US2735960A

US2735960A - h hamilton

Info

Publication number: US2735960A
Authority: US
Publication date: 1956-02-21
Anticipated expiration: 1973-02-21

Description

Feb. 21, 1956 w. s. H. HAMILTON 2,735,960

STARTING AND OPERATING CIRCUITS FOR FLUORESCENT LAMPS 3 Sheets-Sheet 1 Filed Sept. 29, 1950 WC 0/? DC INVENTOR, William 5- H Hami1tnn,,

ATTORNEY.

1956 w. s. H. HAMILTON 2,735,960

STARTING AND OPERATING CIRCUITS FOR FLUORESCENT LAMPS 3 Sheets-Sheet 2 Filed Sept. 29 1950 INVIIINTORJ William E-H.Hamilt an ATTORNEY Feb. 21, 1956 w. s. H. HAMILTON 2,735,960

STARTING AND OPERATING CIRCUITS FOR FLUORESCENT LAMPS 5 Sheets-Sheet 3 Filed Sept. 29 1950 TEJEL INVENTORZ William 5 H- Hamiltnn a. BY.

ATTORNEY United States Patent STARTING AND OPERATING CIRCUITS F OR FLUORESCENT LAMPS William S. H. Hamilton, Larchmont, N. Y. Application September 29, 1950, Serial No. 187,593

6 Claims. (Cl. 315-100) This invention relates to starting and operating circuits for fluorescent lamps, and particularly for A. C. and D. C. fluorescent lamps of the types which are more or less difficult to start and require an inductive kick to start them.

One object of the present invention is to provide an integral combined thermal and magnetic type starter for use in connection with various circuit organizations, said starter having normally closed starting contacts and a coil adapted to be connected in series with the lamp to hold the starter in operating position and allow cooling of its thermal element without reclosing the starting contacts, the circuits including reactance means for giving an inductive kick when the starting contacts open, whereby the lamp, if in normal condition, will fire and the lamp will burn until the line voltage is interrupted, when the coil will be deenergized and the starting contacts will immediately reclose, enabling the lamp to restart without delay upon another application of line current.

Another object of the present invention is to provide a starter of the character described having one or more sets of contacts adapting it for use in difierent circuit arrangements for A. C. and D. C. fluorescent lamps.

Still another object of the invention is to provide a starter of the type described adapted for use with different A. C. and D. C. circuits and embodying means for locking out the starter after a series of attempts to start the lamp, in the event that the lamp is defective or has reached the end of its useful life period.

A still further object of the invention is to provide a starter which is of simple construction for its purpose or purposes, and may be designed for use for a simple form in connection with different circuits or in varied forms, as regards the number and operation of contacts employed for specifically dilferent circuits, and which admits of its connection and use in a circuit for any particular type lamp in an economical manner.

In the accompanying drawings forming part of this specification- Figs. 1 to 5, inclusive, are diagrammatic views of different lamp circuits as designed and adapted for use in conjunction with a starter of the type described, and

Figs. 6, 7, 8 and 9, respectively, are sectional views of the starter as designed for use with the several circuits disclosed, Fig. 6 showing the positions of the starter contacts with the current off, Fig. 7 the positions of the starter contacts with current. on in normal operation, Fig. 8'

showing the starter contacts in their initial positions for a lock-out action when the starter includes a lock-out means, and Fig. 9 showingtheir position after the lockout has taken place for more than a few seconds.

In carrying my invention into practice, I make use of a thermal-magnetic starter S, which may be of the construction shown in Figs. 6, 7, 8 and 9 and which, as showmincludes a can or casing A having a base B provided with contact prongs of which four, denoted p, p3, p are shown, for ready connection of the starter ice with a socket which is permanently connected into the lamp circuit but which may vary in number and arrangement as occasion requires. for general use may be employed to provide a starter capable of being fitted in a socket suitable for an FS-44 starter and of being reversible therein. This four-pronged construction of the starter may be used with a socket of the type described in all the circuit organizations herein disclosed except those hereinafter particularly referred to where it is necessary to polarize or otherwise fix the relation of the parts of the starter to the parts of the circuit more definitely than can be done with a starter having a feur-pronged base, and which is normally reversible in its position in the socket.

In the casing A is enclosed the starter proper, which comprises an insulated body or support D carrying three sets of contacts S, S S a primary heater H, secondary heaters H H a quick snap action concavo-convex bimetallic dis" I, a magnet coil C and core J, and an armature l. The set of contacts S consists of a fixed contact k and switch member k, the set of contacts S of a fixed contact I (Fig. 8) and switch member I and the set of contacts S of a fixed contact in and a bimetallic switch arm in, which sets of contacts are normally closed with current oil, as shown in Fig. 6. Contact switches k and Z are insulated from each other but are connected to each other mechanically and to the disk I and armature J by the rod or stem j, but are not connected in any way with the contact In or bimetallic switch arm 11;. The contacts S, S are normally operated together by the action of the thermostatic disk I when the disk is heated by the primary heater H to reverse its curvature. This causes the contacts S, S to open and the armature I to come into close proximity to the coil so that the current through the coil can hold it with the contacts in the running position shown in Fig. 7. Contacts S are arranged to break their circuit slightly ahead of contacts S, so that the magnetic pull of the coil will assist in opening contacts S, thus giving a rapid break to contacts S. Contacts S are arranged so as to open their circuit only under the action of the secondary heaters H and H as shown in Figs. 8 and 9. A condenser C is provided across contacts S' to absorb the arcing when they open.

In the operation of the starter, when the heater H has heated the starter sufiiciently, contacts S S move to the position shown in Fig. 7 to fire the lamp. If the lamp fires, the contacts are held in this position by the coil C acting on the magnetic structure, and the heater H cools off Without any effect on the operation. The secondary heater H when used, is of sufficient capacity to generate enough heat to move switch S to the open position for a lock-out action, as shown in Figs. 8 and 9, without a flow of current through the coil.

While the thermostatic portion of the starter described above is similar in principle to the one described in my Patent No. 2,667,604, any kind of thermostatically responsive element of the quick acting type (i. e. one whose element moves quickly from one position to the other, after it has been sufiiciently heated) can be used in the various starters described herein provided it is of sufiiciently small physical dimensions.

The starter S as shown is adapted for use in connection with difl'erent circuits in which contacts S alone or in conjunction with contacts S may be used with or without contacts S The contacts S and heaters H and H may be omitted from the starter structure where the lock-out feature is not used in the lamp circuits, and contacts S may be used alone or in connection with contacts S in various hard starting circuits having quick restarting actions but not designed to use a lock-out feature. I accordingly do not limit the invention to the A four-pronged socket number of sets of contacts which may be used in the starter. By the use of a starter of the construction disclosed, however, a type of starter is provided which is adapted for use in hard starting lamp circuits of various types for A. C. and D. C. lamps and employing or not the lock-out feature as desired.

In Figures 1 to 5, inclusive, I have shown various types of circuits for fluorescent lamps in which the starter S in the form disclosed, or in the modified forms above described, may be used to secure prompt starting and rapid restarting actions of a lamp in good order and which may be used to secure a lock-out action of the starter to avoid undue repetitive starting attempts if the lamp is defective or has reached the end of its useful life period.

Furthermore it is to be understood that in circuit organizations such as shown in Figs. 1 and 3 the positive filament 3 may be short-circuited or open-circuited as shown, depending on the characteristics of the lamps used in the circuits.

Fig. 1 shows a circuit without the lock-out feature and designed for use on 60 or 110420 volts D. C., which consists of an electric discharge device of the low pressure type, suchas fluorescent lamp, 1 having negative and positive electrodes 2 and 3, a starting or preheating conductor comprising sections 4 and 5 connected respectively with the positive line terminal and the electrode 2, the return conductor 6, a starter S having a single set of contacts S and the heater H interposed between the conductor sections 4 and 5, the magnet coil C arranged in the operating conductor 7 connecting conductor 4 with the electrode 3, a reactor R in the conductor 4, a current limiting resistance R in the conductor 5, and a ballast lamp L in the conductor 7. it will be understood, of course, that the starter in this circuit also includes the bimetallic disk I, armature 1, rod j coupling the parts k, I and I together, the magnet coil C and the core I.

In operation when the line switch is closed, current flows through the reactor R, through contacts 8 of the starter, heater H of the starter, resistance R and the negative filament 2 of the lamp back through conductor 6 to the negative side of the line. At the same time a circuit is established through conductor 7 from the low side of the reacator R, through the ballast lamp, through coil C of the starter and to the positive side of filament 3 of the fluorescent lamp, which is shown as being short circuited.

After the starter has been heated by the heater H for a predetermined length of time, which corresponds to the appropriate time for heating the negative filament preparatory to starting the lamp, the bimetallic disk reverses itself from the position shown in Fig. 6 so that it assumes the position shown in Fig. 7 and opens contacts S. Spening of contacts S causes the reactor R to give an inductive kick to fire the lamp, and the lamp, if in normal condition, fires. When this occurs, current flows from the positive side of the line through the reactor R, through the ballast lamp L, through coil C of the starter, through the positive filament 3 of the lamp, through the gas in the lamp to the negative filament 2 of the lamp and back through conductor 6 to the negative side of the line. Coil C is purposely made strong enough so that it'will hold contacts S open after heater H has cooled off, to prevent contacts S from reclosing. The lamp will then continue to burn until the line voltage is interrupted, when the coil C will then be deenergized and the starter will immediately return to its starting position (Figs. 1 and 6) ready to restart the lamp without delay upon another application of line current.

If the lamp does not fire, the starter will continue to attempt to start it until either the lamp starts or the starter continues indefinitely in attempting to start it until the line voltage is cut oil.

Fig. 2 shows a circuit without the lock-out feature which is designed for use on A. C. or D. C. and which employs a starter having two sets of contacts S, S in connection with the magnet coil C, bimetallic disk, I, armature 1, rod j and heater H. In this circuit arrangement both lamp electrodes 2 3 are heated. As shown, conductor 4 is arranged to cooperate with switch contacts S and to be connected thereby with the end of conductor 7' connected in series with the electrode 3 and through the same with a conductor 8 arranged to cooperate with contacts S and a conductor 9 connected with heater H in the conductor 5 in operation, current fiows from one terminal of the line through the reactor R, through contacts S of the starter, through filament 3 of the fluorescent lamp, through contacts S of the starter, through heater H of the starter, through limiting resistance R1, through the other filament 2 of the fluorescent lamp back to the other terminal of the line. It will be noted that during this condition the coil C of the starter and the ballast lamp are short circuited by contacts S so that they take no part in the starting action.

When the starter has been heated sufiiciently by heater H, the disk reverses and contacts 5 and S open. Contacts S on opening, cut the ballast lamp L and the coil of the starter C into circuit with the lamp, while contacts S, on opening, break the inductive circuit through the reactor R, which gives a kick to fire the lamp. if the lamp fires, current flows through the reactor R, through the ballast lamp L, through the coil C of the starter, through filament 3s of the fluorescent lamp, through the gas in the fluorescent lamp to the other filament 2 and back to the other side of the line. So long as the lamp continues to burn, the starter remains in this position, for when heater element H cools off, the coil C still keeps the contacts S and S in their open position as shown in Fig. 7. If the line current is cut off, the starter immediately returns to the position shown in Figs. 2 and 6, ready for another start as soon as the line circuit is reclosed.

If the lamp fails to fire, however, the starter makes repeated efforts to start it, and the lamp either fires or the repeated efforts cause flashing in the lamp that calls attention to the fact that there is a defective lamp.

The starter employed in the circuit organization of Fig. 2 requires the use of five instead of four prongs for connection with a receiving socket of special construction.

Fig. 3 shows a special D. C. circuit without the lockout feature and which is designed so that it can be installed in place of certain starters now in use on railway passenger cars having 60 volt D. C. lamps. As shown, preheating conductor sections 4*, 5 are normally connected by switch contacts S. Conductor 4* contains the reactor R and ballast lamp L is connected through a bypass conductor 8 with the lamp electrode 3, which is shown as being short circuited, while the heater H is arranged in the conductor 6 and coil C is arranged so as to be connected across heater H by the switch contacts S As stated, the starter is special in that contacts S are reversed from the arrangement shown in Fig. 6, i. e., they are normally open and close when the starter operates. v

When contacts S close, they connect the coil of the starter across the heater H, and by properly proportion-j ing the coil resistance to the heater resistance, the combined resistance of the two will be low, which is desired in this particular circuit. Also the heat retained in heater H will be small, so that there will be little, if any, delay in the return of the starter to the position shown in Fig. 3 when the line current is turned off.

In operation, current flows from the positive side of the line, through the reactor R, through the ballast lamp L, through contacts S of the starter, through the negative filament 2 of the fluorescent lamp, through the heater H and back to the negative side of the line. At the same time a circuit is established through the conductor 8* agzsaaeo from the ballast lamp to the positive filament 3 of the fluorescent lamp, which is shown as being short circuited.

When heater H has heated the starter sufii'ciently, it causes contacts S to open and S to close. The opening of contacts S breaks the inductive circuit through the reactor R, and fires the lamp, ifit is in normal operating condition. The closing of contacts S puts the coil C of the starter in parallel with the heater H, and by proper design, the coil can be made strong enough to hold the starter in the position in which contacts S are open and S are closed, after heater H cools off. The lamp continues to burn with current flowing from the positive side of the line, through the reactor R, through the ballast lamp L, to the positive filament 3 of the lamp, through the gas in the lamp to the negative filament 2 and back through the coil C, contactsS with heater H in parallel with the coil C.

When the line current is interrupted, the coil C is deenergized, and while there may be a slight delay due to some residual heat in heater H, the starter will return to the position shown in Fig. 3 almost, if not immediately, ready to restartthe lamp.

Fig. 4 shows a starter without the lock-out feature and a circuit arrangement especially adapted for operating long A. C. lamps, such as the 40 watt, 48", T-12 lamps which are extensively used in industrial service. The circuit includes a reactor-transformer R-T whose primary is connected across the A. C. line and whose secondary is arranged in the output portion 5 of the preheating conductor connected with electrode 2. The starter S here is arranged so that its contacts S normally connect that portion of conductor 6 leading from electrode 2 in series with heater H and one end of the electrode 3 and so that its contacts S normally short circuit the coil C and connect the operating conductor branches 7 with each other, and through conductor 6 to one side of the line, the arrangement being such that both electrodes 2 3 are heated in the lamp starting operation.

In operation, current flows from the terminal of the A. C. line marked B, through one winding of the reactortransformer to the other terminal of the A. C. line marked W. Current also flows through the other winding of the reactor-transformer, steppingup the voltage to above that of line voltage, from whence it flows through filament 2 of the fluorescent lamp, through conductor to and through contacts S of the starter, through heater H, through the filament 3 of the fluorescent lamp, through contacts S of the starter, through

conductors

7 and 6 back to the terminal W of the A. C. line. It will be noted that during this condition coil C of the starter is short circuited by contacts S and takes no part in the starting action.

When heater H has heated the thermal disc of the starter sufiiciently, it opens the contacts S and S. Contacts S opening remove the short circuit around the coil C of the starter so that the coil will assist in opening contacts S, and so that it can function to hold the starter open when heater H cools off, while contacts S opening, give an inductive kick to fire the lamp, if it is in normal operating condition. As long as the lamp continues to burn, current fiows from the secondary of the reactortransformer R-T, through filament 2 of the fluorescent lamp, through the gas of the lamp to the other filament 3, through the coil C and back to the other side of the A. C. line at the terminal W.

As long as the line circuit is closed, the lamp continues to burn under this condition. If the line circuit is opened, coil C is deenergized, which permits the starter to return to the position shown in Figs. 4 and 6, in which it is ready to commence another starting action.

If the lamp does not fire, thestarter continues to repeat attempting to start it, until it either fires, or attention is drawn to its being a defective lamp by the repeated flashes.

Fig. 5 shows an A. C. circuit for 48 T'-l'2-lamps which is similar to the circuit shown in Fig. 4, but in which the starter has the lock-out feature which is accomplished by the addition of switch S3 and heaters H2 and H3. Heater H2 is connected across switch S and heater H, and is thus responsive to lamp voltage when the lamp has fired and is operating normally. Heater H3 is connected across the contacts of switch S3 and hence is short circuited until their contacts open. With this construction heater H2 need be only of sufficient capacity to open contacts S3 in a cut out action and heater H3 need be only of sufiicient capacity to hold contacts S3 open after they have been opened by heater H2. When the lamp is operating normally not sufficient heat is generated in heater H2 to cause switch S3 to open, but if the lamp does not fire then line voltage instead of lamp voltage is impressed across H2 while contacts S are open and as this is considerably greater than lamp voltage, one or more applications will be sufiicient to heat H2 enough to cause contacts S3 to open. Once open, heater H3 generates sufficient heat to keep them open, as will be more fully described below.

In operation, current flows from terminal B of the A. C. line through the primary of the reactorransformer R-T back to the other terminal W of the A. C. line. It also flows through the secondary winding of the reactortransformer, which steps up the voltage above that of the line voltage, to the conductor 5. From conductor 5, current flows through filament 2 of the fluorescent lamp, through contacts S1 of the starter, through heater H1 of the starter, through the other filament 3 of the fluorescent lamp through contacts S2 and S3 of the starter and back through conductor 6 to the terminal W of the A. C. line.

The second heater H2 is connected across contacts S1 and heater H1 and is arranged to influence switch S3 only as described above. This heater H2 is of very high resistance compared to that of heater H1, and while the starter is heating for initial firing of the lamp, it is practically short circuited by switch S and heater H and has no effect on the initial firing.

The third heater H3 is also of very high resistance the contacts of switch S3 and is short circuited until they open and so has no effect on the initial firing.

Current flows through the circuit until heater H1 heats the starter disc sufficiently to cause contacts S2 and S1 to open. Opening of contacts S2 allows coil C of the starter to be energized to assist in opening contacts S1 rapidly and to permit holding the starter in the position shown in Fig. 7 after heater H1 cools oii. Opening of contacts S1 breaks the inductive circuit through the reactor-transformer R-T and causes the lamp to fire, if it is a normal one in good condition. When the lamp fires, current flows from lead 5, through filament 2 of the lamp, through the gas in the lamp to the other filament 3 and back through the coil C of the starter and contacts S3 through conductor 6 to the line terminal W. The starter assumes the position shown in Fig. 13.

It will be noted at this time that heater H2 has some heat applied to it due to the voltage drop across the lamp but this does not generate sufficient heat to cause it to open switch S3 and has little or no effect on the thermal disc I so that it does not seriously retard the prompt return of the starter to the position shown in Figs. 5 and 6 when current is cut oil' the line circuit. if the line circuit is interrupted, the starter then returns to the position shown in Figs. 5 and 6, ready for another starting action when the line circuit is rcclosed.

However, if the lamp is defective, or has reached the end of its useful life, it wiil not the on the first opening of the starter contacts S, and may not fire on several other openings. The repeated opening and closing of the starter contacts S gradually builds up heat in heater H2, inasmuch as the voltage applied across heater H2 is much greater when contacts S1 are open and the lamp has not fired, than when contacts S1 are open and the lamp has fired. The accumulative effect of this heat finally causes switch S3 to open as shown in Fig. 8, upon which the starter then moves to the position shown in Fig. 9 where the contacts 52 and Si are closed but switch S3 remains open.

The contacts of switch S3 interrupt the circuit to one filament of the lamp by opening the connection between conductor 7c at filament 3a and conductor 60, except for the small amount of current passing through heater H3. As this heater H3 is a carbon rod in the order of 30,000- 50,000 ohms it will not pass sufficient current to support ghost lighting. This is the passage of current through.

the lamp at low values about A to /2 of normal current, which is usually variable and causes disagreeable flickering. This ghost lighting occurs with many starters of the cut out type, which while they open the starting circuit and prevent further attempts to start the lamp, do not remove potential from across the terminals, and with a reactor-transformer this is usually about double the line voltage when the lamp has not fired.

Heater H3 is located in close proximity to contacts S3 and designed to be of sufiicient capacity to only hold contacts S3 open.

Under this condition heaters H1 and H2 will cool oli and when they have cooled sufiiciently, the starter contacts S2 and S1 will return to the position shown in Fig. 9. The reclosure of contacts S essentially short circuits the lamp between the electrodes 2 and 3 so that there is practically no voltage impressed between them. The entire voltage output of the reactor transformer is then absorbed in heater H3.

Under these conditions there will be a delay if the line circuit is opened before the starter returns to the position shown in Figs. 5 and 6. However, if it is noted that there is a defective lamp in the circuit, and the lamp is replaced by a new one, then it is either necessary to remove the starter from its socket for a few minutes or to replace it with a new starter for the new lamp to properly start and go into operation. If the starter is removed, it can be used again with some other lamp, as it will not have been damaged in any way.

It will be evident that the use and arrangement of the coil C and armature core I and armature J in the starter structure and circuits described is such as to ensure full movement of the thermostatic element when the coil is energized from its inactive to its active position to fully open the switches and hold them open.

While the circuit of Fig. 5 shows a reactor-transformer, this can be changed to a plain inductive reactance as in Figs. 1, 2 and 3 when it is desired to use this starter with a lamp which does not require a reactor-transformer. It is to be understood that appropriate changes would also be made in such a case in the values of H, H2 and H3 corresponding to the particular characteristics of the lamp used. The starter of Fig. 5 can also be used on D. C. in the circuit of Fig. 3.

From the foregoing description, taken in connection with the drawings, it is believed that the construction and operation of the starter in connection with the various circuits disclosed will be readily understood without a further and extended description, and it will be seen that the invention provides in each circuit starting means of a simple and economical type which facilitates and renders easier the starting of hard starting lamps and which also provides means for locking out the starter from continuous or undesired repeating actions when such a feature is desired to prevent damage to the starter or other parts of the equipment in the case of a defective lamp, or a lamp which has reached the end of its useful life. While the constructions shown for these purposes are preferred, it is to be understood that they are merely exemplificative and that changes in form, arrangement and construction of parts, falling within the scope of the appended claims, may be made without departing from the spirit of the invention.

Having thus described my invention, I claim:

1. A starter for an electric discharge device comprising a thermostatic operating mechanism including a heater therefor, at least one normally closed movable switch member and cooperating contact, an actuating thermostatic element of the quick acting type, and an electromagnetic coil and cooperating armature, an electrical discharge device of the low pressure type having two electrodes, at least one of which is constructed to receive preheating current, a supply circuit, means including an inductive reactance device and said coil for connecting said electrodes to said supply circuit, a preheating circuit for said discharge device extending from one side of a voltage source to the opposite side of said source and including in series connection the electrode to be preheated, said normally closed switch member and contact and said heater, the operation of said quick acting thermostatic element causing said switch member to move away from said contact so as to break the preheating circuit and fire the discharge device by an inductive voltage surge, and said armature being mechanically connected to said switch member and said thermostatic element so that said armature is moved by said thermostatic element into proximity to said coil and so that said switch member is held in actuated position when said coil is energized by the operating current through said discharge device, thus holding said switch member in actuated position after the heater for the thermostatic operating mechanism cools.

2. A starter as claimed in claim 1 in which the thermostatic operating mechanism has a second normally closed switch member and cooperating contact, said switch member being actuated by said thermostatic element, said switch member and contact being connected in shunt to said coil so as to short-circuit said coil during the preheating operation, the opening of said second switch member removing said short-circuit to allow said coil to be energized by the operating current of the discharge device.

3. A starter as claimed in claim 2 having a third thermostatically actuated normally closed switch independent of said thermostatic operating mechanism, said third switch having two heaters, one of said heaters being connected across the electrodes of the discharge device and generating sufiicient heat, if the discharge device does not fire, to cause the opening of said third switch, the second heater for said third switch being connected across said switch, said third switch forming part of said means for connecting the electrodes of the discharge device to the supply circuit so that the opening of said third switch in effect cuts off the discharge device from the supply circuit, inasmuch as the resistance of said second heater connected in series with said discharge device by the opening of said switch permits only a small current to flow, the heat generated by said second heater being suflicient to maintain said third switch in open position so as to keep the discharge device operatively disconnected from the supply circuit.

4. A starter as claimed in claim 1 wherein the said coil does not initially connect the discharge device with the supply circuit and in which the thermostatic mechanism has a second normally open switch member and cooperating contact, said switch member being actuated by said thermostatic element, said switch member and contact being connected in a circuit including said coil, said circuit being connected in shunt to said heater so that the actuation of said second switch member closes said shunt circuit and connects said coil across said heater circuit so that said coil is energized by the operating current of the discharge device.

5. A starter as claimed in claim 1 in which one of the electrodes is connected to one side of the supply circuit through a circuit including in series connection an inductive reactance device and the said coil, and the other electrode is connected directly to the opposite side of the supply circuit.

6. A starter for an electric discharge device as claimed in claim 1 in which one of the electrodes is connected to one side of the supply circuit through a circuit including an inductive reactance device, and the other electrode is connected to the opposite side of the supply circuit through a circuit including the said coil.

References Cited in the file of this patent UNITED STATES PATENTS Hays Dec. 23, 1941 Abbott Dec. 30, 1941 Babb Feb. 15, 1944 Vaughan May 15, 1945 Hehenkamp Mar. 30, 1948