US2089325A - Discharge electric lamp - Google Patents

Description

1937. J. N. ALDINGTON 2,089,325

DISCHARGE ELECTRIC LAMP Filed Dec. 4, 1934 //VV/V70/? ATTORNEY Patented Aug. 10, 1937 DISCHARGE ELECTRIC LAIMP John Norman Aldington, Ashton-on-Ribble, England, assignor to Siemens Electric Lamps and Supplies Limited, London, England, a

British company Application December 4, 1934, Serial No. 755,989 In Great Britain December 6, 1933 3 Claims. (Cl. 176 -122) This invention relates to electric discharge lamps of the hot cathode type, that is to say, lamps in which one or more cathodes emit electrons by reason of being separately heated or heated as the result of the discharge in the lamp and in which the discharge takes place through metallic vapour at comparatively high vapour pressure the vapour arising from a separate filling and not fromthe material of the electrodes.

Lamps of this kind already known in the market have a pair of electron emitting cathodes a filling including a small quantity of mercury and to facilitate starting some rare gas such as neon or argon is contained in the lamp. To further facilitate starting it is most usual to surround the container of the lamp with a helix of wire which is connected to one electrode and terminates short of the other electrode.

In operation the mercury vapour attains a high temperature and vapour pressure and the.

lamps are known as high pressure lamps.

- The light emitted by such lamps however is not white and is highly deficient in red rays.

One object of the present invention is an electric discharge lamp of the hot cathode high pressure kind in which the light emitted will approximate more to white light by reason of having a greater proportion of red rays.

To this end in addition to mercury the filling also comprises another metal or metals which as the lamp heats up due to the discharge through the mercury vapour give off vapour and take part in light emission the spectrum of the emitted light complementing that of the mercury.

The choice of the additional metals is limited in the first place by the temperature that the transparent container can be allowed to assume, and with present day glass assuming that the glass must be able to sustain without distortion a difference of pressure between the inside and outside of one atmosphere, a temperature of probably between 500 C. and 600 C. appears to be the maximum allowable.

The choice is accordingly limited to metals giving an appreciable vapour pressure at or about 500 C., such as zinc, cadmium, bismuth.

For the production of complementing spectra, cadmium and/or zinc are very suitable, cadmium and zinc supplying prominent blue, green and red lines. These metals also have the characteristic that their ionization potentials are not widely different to that of mercury so that the tendency for the vapour spectrum of the one metal to predominate is reduced.

The temperature in the container should be high enough to produce such partial pressures of the vapours of the additional metals that these contribute spectral lines to the light emitted by the lamp. As a guide in the case of cadmium and zinc as additional metal the watts dissipation per cubic centimetre of the container should be of the order of 3.5 watts. A higher wattage per cubic centimetre is allowable and whilst in general this would carry with it a higher efliciency yet if carried far the danger would arise of trouble due to softening or cracking of the container and furthermore a lessening of the useful life of the lamp.

With a lesser wattage per cubic centimetre the temperature may be too low to allow of sufi'icient vapourization of the additional metals with the consequence that little red light is added to the mercury light. If the light transmitted through a No. 25 Wratten (red) filter or other filter designed to pass light only of above 5800 Angstrom units is not 2% of the lumens emitted by the lamp the complementing effect is too little iorpractical purposes and such a lamp is excluded from the scope of this invention.

It should be borne in mind that a temperature that plays a great part is that of the coolest portion of the container as it ishere that condensation of metal vapour is more likely to take place when the lamp is in operation and since the less volatile metals will condense first the colour complementing efiect will steadily diminish.

It is accordingly necessary to try to avoid pockets where a lower temperature may prevail and also to position the electrodes as close as possible to the ends of the container.

The filling is introduced in the form of an amalgam. In a mercury-cadmium-zinc amalgam the amount of mercury requires to be approximately the same as when a plain mercury filling is used if the voltage across the lamp terminals is to be the same for the same wattage. The cadmium and zinc present apparently reduce the vapour pressure of the mercury but the vapour pressures of the cadmium and zinc have a compensating effect. The relation between the weights of the constituents depends on the colour correction required.

To attain light approximating to north daylight the following weights may be used via, two or three units of mercury to one unit of cadmium and one unit of zinc. The actual weights used will depend on the wattage of the lamp and the intended internal volt drop. A greater proportion of cadmium and zinc would result in an increase in red and blue components of the discharge spectrum and a decrease in the yellow components assuming that the wattage remains the same.

Too little cadmium and zinc would bring about little colour correction and an amalgam consist- 5 ing of five units of mercury, one of cadmium and one of zinc is one which gives the colour correction about the minimum included hereunder.

An increase of the proportion of cadmium :and

zinc with a corresponding increase in the proportion of red light is accompanied by a reduction in lumens per watt.

' If the colour correction is carried so far that the lumens emitted by the lamp are of the order of 60% of the lumens that would be emitted by the same lamp with the same weight of mercury as before but without the cadmium and zinc a point is being approached where the increase in blue light renders the increase of red light of little value.

The internal volts drop in the lamp needs to be designed having in view the voltage of the circuit on which the lamp is to be used.

If the volts drop is too high the lamp becomes unstable in operation: if too low the power factor is too low for commercial requirements and to this extent it should not be less than 50% of the mains voltage. The internal volts drop is controlled by the amount of amalgam forming the filling an increase in the amount causing an increase in the internal volts drop for the same wattage.

It may be taken as a guide that if cadmium is alone added to the mercury or zinc alone is added to the mercury the zinc or cadmium as the case may be of the mercury-cadmium zinc amalgam is replaced by an equal weight of the other of them. Zinc appears to be better than cadmium from the point of view of red correction but not so good from the point of view of etficiency.

As a guide to the production of a 400 watt lamp the following details are given.

The container or inner tube of the lamp may be 180 m. m. long inside with an interior diameter of 30 m. m. A main electrode comprises a spiral of m. m. tungsten wire enclosing a core of electron emitting material, such as barium silicate, and is arrangedcross wise to the tube being supported by lead-in wires. The electrodes may be 160 m. m. apart. An auxiliary electrode is provided in the shape of a tungsten wire helix with close turns and surrounds the main electrode the nearest approach being say 1 /2 in. m. The axis of the helix lies along the longitudinal axis of the tube.

The auxiliary electrode should have a substantial thermal capacity so that heated by the discharge it tends to maintain a steady temperature at the end of the container and with the heat radiated by the incandescent main electrode the temperature of the end is above the temperature of, that portion of the container immediately in front of the main electrode. The helix may consist of 5 turns of 0.8 m. m. tungsten wire wound on a 10 m. m. mandrel. The effect of this arrangement is to cause any condensate to be deposited in front of the electrodes where it is subject to contact by the convection currents thereby maintaining a stable equilibrium between the various components of the vapour phase.

For exhausting the tube, it is sealed onto an exhaust pipe which is provided with a downwardly depending branch closed at its lower end in 7 which the amalgam to be introduced is placed.

The amalgam is formed of three units of mercury, one of cadmium and one of zinc amounting in total to 250 milii-grammes.

After exhaustion with baking of the tube, neon gas to a pressure of between 5 and 10 m. m. of 6 mercury is introduced together with argon at say a pressure of 20 to microns. The branch containing the mercury amalgam is kept very cool during the pumping process. The tube is then sealed oif from the exhaust pipe and in- 10 verted to run the amalgam down to the lamp heat being applied to facilitate this. The final sealing is then performed.

It should be noted that the sealing pip forms a pocket which is likely to bring about condensa- 15 tion. It should be made as small as practicable and be situated slightly in front of the electrode to which it is adjacent. If too far forward it may crack:' if behind the electrode it may not be sufiiciently heated in the interior by convection 20 currents.

The tube is then sealed into the other contain er the space between being exhausted. The auxiliary electrodes are connected together through a high resistance to facilitate starting of the 25 lamp.

The above described lamp would be suitable for a 230 volt alternating current supply a choke being provided to limit the watts dissipation in the lamp to 400 watts. 30

For lamps of difierent wattage a general guide is that the internal capacity of the container or inner tube and the weight of amalgams should be increased or decreased as the case may be corresponding to the wattage. The distance be- 5 tween the main electrodes should be varied as the square root of the wattage figure rather than as the wattage figure.

The drawing shows the general arrangement of a discharge lamp embodying my invention. 0

In the drawing I is the outer container 2 the screw lamp cap, 3 is the inner container the sealing pip thereof being indicated at 4. The inner container has pinched seals 5 and 6. In seal 5 are three leading-in wires 1, 8, and 9. Wires 4 1 and 9 support and connect to the tungsten wire heliv I0 which encloses a core of barium silicate. Wire 8 connects to a helix of tungsten wire H with closely adjacent turns forming the auxiliary electrode (shown in cross section). Wire 8 also 50 connects to a resistance element I2. Wires 1 and 9 are welded to a diametral tie member of a ring of wire iii to which member is also welded the leading in wire it. The ring. l3 closely fits the inside of the outer container. 55

Wires l5 and I! in the pinch 6 are likewise welded to a diametral tie member of a ring l8 which ring is connected by wire is to lead-in wire 20. Wire IS in the pinch 6 connects to the resistance element I2. 60

It will be noted that the sealing pip of the inner container is positioned approximately level with the electrode though slightly in advance, also that an auxiliary electrode forms a kind of shield of substantial thermal capacity tending to keep 65' the temperature at the end of the tube concerned steady.

What is claimed is:-

1. An electric discharge lamp of the high pressure type and such that the light transmitted 70 through a filter designed to pass light only of above 5800. Angstrom units is at least 2% of the lumens emitted by the lamp comprising a translucent sealed container, main electrodes of refractory material one at least having a core of 7 electron emitting material, a filling comprising mercury cadmium and zinc in relative weights 3 to 1 to 1 respectively, said electrodes being arranged transversely to the longitudinal axis of the container, an auxiliary electrode for each main electrode of refractory metal surrounding the main electrode but being spaced therefrom, said container being provided with a sealing pip approximately level with one electrode substantially as described.

2. An electric discharge lamp of the high pressure type and such that the light transmitted through a filter designed to pass light only of above 5800 Angstrom units is at least 2% of the lumens emitted by the lamp comprising a translucent sealed tubular container the volume of which is related to the wattage of the lamp, electron emitting cathodes situated closely adjacent to the ends of the chamber, a filling including mercury, cadmium and zinc, an auxiliary electrode of refractory material surrounding each cathode and consisting of a helix of tungsten wire of closely adjacent turns and having substantial thermal capacity.

3. An electric discharge lamp of the high pressure type and such that the light transmitted through a filter designed to pass light only of above 5800 Angstrom units is at least 2% of the lumens emitted by the lamp, comprising a translucent sealed container, electron emitting cathodes situated closely adjacent to the ends of the container, a filling partly comprising mercury, further metal in the filling giving rise to appreciable vapour pressure at 500 C., luminous vapour of said further metal having prominent red rays in its spectrum, an auxiliary electrode for each cathode of refractory metal surrounding the cathode but being spaced therefrom and being of substantial thermal capacity, said auxiliary electrode consisting of a helix of tungsten wire with closely adjacent turns.

JOHN NORMAN ALDINGTON.

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