Starting and operating of fluorescent lamps

Oct. 14, 1947. r. w. Keiser, et al 2,429,162

STARTING AND OPERATING OF FLUORESCENT LAMPS

Filed Jan. 18, 1943

Sheets-Sheet 1

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Oct 14, 1947. r. _w keiser etal 2,429,162

STARTING AND OPERATING OF FLUORESCENT LAMPS

Filed Jan. 18, 1943 3 Sheets-Sheet 2

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Oct. 14, 1947. r. _w. keiser etal 2,429,162

STARTING AND OPERATING OF FLUORESCENT LAMPS Filed Jan. 18, 1943 3 Sheets-Sheet 3

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Patented Oct. 14, 1947

2,429,162

UNITED STATES PATENT OFFICE

2,429,162

STARTING AND OPERATING OF FLUORESCENT LAMPS

Russen William Keiser and Charles Philippe Boucher, Fostoria, Ohio, assignors to Boucher and Keiser Company, Atlanta, Ga., a partnership

Application January 18, 1943, Serial No. 472,768 (Cl. 315—278)

Claims.

. 1

This invention relates to fluorescent lamps and more particularly to circuits and apparatus for starting and operating such lamps from normal sources of power supply.

It is a general object of the present invention to Provide novel and improved apparatus and circuits for cold cathode starting and normal oper^{one or more lamps} of the fluorescent type ^f om a commercial source of alternating current.

More particularly it is an object of the invention to provide circuits and apparatus capable of Providing the necessary excess potential for the cold cathode starting of lamps of the fluorescent type and for subsequently providing the exact operating potential and current for the normal lighting of these lamps.

It is an important object of the invention to provide the additional lamp starting potential without using any increased quantity of copper or iron above that necessary to provide the normal operating voltage and current for the lamns under optimum conditions.

It is an important feature of the invention to provide a transformer, adapted to be interposed between a normal source of alternating current at commercial potential and one or more lamps of ^tbe fluorescent type whereby for regular operation ^^aI?^PS L^SuitabIe P^otential and current is provided, together with automatic means to temporarily give an increased turns-ratio between the primary and secondary of the transformer for supplying a substantially higher potential available to start the lamps by the cold cathode method

Another important feature of the invention consists in the switching arrangement, which under cold lamp conditions so associates the parts of the circuit, that, upon closing the main power switch a high starting potential is applied to the lamps and is automatically reduced upon lighting f the lamps to that necessary for their normal 40 operation.

An additional object of at least one embodiment of the invention consists in the automatic switching of current limiting resistors into the lamp circuits during the starting operation to limit the ⁴⁵arc currents to values low enough to ensure against damage to the filaments of lamps built ^f°v hot cathode ^starting but used in accordance with the present invention for cold cathode starting.

A still further object of that particular embodiment includes delayed action switching which is timed to occur when the filaments of the electure^eas^h_a^aresmt^aji^iethA^{a P}t°^Per ®™^ssive tempera- power source u e as a result of the action of the striking arc. 55 heated. After

Other and further features and objects of the invention will be more apparent to those skilled in the art upon a consideration of the accompanying drawings and following specification wherein are disclosed a number of exemplary embodiments of the invention with the understanding that they may be so modified or combined as to fall within the scope of the appended claims without departing from the spirit of the invention.

In said drawings:

Figure 1 is. a schematic and diagrammatic showing of a circuit and apparatus for starting and operating a pair of fluorescent lamps from a conventional source of alternating current, making use of a heat actuated switching device for effecting the voltage changes.

Figure 2 is a view similar to Figure 1 but showing a relay type magnetic switching device;

Figure 3 is a view similar to Figure 1 but show20 ing a modified thermal type of starting switch;

Figure 4 illustrates a circuit in which the reactances normally used for regulating the current flow to the several lamps are combined with the secondaries of the transformer;

Figure 5 shows a further embodiment of the invention;

Figure 6 shows a still further embodiment for the operation of a plurality of lamps;

Figure 7 is a diagrammatic arrangement of a ³⁰ core structure which may be used when magnetic shunts are indicated in the transformer magnetic circuits; and

Figure 8 illustrates a further embodiment of the invention incorporating delayed action switching from starting to operating conditions together with automatic current limiting features active only during the starting operation.

With the increasing use of fluorescent lamps for general lighting purposes there has been an insistent demand for an improved system for starting such lamps, since the ones in vogue at the present time interpose too much delay between the closing of the power switch and the actual striking of the arc which causes the illumination in the lamp.

Most lamps are now lighted by making use of starting systems involving the filaments in the two ends of each lamp which are first heated to incandescence after which a high potential resulting from a reactance kick following a circuit change is relied on to establish the arc. Certain automatic starting switches are provided which first connect the two filaments in series into the ^POTZ.^e2L ^s01^^{ce un}til they become adequately a certain time delay the starting

2,429,162 tween terminals 24 and 26 representing the normal or operating primary. A source of alternating current at a suitable commercial potential, is shown at 28 connected through a switch 29 ana wire 30 to the terminal 26. In normal operation the terminal'24-is also connected to'the source of power 28 through wire 31, contact 32 of switch 33, movable contact 34 of this switch, and conductor 35. Under these conditions the turns•10 ratio between the primary 24, 26, and the secondary 23, 24, is such as to provide the exact optimum operating potential from the source 28 to the lamps 10 and II, the current being held to the -predetermined proper amount by the design of the 15 reactors 16 and 17. ,,

For starting fluorescent lamps by the cold catnode method a voltage at least 50% higher than that necessary or desirable for normal operation is required and in this embodiment of the invention it is provided by changing the turnsratio between the effective portion of the primary ahd the secondary. This-means a reduction in the number of turns on the active section of the primary and for this purpose a tap is made at 37 and connected by wire -38 to contact 39 of switch 33. Under the proper positioning of the contacts in the switch, as shown in the drawing, this connects the source 28 to the section 26, 37 of the primary, having less turns than the whole or normal operating portion of the primary and because of the now higher turns-ratio between secondary and primary an additional voltage is supplied to the lamps for striking the arcs and starting them.

, The switch illustrated -at 33 is only one of a number of types suitable for changing from starting to operating conditions. The switch shown is preferably enclosed in an evacuated bulb _ to reduce sparking or arcing and to thermally insulate a bi-metallic element therein from the surrounding atmosphere. The switch comprises three contacts 32, 34, and 39 each carried on the end of a long arm. The supporting arm 40 for •the -contact 34 is a; bi-metallic element, and is 45 initially adjusted to close the circuit between contact 34 and contact 39 when the element 40 is cold This conditions the circuit for starting the lamps arid the arcs jump through the lamps upon closing the switch 29 because the curtailed section of the primary is energized and produces -a high secondary voltage. During this starting operation current flow to the primary is held to desired safe limits by the resistor 42 interposed in conductor 38.

The current flowing to the primary through the bi-metallic element 40, resulting from the operation of the two starting arcs is sufficient to heat the arm 40 and cause the contact 34 to move over and engage contact 32. Since this is a relatively slow operation, it is desired to have contact 39 remain in engagement with contact 34 until the circuit 32, 34, is closed, thereby preventing extinguishing of the arc. This is readily effected by having the supporting arm 45 for contact 39 of spring material and ' biased toward contact 32. Continued movement of the arm 40 toward the right after 34 engages 32 causes the spring support 46, for contact 32, to be pressed sufficiently far to the right so that contact 34 is disengaged from contact 39. The type of switch represented by 33 may be said to comprise what is known as a “make before break’’ relay. If high speed operation can be effected, the starting circuit can -be broken just instantaneously before closing the switches eliminate the filaments, as such, from the circuits and connect them so that they act as electrodes, at opposite ends of the lamp, between which the arc is struck.

In accordance with the present invention the filaments are use.d_;.solely as electrodes and,are never heated by the passage of current through them as filaments. The arc is struck in each tube by applying a voltage between the opposite filaments of a potential much higher :than the nor- . mal operating potential, and as soon as . the arc is struck, this voltage is reduced to just the requisite amount to maintain-that, characteristic of arc which will provide the.desired light Λ™?’ sion from the lamp as well as the length of life commensurate with that provided by the use of the previously described type of starting operaIt has heretofore been proposed to start fluorescent type lamps by the use of high potentials with cold, cathodes, but these systems have either failed.to properly reduce the voltage for normal operation, whereby the life of the lamp was shortened, or have necessitated the use of too great quantities, of,copper .and iron, in the starting and operating apparatus. .In accordance with the present invention.a minimum use is made or these critical .materials,and;no additional .material, above that required for normal operation, is-included in the apparatus in spite of its ability 30 to provide a high striking voltage for starting

Ref er ring, fir st to Figure 1 for _rone.embodiment of the invention, there .is .illustrated a pair pf conventional fluorescent .lamp tubes ,10 and 1.1 each having , end filaments 1,4 .each of which is connected to a pair of pinslS.at .one end of the tube. These pins are ..connected together, as shown in accordance with,the present invention, so that the .filaments, are individually shorted and act-solely:'as electrodes. To.limit the current supplied to the .fluorescent type lamps, which have what is known as “.negative resistance,” suitable reactors IS and 17 are associated with the lamps.

Where two similar temps are simultaneously lighted on one circuit it is preferred to use a dual form of reactor such as .illustrated in the co-pending application of Russell W. Keiser, Serial No. 445,047, filed May .29, 19,42, for .Reactances, .which issued October 30, 1945 as United States Patent No. 2,387,797, in which the .two windings such as 1.6 and .1.7 are mounted on a single core provided with appropriate air gaps to produce high leakage reactance. Obviously the lamps are connected in parallel so that either one alone : could be independently operated if desired without unbalancing the system.

One end of one of the windings 16 is directly connected by conductor 19 to one end of the tube 10, while the other winding 17 has one end connectedly conductor .19 to .one end ,of tubp 11. a suitable condenser .20 is Interposed in the ^conductor 19 to improve the power factor oi tne whole circuit and obviate stroboscopic effects from the lamps as is well-known.

The .common terminal 21 of the two reactors is connected by wire 22 to one end 23 of the secondary of a special transformer and the opposite end 24 of this secondary, is connected by Wire 25 to the .remaining terminals at the opposite ends of both lamps. .

In this embodiment the transformer is of tne .autOTCbnnected type, the full number of turns of ;the winding between terminals 23 and 24 representing the secondary and a section thereof be40

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3,429,162 operating circuit and the arc will not be extinguished. The resistor ¢2 previously mentioned prevents a complete short circuit of that section of the primary between 24 and 37 while all contacts 32, 34, 39 are in engagement. 5

The starting operation is of extremely short duration so that the momentary overloading of both the primary and the secondary as well as the core of the transformer has no detrimental effect so that this transformer can be designed 10 to have only the requisite amount of both copper and iron to take care of its normal operating load. By using this system starting is achieved in an improved manner without the necessity of using any additional material in the system over 15 that used for normal operation since the starting switch 33 need not be substantially larger than the usual starter, one of which is supplied with each lamp in the hot cathode starting sys^tem· 20

The circuit of Figure 2 does not differ substantially from that of Figure 1 and a detailed description of the whole circuit is not essential. The main difference is in the switch which is of the magnetic type rather than the thermally _2goperated type of Figure 1. It comprises a relay-Iike device having an operating coil or magnet 60 interposed in the conductor 6i leading from terminal 24 of the secondary of the transformer to the left hand ends of both lamps. 30 The armature 62 of the relay is connected by conductor 63 to the source of power 28 in the same manner as the bi-metallic arm of switch 33 while the opposite end of this source of power is connected to the primary terminal 26 of the 35 transformer. The back contact 64 of the relay, with which the armature is in engagement when the magnet is unenergized, is connected through resistance 42 and wire 65 to the tap 37 on the primary of the transformer. The front contact 40 66 of the relay is connected to the wire 61 which leads from the coil 60 directly to the terminal 24 of the secondary of the transformer.

In operation, with the parts in the position shown, a closing of the main switch energizes the 45 section of the primary between the terminals 26 and 37 and produces a high secondary voltage which flows to the lamps striking arcs therein. When these arcs strike, current flows in the winding 60 from the secondary of the trans- 50 former, energizes the magnet and transfers the armature from the back contact 64 to the front contact 66 so that current from the source 28 is transferred from the tap 37 on the primary to the terminal 24 thereof. There is always a hold- 55 ing current which flows in the magnet 60 to maintain it in operating condition as long as the lamps are lighted. The contacts of the relay may either be arranged, in any well-known manner, to close at 66 before breaking at 64 or re- 60 course may be had to the speed of operation to eliminate the necessity for this. In the event that it is desired to permit the starting arcs to flow in the lamps for a short interval, to heat up the tubes to insure operation at the lower normal 65 temperature, the relay may be made of the slow acting type, by any of the well-known expedients, such as the placing of a large copper slug 68 on the core thereof.

In Figure 3 is shown a system for obtaining a 70 change in turns-ratio between the primary and secondary windings wherein the source of power 28 is arranged to be connected always to the whole primary winding between terminals 100 and 101. An auto transformer is again shown. 75

The secondary of this transformer is represented by the whole winding between 101 and i02. The secondary is tapped at 103 to produce a section 101, 103 for supplying the normal operating potential to the lamps. This is indicated .as having fewer turns than the primary for use with lamps requiring less operating potential than that provided from the source.

The two lamps and their reactors are connected exactly as in Figure 1 and it remains only to describe how either the whole secondary or a portion of the same is used for energizing them. The lead (04 from the center of the reactors is connected to the bi-metallic element 105, of an evacuated switch 106, which carries the contact 107 normally in engagement with contact 108 connected by wire 109 to the end of the secondary at 102 through the interposed resistor 110. Under these conditions a high secondary to primary turns-ratio exists and a striking potential is provided between points 101 and 102 of the transformer for starting the lamps. Within the envelope 106 is glow terminal 112 connected to an intermediate position 113 on the secondary of the transformer. This electrode, being at a different potential than any of the other electrodes in the switch envelope, which may be suitably filled with a quantity of ionizable gas and provided with an emissive material causes a glow discharge to be set up, the heat from which causes the bi-metallic supporting arm (05 of contact 107 to move toward the right and eventually engage contact 115 subsequent to which it disengages contact 108 in the same manner as described in connection with Figure 1. When this operation is completed only that portion of the secondary between 101 and 103 is applied to the lamps for operating them. There still remains within the switch a difference of potential between the glow electrode and the others which maintains the glow to hold the bi-metallic element stressed toward the right during all of the operation of the lamps. When the main circuit is opened to extinguish the lamps the glow is extinguished and the contact 107 returned to the position illustrated in the drawing ready for a restarting of the lamps.

In Figure 4 is illustrated a circuit in which a transformer having a primary separate from· the secondary is used. This circuit also leads to the elimination of the reactors for the lamps, their function being performed by the separate secondaries one for each lamp in a manner which will be later described. Referring now to this drawing it will be seen that the separate primary winding comprises a number of turns between terminals 200 and 201, there being an intermediate tap at 202. The source of power 28 is arranged to be connected by suitable switching contacts 204 to either the whole primary or that portion of the same between 200 and 202. When in the second position the resistor 205 is interposed in the circuit.

Two identical secondaries, where identical lamps are used, are represented at 207 and 208, the first being connected through a condenser 209 to lamp 2(0 and the second being directly connected to lamp 2((. The two lamps are connected together at 2(2 and through the small winding 2(3 back to the connected ends of the secondaries. The winding 213 represents diagrammatically the magnet for operating the contacts 204 in the same manner as the magnet 60 in Figure 2.

It will be seen that when the arrangement of

2,429,162 'contacts is such that the source of power is connected between 200 and 202 a high -secondary to primary turns-ratio exists and striking voltages are provided in each secondary, whereas when the whole primary is energized a lower or operating voltage is provided for each of the lamps.

In order to prevent excessive current flow to each of the negative resistance -lamps, suitable magnetic shunts are arranged in the common core of the transformer to provide a high leakage flux : which produces a counter-E. M. F. in the primary winding and hence reduces the current flow and the output of the -transformer. This leakage reactance may be achieved through the use of a core section somewhat as illustrated in Figure 7. Here the core includes three connected legs 215, 246, and 247, the center one of which is provided with the primary winding and the outer ones with the secondary windings respectively. The connecting portions of the core structure at the top and bottom are designated 2IB and 219 respectively. The magnetic shunts are shown at 220 and 22 ί and may be provided with air gaps as at 222 if desired. These same shunts are diagrammatically represented in Figure 4 at 220 and 221 as existing between the sections of the coils on -the common core.

The resistor 205 shown in Figure 4 serves the same purpose as the resistor 42 in Figure 3, that of limiting the current when only a portion of the primary is in operation. The circuit of Figure 4 uses a minimum of critical materials since no separate reactors are required and no additional metal is needed in the transformer above the absolute minimum used for normal operating.

The circuit of Figure 5 is not substantially different from that of Figure 2 except that the primary 300, which is tapped to change the turnsratio in the manner described in connection with Figure 4, is entirely separate from the secondary 304 instead of being arranged as an auto transformer as shown in Figure 2. The operating coil 303 for the relay is shown connected between one end of the secondary and the intermediate tap on the reactors 304.

The circuit of Figure 6 is similar to that of Figure 5 but here the single secondary 400 is connected to one end of each of three reactors 401, 402, and 403. These may be on a common core, as shown and constructed in accordance with the 50 ^prllnaryj previously mentioned application or they may each be independent. The opposite end of the secondary is, through wire 404, connected to one end of each of the three lamps 405, 406, 407. The .opposite ends of each of the -reactors are connected to the remaining ends respectively of the several lamps. The circuit of at least one of the lamps is provided with the power-factor changing condenser 408. The relay operating coil 410 is so interposed in the secondary circuit that current to all three lamps must flow through it. In this manner, by properly designing this coil, the circuit may be arranged so that there is no transfer of the source of potential from the partial to the whole primary until an arc has struck in each 05 of the three lamps. This prevents premature operation which might leave one or two lamps unilluminated. It will be noted that this characteristic also applies to the arrangements of some ment electrodes intended to be heated for starting, and it has been found particularly desirable, especially for outdoor operation, to use high current densities for their starting since this vaporizes the mercury more readily and heats up the gas within the tube quickly. Such lamps, however, are not readily available on the market and recourse is usually had to the so-called filament electrode type of lamp intended for hot cathode starting by heating the two filaments to incandescence. These filaments are coated with an electron emissive material and are readily damaged by excess operating temperatures so that care must be exercised, in starting by the cold cathode method, to limit the current densities to those which are safe and will provide normal or extra life for the filaments. Unfortunately the current limiting devices, such as reactors or the like, used to hold down the normal operating current to a safe value are of insufficient impedance to limit the starting current at the much higher starting voltages.

In the embodiment of the invention illustrated in Figure 8 a circuit arrangement is shown which provides additional impedance to the current only during the starting operation and switching mechanism is arranged not only for inserting and removing this impedance but for making the change-over from the starting to the operating 1 voltage and vice versa in a manner somewhat similar to those already described.

Referring now to the figure, it will be seen that it makes use of an autotransformer 500 although obviously separate windings might be used in the manner illustrated in Figures 4 and 5. This transformer contains a two-section primary, that bearing the reference character 501 being used for starting purposes, and this portion plus the section 502 being used for operating purposes whereby, under the latter condition, the ratio of turns to that of the secondary winding 503 is lower.

A source of alternating current 504 and a main switch 505 are connected to the end -506 of the winding on the transformer and, through an aumatic switch including contacts 507 and -598 which are always closed when the switch 505 is open, this source of current is also connected to the terminal 507' of the starting section of the . The terminal 58S of the transformer is connected by conductor 509 to the left-hand ends of the lamps 510 and 512, the current passing by way of conductors -514 through a relay coil 515. The secondary terminal 5*8 is con55 nected by a wire 519 to the junction 520 of a pair of starting resistances 521, whose opposite ends are respectively connected to the two ends of the balanced reactor coils 522 and 524, exactly like those shown in Figure 1. The remaining ter60 minals of the reactor coils 522 and 524 are connected to the opposite ends of the .lamps 510 and 512, a condenser-525 being interposed in one of the circuits to improve the-power factor and eliminate stroboscopic effects.

Under the conditions illustrated and just described, the closing of switch 505 energizes -primary section 501 of the transformer and a high potential is induced between the terminals S3» _________________________ and 518 of the transformer which is applied to of the other figures and is largely controlled by 70 the opposite ends of the -lamps through the cur_ , — — , ..... Lil.. ft- — ft ZX nLftuft EO O Λ xi CO Λ ΛΛ 7 VX OZXVl ΛΓ· the use of so-called marginal relays which do not close unless the -current flow through them exceeds a predetermined minimum.

Fluorescent lamps which are especially de40 rent limiting-reactors 52-2 and 524 each in.series with one of the resistors 521. The reactors 522 and 524 are designed to hold down the operating current to the-desired amount when the voltage signed for cold cathode starting do not have fila- 75 is relatively-low,-and are thus insufficient to limit

2,4129,162 the starting current to the desired low figure when the voltage is much higher. They are therefore augmented by the simple resistors 52 i in series therewith for this purpose. For operating the lamps, however, it is desired to increase the number of turns in the primary so that its ratio to the secondary shall be lower, whereby the overall induced voltage applied to the lamps will be sufficiently low for continuous operating purposes, when the reactors alone are adequate to limit the current. The switching operation to transfer from starting to operating conditions must therefore include means to remove the resistors 521 from the lamp circuits.

A composite relay shown in the dotted line box 530 takes care of all the switching. It is actuated by the coil 515 which is adequately energized only when the arcs strike in the two lamps, since it is inserted in the main conductor 509 leading thereto. When this coil is energized, it pulls down its core attached to the insulating frame 531 which closes the spring mounted contacts 532 and 533, the former being connected by wire 534 to terminal 535 at the end of the operating primary comprising the sections 501 and 502. Lower contact 533 is mounted on the same spring as the lower contact 508 on the switch which closes the starting section of the primary. When the spring carrying contact 532 moves downwardly, a circuit is closed at 532, 533, and one is opened at 507 and 508, thereby applying the source of alternating current across the two primary sections 501 and 502 in series, and disconnecting it from the single section 501. This provides proper operating voltage from the whole winding of the transformer.

A three-contact switch is also operated by the frame 531 on its downwardly movement and serves to shunt the two resistors 52 i for which purpose the middle connection 520 thereof is connected to the spring carried contact 537, while the opposite ends of the two resistors are connected respectively to spring mounted contacts .538 and 539. When the frame 531 is drawn downwardly by the relay winding 515, the three contacts 537, 538, and 539 are drawn together, shunting out the resistors and connecting the inner ends of the reactor windings directly to terminal 518 of the secondary. Operation is now exactly as illustrated in connection with Figure 1 or 2.

For starting a hot cathode type of lamp by the cold cathode method, it has been observed that the best results are obtained if this is achieved in three steps: first, the striking of an arc of just sufficient intensity so that it will locate itself at some point on the filament of the electrode which is coated with emissive material; second, the regulation of the arc current to just a sufficient intensity to bring the striking point on the filament to an electron emissive temperature in a predetermined time; and third, the transfer from starting voltage to operating voltage only after the filaments have attained this emissive temperature. The circuit just described in connection with Figure 8 permits obtaining these desired results since the intensity of arc current can be regulated by changing the size of the resistors 52 i. The time elapsed between closing the starting circuit and transferring to the operating circuit can be regulated by means of the relay 530 which may be of the time delay type. The time delay may be obtained by any well known means, such as the use of a thermal relay, a dash pot, a mercury pocket, or the well known use of a heavy metal slug such as shown at 540 associated with the core or winding of the relay. It will be appreciated of course that there is no current flow in the relay winding until the striking of the arcs in the 5 lamps, although the starting current may have been applied for several seconds. The time delay interval will therefore occur between the instant of striking of the starting arcs or arc and the time of transfer from the starting to the operating 10 condition. This period can be regulated, in the initial design of the relay, or can be made variable to take care of different types of lamps. If a cold cathode type of lamp is used, no time delay is necessary and the transfer can take place 13 immediately upon the striking of the starting arc.

The above described circuit arrangement is of importance, for the contacts are so disposed in the circuit that only one pair of them opens under load. The contacts 507 and 508 are separated 2o while carrying current. The others are only closed under what might be termed “load conditions,” but since contacts 532 and 533 practically shunt 507 and 508 while they are being opened, there is not much chance for these latter con25 tacts to burn. The remaining contacts do not open until the main switch 505 has been opened and all current has stopped flowing. They can therefore be relatively small, not only because they do not open under load but because they 30 carry relatively low currents and for only brief intervals during the starting operation. A simple and compact relay of the telephone type can therefore be used which is good for millions of cycles of operation.

As an example of the values of voltage, current, resistance, and the like, the following may be noted as an optimum arrangement for controlling two 40-watt fluorescent lamps as arranged in the circuit of Figure 8:

Operating alternating current source—110 volts Operating voltage across the lamps—200 Striking voltage across the lamps—400 Resistance of resistors 521—1200 ohms each _4g Capacity of resistors 521—20 watts each

Striking current—.2 ampere for each lamp Operating current—.4 ampere for each lamp Current in relay coil 515—.4 ampere Resistance of coil 515—8 ohms ₅₀ Operating loss in relay—3.2 watts Time delay in relay—2.5 seconds

The arrangements of circuits and apparatus as shown are suitable for the operation of any desired negative loads and cannot be the cause of 55 any uncontrollable potential or current disturbances. Under ordinary circumstances starting voltages are needed only for short periods, say for considerably less than one second to a maximum of 30 seconds. Such voltages are not dan60 gerous in equipment designed for normal operation at lower voltages because of the short time interval precluding any undue heating in the copper or iron.