US2748308A

US2748308A - Low-pressure arc-discharge tube supplied with direct current

Info

Publication number: US2748308A
Application number: US361342A
Authority: US
Inventors: Kolkman Dirk; Visser Adolf Jan De
Original assignee: Hartford National Bank and Trust Co
Current assignee: Hartford National Bank and Trust Co
Priority date: 1952-08-05
Filing date: 1953-06-12
Publication date: 1956-05-29
Anticipated expiration: 1973-05-29

Description

D. KOLKMAN ET AL 2,748,308

LOW-PRESSURE ARC-DISCHARGE TUBE SUPPLIED WITH DIRECT CURRENT May 29, 1956 Filed June 12, 1955 INVENTORS DIRK KOLKMAN ADOLF JAN DE VISSER AGENT United States Patent LOW-PRESSURE ARC-DISCHARGE TUBE SUP- PLIED WITH DIRECT CURRENT Application June 12, B53, Serial No. 361,342

6 Claims. (Cl. 313-185) This invention relates to low-pressure arc-discharge tubes supplied with direct current, more particularly fluorescent lamps, which comprise a gas-filling having a pressure of 2 to 5 mm. of mercury, two activated main electrodes located at the ends of the tube, and two strip-shaped auxiliary electrodes consisting of resistance material and provided on the inner wall of the tube. Said auxiliary electrodes extend substantially throughout the length of the tube, are each time conductively connected to one of the main electrodes within the discharge space and insulated from the other main electrode.

Upon connecting such tube through a series resistor to a suitable direct voltage a glow discharge is struck between the main electrode connected as a cathode and the auxiliary electrode insulated from it and connected as an anode, the glow discharge propagating along the auxiliary electrode and finally finding its way to the main electrode connected as an anode. The glow discharge changes into an arc discharge upon the activated cathode attaining its emission temperature.

The tubes comprise two main electrodes and two auxiliary electrodes so that, independently of the polarity of the source of direct current, always either of the main electrodes acts as a cathode and either of the auxiliary electrodes acts as an anode of the glow discharge.

This further permits of inversing the polarity of the tube during operation or during idle hours, thus preventing the metal vapour, for example in fluorescent lamps, from concentrating near the cathode.

Inversion of polarity is mainly effected in the idle hours by means of a suitable main switch in such manner that the tube is each time differently polarised in the successive operating conditions usually lasting at most several hours.

It has now been found, that ignition difficulties may arise in connection with the inversion of polarity. If the tube has been operated only for a few seconds, followed by several idle hours, and the tube is subsequently switched on in the condition of inversed polarity, it may namely happen that the tube fails to ignite.

The present invention has inter alia for its purpose to mitigate this inconvenience.

According to the invention only one end of each main electrode is connected to an external contact of the tube, the other end being provided with an auxiliary conductor which extends in the direction of the auxiliary electrode insulated from the said main electrode.

During operation the auxiliary conductor of the main electrode, which electrode is connected as an anode, takes a part of the arc-discharge current and passes this part via the non-negligible resistance of the main electrode to the external contact of the tube end containing the main electrode acting as an anode, thus heating the activated part of said main electrode. It has unexpectedly been found that this permits of obviating the aforesaid ignition difiiculties.

The auxiliary conductor moreover fulfills another im- 2,748,308 Patented May 29, 1956 'ice 2 portant function. Since it extends in the direction of the auxiliary electrode, which is insulated from the main electrode to which the auxiliary conductor is connected, the auxiliary conductor, if connected as a cathode, promotes the striking of the glow discharge.

The lowest voltage at which the glow discharge strikes is attained if the minimum distance in millimeters between the auxiliary conductor and the auxiliary electrode insulated from it is between l2/p and 40/ p, preferably between 15/ p and 30/ p, where p represents the pressure of the gasfilling in millimeters mercury at room temperature.

The auxiliary conductor, viewed from the discharge space, is advantageously situated behind the activated part of the main electrode connected to it. Thus it is ensured that blackening of the tube wall, due to the presence of the auxiliary conductor, occurs where the emission of light or other radiations must not be impeded, the auxiliary conductor yet receiving a part of the discharge current sufficient to heat the main electrode adequately.

Activated electrodes are often surroundedby an insulated screen to collect the particles disintegrated from them. When using such screens in a tube according to the invention it has been found imperative that the auxiliary conductor should terminate withinthescreen near the main electrode end connected to the contact, lest the passing over of the glow discharge into the arc discharge be retarded or even prevented.

The auxiliary conductor preferably consists of nickel or iron. If made of tungsten, for example, it has been found that it fulfils its functions worse.

In order that the invention may be readily carried into effect, it will now be described with reference to the accompanying drawing which represents one example thereof and in which Fig. l is a perspective view of a discharge tube according to the invention, Fig. 2 being a plan view of the lower part of this tube.

With reference to the drawing, the reference numeral 1 denotes the cylindrical, glass discharge vessel approximately cm. long and approximately of 35 mm. inner diameter of the tube. The discharge vessel is filled with argon gas under a pressure of approximately 3 mm. at room temperature and contains a small quantity of mercury (approximately 30 mg). The inner wall of the discharge vessel is provided with a layer (not shown) capable of converting ultraviolet rays of the lowpressure mercury-vapour discharge into radiations of greater wavelength during operation.

The discharge tube comprises two coiled-coil main electrodes 2 and 3 respectively which are activated by the conventional alkaline earth compounds and the ends of which are secured to supporting wires 4, 5 and 6, 7 respectively. The supporting wires are carried by the usual pinch constructions closing the discharge vessel in a vacuum-tight manner. The supporting wires 5 and 7 respectively are conductively connected to contacts 8 and 9'respectively fitted to the-outer wall of tube feet 10 and 11 respectivelyat the ends of the tube.

The supporting wires 4 and 6 respectively, not connected to a contact, are linked to band-shaped auxiliary electrodes 16 and 17 respectively through metal wires 12 and 13 respectively and quadrantal metal strips 1 and 15 respectively. Said auxiliary electrodes consist of a mixture of graphite and glaze, have a resistance of approximately 25 ohms per cm. length of the band and extend parallel with the axis of the discharge vessel on its inner wall over a width of approximately 3 mm. and substantially throughout the length of the discharge vessel.

Thus, the auxiliary electrode 16 is conductively connected to the main electrode 2 and insulated from the main electrode 3, whereas the auxiliary electrode 17 is conductively connected to the main electrode 3 and insulated from the main electrode 2.

Welded to the metal wire 12 of the main electrode 2 is an auxiliary conductor consisting of nickel wire approximately 2 mm. thick, which comprises three

horizontal parts

18, 19, 20, and a vertical part 21. The horizontal parts are U-shaped, their plane extending substantially at right angles to the plane through the supporting Wires 4 and 5 of the main electrode 2. The vertical part extends substantially parallel with the supporting wire 4 and may be situated in the plane through the supporting wires 4 and 5.

Secured to the metal Wire 13 of the main electrode 3 is a corresponding auxiliary conductor also comprising three

horizontal parts

22, 23 and 24 and a vertical part 25 (see also Fig. 2).

The part 19 of the auxiliary conductor of the main electrode 2 and the part 23 of the auxiliary conductor of the main electrode 3 are nearest to the auxiliary electrodes 16 and 17 respectively insulated from the main electrode concerned. The minimum spacing between the auxiliary conductor and the auxiliary electrode (the spacing between the part 23 and the auxiliary electrode 16 in Fig. 2) is approximately 8 mm., hence this spacing corresponds to approximately 24/p, where p represents the pressure of approximately 3 mm. of the gasfilling.

Each main electrode is surrounded by the usual, annular, metal, disintegration screen, shown only in Fig. 2 as the screen 26 of the main electrode 3. This screen comprises a tag 27 which is soldered to the supporting wire 28 (also shown in Fig. 1). Fig. 1 shows a similar supporting wire 29 which is sealed into the pinch of the main electrode 2 and serves for the connection to the tag of the screen for the main electrode 2.

The tube shown is a 40 w. fluorescent lamp which, upon connection via an incandescent lamp of 110 v., 40 w. to a direct voltage of approximately 220 v. has a tube voltage of approximately 110 v. and takes an arc-discharge current of approximately 360 ma.

Measurements have shown that the auxiliary conductor connected as an anode takes approximately 150 ma. of the arc-discharge current during operation and passes it through the coils of the associated main electrode to the positive terminal of the source of current. Due to the resulting heating of the main electrode the ignition difiiculties referred to are obviated.

By the said spacing of approximately 8 mm. of the auxiliary conductor from the auxiliary electrode it is achieved that the tube strikes at a direct voltage as low as 180 volts and attains the arc-discharge condition, so that the tube is substantially independent of voltage drops that may occur in direct current mains of 220 volts nominal.

When using insulated disintegration screens it has found to be necessary to provide the auxiliary conductors with the vertical parts 21 and 25 respectively and to locate these parts within the screen near the supporting wires 5 and 7 respectively connected to the external contacts 8 and 9 respectively, since otherwise the arc-discharge condition is not reached or only very slowly, which detracts from the life of the tube.

The horizontal parts of the auxiliary conductors not surrounded by the disintegration screen are nearer to the tube end concerned than the associated main electrode,

thus achieving that blackening of the discharge vessel from the auxiliary conductors can only occur beyond the discharge path. The fact that the auxiliary conductors point towards an auxiliary electrode moreover involves that any blackening from the auxiliary conductor mainly occurs close to said auxiliary electrode. However, the discharge tube is preferably positioned so as to remove the auxiliary electrodes from sight, hence any possible blackening is harmless.

What is claimed is:

1. A low-pressure arc-discharge tube for direct current operation, comprising an elongated envelope having a gas-filling at a pressure of about 2 to 5 mm. of Hg., a pair of activated main electrodes each having two terminals and each being located at one end of said envelope and both defining a discharge space therebetween, a pair of strip-shaped auxiliary electrodes each constituted of resistance material, each of said auxiliary electrodes being mounted on the inner wall of the envelope and extending substantially throughout the length of the tube, each of said auxiliary electrodes being electrically connected to one of said main electrodes within the discharge space and being insulated from the other main electrode, only one external contact member mounted at each end of the envelope, only one terminal of the main electrode adjacent each contact member being electrically connected thereto, and an auxiliary conductor electrically connected to the other terminal of the main electrode and extending in the direction of but spaced from the auxiliary electrode insulated from said main electrode.

2. A discharge tube as claimed in claim 1, in which the minimum spacing in millimeters of the auxiliary conductor from the auxiliary electrode insulated from it is between 12/p and 40/p, p representing the pressure of the gas-filling in mm. mercury at room temperature.

3. A discharge tube as claimed in claim 2, in which the auxiliary conductor, viewed from the discharge space, is situated behind the activated part of the main electrode to which it is connected.

4. A discharge tube as claimed in claim 1 in which a main electrode is surrounded by an insulated screen, and the auxiliary conductor terminates within the screen near the main electrode terminal connected to the contact member. I

5. A discharge tube as claimed in claim 4 in which the auxiliary conductor consists of a metal selected from the group consisting of nickel and iron.

6. A low-pressure arc-discharge tube as claimed in claim 1 wherein the auxiliary conductor comprises a U- shaped wire extending transverse to the envelope axis and having one end connected to said other terminal of said main electrode and the other end terminating in a short axial portion adjacent said one terminal of said main electrode, the plane of said U-shaped wire lying behind the activated part of said main electrode.

References Cited in the file of this patent UNITED STATES PATENTS 1,961,618 Machlett June 5, 1934 2,042,147 Fairbrother May 26, 1936 2,094,647 Freitag et al Oct. 5, 1937 2,291,965 Iancke et a1. Aug. 4, 1942 2,491,854 Force Dec. 20, 1949