information sought. This order of things will probably avoid the troubles now often caused by overlooking certain points that should be considered.
Aisle Light.—Figure 151 is an illustration of an aisle light. Such lights are often placed along steps and aisles. The light illuminates only the floor. Aisle lights should be arranged on a separate circuit and controlled by a switch at the door.
Alternating Current.—All wires of any circuit or mains or sub-mains of any system must be run in the same conduit. Failure to do this will cause an unnecessary drop in voltage and heating of the conduit.
Arc Lamps.—For treatment and construction of portable arc lamps, see the chapter on “Portable Stage Equipment”.
Permanently located arc lamps are used about theaters mostly for out-door lighting. Very often, two or more lamps are arranged in front of the house. Such lamps are mostly of the flaming arc lamp type and are hung up high.
FIGURE 151.
In some of the cheaper theaters a pair of arc lamps is used on the stage, but they do not give satisfaction. The light is not even and steady enough and cannot be properly “dimmed”. Where arc lamps are to be arranged for stage illumination they must be suspended amid the scenery and enclosed with wire guards. In some cities the use of arc lamps suspended above the stage is prohibited.
In the auditorium, arc lamps are sometimes installed, but this practice can not be recommended and, with the present high efficiency incandescent lamps, there is but little excuse for it. The only advantage in using arc lamps lies in the first cost of wiring, and this is more than balanced by the difficulties of trimming lamps located in such places. Wherever arc lamps are used it is essential that they be hung high and those that do not naturally throw the light downward must be equipped with suitable reflectors. The question of drop in voltage need not be considered with arc lamps unless runs are very long.
Must be provided with reliable stops to prevent carbons from falling out in case the clamps become loose.
All exposed parts must be carefully insulated from the circuit.
Must, for constant-current systems, be provided with an approved hand switch, and an automatic switch that will shunt the current around the carbons, should they fail to feed properly.
The hand switch to be approved, if placed anywhere except on the lamps itself, must comply with requirements for switches on hanger-boards.
Terminals must be designed to secure a thoroughly good and permanent contact with the supply wires, which contact must not become loosened by motion of the lamp during trimming.
Spark arresters must so close the upper orifice of the globe that it will be impossible for any sparks, thrown off by the carbons, to escape.
Must be carefully isolated from inflammable material.
Must be provided at all times with a glass globe surrounding the arc, and securely fastened upon a closed base. Broken or cracked globes must not be used.
Must be provided with a wire netting (having a mesh not exceeding one and one-fourth inches) around the globe and an approved spark arrester, when readily inflammable material is in the vicinity of the lamps, to prevent escape of sparks of carbon or melted copper.
Outside arc lamps must be suspended at least eight feet above sidewalks. Inside arc lamps must be placed out of reach or suitably protected.
Arc lamps, when used in places where they are exposed to flyings of easily inflammable material, must have the electrodes enclosed completely in a tight globe in such manner as to avoid the necessity for spark arresters.
“Enclosed arc” lamps, having tight inner globes may be used, and the requirements of b and c above would, of course, not apply to them.
Where hanger-boards are not used, lamps must be hung from insulating supports other than their conductors.
Lamps when arranged to be raised and lowered either for carboning or other purposes, shall be connected up with stranded conductors from the last point of support to the lamp, when such conductor is larger than No. 14 B. & S. gauge.
Must have a cut-out for each lamp or each series of lamps.
The branch conductors must have a carrying capacity about fifty per cent in excess of the normal current required by the lamp.
Must only be furnished with such resistances or regulators as are enclosed in non-combustible material, such resistances being treated as sources of heat. Incandescent lamps must not be used for this purpose.
Must be constructed, so far as practicable, similar to arc lamps of theaters, and wiring to same must not be of less capacity than No. 6 B. & S. gauge. See “Portable Stage Equipment”.
Must be of approved type, insulated from ground and controlled from switchboard, each receptacle to be of not less than 35 ampere rating for arc lamps nor 15 amperes for incandescent lamps, and each receptacle to be wired to its full capacity. Arc pockets to be wired with wire not smaller than No. 6 B. & S. gauge and incandescent pockets with not less than No. 12 B. & S. gauge.
Plugs for arcs and incandescent pockets must not be interchangeable.
Armored Cable.—All wires in the stage part of theaters must be enclosed in conduit or armored cable. Armored cable is thus the only flexible conductor allowed for permanent work. This cable is very convenient where wires must be “fished”, or run around beams or other obstacles making many bends necessary. It should, however, be used only where the rigid conduit cannot well be installed and it is advisable to use the latter, even where the additional expense is considerable. Wires run in rigid conduit can be taken out and replaced by new ones at any time, while this is not the case with armored cable. Should a wire incased in armored cable develop a serious fault, the old cable would have to be abandoned and a new circuit run, which in many cases would mean the tearing up of parts of the building.
FIGURE 152.
Where armored cable is to be used great care should be exercised to see that bends are not made too short, and that each length of cable is tested for grounds, short circuits, and open circuits. Special attention must be given to the wires at the place where the armor has been cut. Careless workmen can do great damage at this point. The manner of cutting the armor is shown in Figure 152. Each strand of the armor is partly cut with a saw and may then be broken off, taking care that no sharp edge is left in position to pierce the wire.
Installation rules are given below. Before installing any armored cable, be sure that it is of approved make and guaranteed to pass inspection.
Must be continuous from outlet to outlet or to junction boxes or cabinets, and the armor of the cable must properly enter and be secured to all fittings, and the entire system must be mechanically secured in position.
In case of service connections and main runs, this involves running such armored cable continuously into a main cut-out cabinet or gutter surrounding the panel board, as the case may be.
Must be equipped at every outlet with an approved outlet box or plate, as required in conduit work.
Outlet plates must not be used where it is practicable to install outlet boxes.
For concealed work in walls and ceilings composed of plaster on wooden joist or stud construction, outlet boxes or plates and also cut-out cabinets must be so installed that the front edge will not be more than one-fourth inch back of the finished surface of the plaster, and if this surface is broken or incomplete it shall be repaired so that it will not show any gaps or open spaces around the edges of the outlet box or plate or of the cut-out cabinet. On wooden walls or ceilings, outlet boxes or plates and cut-out cabinets must be so installed that the front edge will either be flush with the finished surface or project therefrom. This will not apply to concealed work in walls or ceilings composed of concrete, tile or other non-combustible material.
In buildings already constructed where the conditions are such that neither outlet box nor plate can be installed, these appliances may be omitted by special permission, provided the armored cable is firmly and rigidly secured in place.
Must have the metal armor of cables permanently and effectually grounded to water piping, gas piping, or other suitable grounds, provided that when connections are made to gas piping, they must be on the street side of the meter. If the armored cable system consists of several separate sections, the sections must be bonded to each other, and the system grounded, or each section may be separately grounded, as required above.
The armor of cables and gas pipes must be securely fastened in outlet boxes, junction boxes, and cabinets, so as to secure good electrical connection.
If armor of cables and metal of couplings, outlet boxes, junction boxes, cabinets, or fittings having protective coating of non-conducting material, such as enamel, are used, such coating must be thoroughly removed from threads of both couplings and the armor of cables, and from surfaces of the boxes, cabinets, and fittings where the armor of cables or ground clamp is secured in order to obtain the requisite good connection. Grounded pipes must be cleaned of rust, scale, etc., at place of attachment of ground clamp. Connections to grounded pipes and to armor of cables must be exposed to view or readily accessible, and must be made by means of approved ground clamps, to which the ground wires must be soldered.
Ground wires must be of copper, at least No. 10 B. & S. gauge (where largest wire contained in cable is not greater than No. 0 B. & S. gauge), and need not be greater than No. 4 B. & S. gauge (where largest wire contained in cable is greater than No. 0 B. & S. gauge). They must be protected from mechanical injury. The ground for the armored cable system is not to be considered as a ground for a secondary system.
When installed in so-called fireproof buildings in course of construction, or afterwards if exposed to moisture, or where it is exposed to the weather, or in damp places, such as breweries, stables, etc., the cable must have a lead covering placed between the outer braid of the conductors and the steel armor. The lead covering is not to be required when the cable is run against brick walls, or laid in ordinary plaster walls unless same are continuously damp.
Where entering junction boxes, and at all other outlets, etc., must be provided with approved terminal fittings which will protect the insulation of the conductors from abrasion, unless such junction or outlet boxes are specially designed and approved for use with the cable.
Junction boxes must always be installed in such a manner as to be accessible.
For alternating-current systems must have the two or more conductors of the circuit enclosed in one metal armor.
All bends must be so made that the armor of the cable will not be injured. The radius of the curve of the inner edge of any bend not to be less than 11⁄2 inches.
Asbestos.—As all wiring in theaters is required to be run in conduit, and metal cabinets are compulsory in this connection, there is but little opportunity to use asbestos. Wherever the use of asbestos is advisable it must conform to the general requirements as given for wooden cutout cabinets, viz.: “for lining wooden cabinets, one-eighth inch rigid asbestos board may be used when firmly secured in place by screws or tacks”.
Attachment Plugs.—Attachment plugs should be used to connect all portable apparatus. All plugs should be of approved type and constructed so as to pull out in case strain is put on them. On the stage, pin-plug connectors should be used in place of attachment plugs, as none of the latter are sufficiently rugged to withstand the hard usage.
Link fuse attachment plugs of the types now on the market are considered unsafe, as under entirely possible conditions an arc may be produced when the fuses blow, damaging the plug and perhaps causing fire. Attachment plugs are not approved for more than six hundred and sixty watts or two hundred and fifty volts.
Auditorium.—Two separate systems of lighting are required: See emergency or exit lighting. Metal moulding, as well as armored cable or conduit, is permissible in wiring the auditorium part of the theater.
Auto-Starters.—Auto-starters perform the same service with alternating-current motors that resistances do with direct-current motors. They are used with motors from two or three horse power upward, and not generally with the smaller motors.
The following are extracts from the “National Electrical Code” concerning their use:
In all wet, dusty, or linty places, auto-starters, unless equipped with tight casings enclosing all current-carrying parts, must be enclosed in dust-tight fireproof cabinets. Where there is any liability of short circuits, caused by accidental contacts, across their exposed live parts a railing must be erected around them.
The switch on the auto-starter must provide an off position, a running position, and at least one starting position. It must be so arranged that it will be held in off and running position, but not in starting position or without the proper running overload-protection devices in the circuit.
For currents above 30 amperes, lugs, into which the connecting wires may be soldered, or approved solderless connectors must be used. Clamps or lugs will not be required when leads are provided as a part of the device.
The following rules are drawn for rheostats but may also apply to auto-starters:
Where the circuit-breaking device on the motor-starting rheostat disconnects all of the wires of the circuit, the switch called for in this section (to disconnect all apparatus) may be omitted.
Overload-release devices on motor-starting rheostats will not be considered to take the place of the cutout required to protect the motor and the rheostat.
Balconies.—The illumination of balconies is a difficult matter. The ceilings under galleries above are always low, and to obtain even illumination requires the use of many small candle-power lamps. These should be set well back so as not to be visible too much to the audience.
Stage-pocket capacity for one or more arc lamps should always be provided. Where there are galleries above, the arc lamps used for stage illumination are generally placed there, but it often happens that a moving-picture machine must be installed and it is very disadvantageous if this must be located in the galleries. Balconies require the same exit and emergency light service as is required in the auditorium.
Batteries.—See Portable Stage Equipment.
Bells.—Systems of call bells are generally arranged between the box office or the manager’s office and the stage switch board; also from the stage switch board to the fly floor by which signals for raising and lowering the curtain may be given; also to the orchestra leader. In some cities all of this wiring is required to be in conduit. These signaling circuits should be carefully installed, for they are as important as any part of the wiring. Only the very best bells and push buttons should be used, and it is advisable to avoid the use of the ordinary annunciator wire so often seen in connection with bell work. Numerous diagrams and much information concerning bell wiring is given in “Modern Wiring Diagrams and Descriptions”, which should be consulted in case some complicated annunciator system is to be installed. Figure 153 is a diagram of a simple call-and-return-call system.