Electric lamp and discharge devices – With frangible capsule containing getter – gas or vapor...
Patent
1982-06-01
1985-09-17
Moore, David K.
Electric lamp and discharge devices
With frangible capsule containing getter, gas or vapor...
313492, 313326, 313550, 445 9, H01J 9395, H01J 6128
Patent
active
045423199
DESCRIPTION:
BRIEF SUMMARY
This invention concerns a mercury dispenser for electric discharge lamps, especially lamps having a sealed transparent or translucent envelope containing at least one cathode, at least one gas at substantially reduced pressure and a certain amount of mercury. Such discharge lamps include fluorescent lamps and low pressure mercury discharge lamps, but may also include cold cathode glow discharge tubes.
In the manufacture of fluorescent tubes the introduction of an accurately metered amount of mercury into an already sealed and evacuated lamp envelope presents a great problem, not only technologically but also from the point of view of preventing the escape of mercury which is, of course, biologically toxic.
The conventional technique involves the use of an electromagnetic valve dispenser to dispense liquid mercury into a portion of an exhausting machine adjacent the exhaust tube (sometimes referred to as "tubulation") and then blowing or dropping the droplet of mercury into the interior of the envelope by means of a stream of argon, which is also the fill gas. This technique suffers from several drawbacks. Firstly, the dispenser cannot dispense an exact amount of mercury. Secondly, tiny amounts of mercury may never reach the tube envelope but may instead get stuck along the dispensing path, e.g. in the dispenser itself or in the exhaust tube. Thirdly, as the dispensing takes place in a hot environment, evaporation losses may occur. Because of these disadvantageous factors the amount of mercury usually dispensed considerably exceeds the actually desired amount and this is wasteful of a not-inexpensive raw material of finite abundance. Furthermore, on breakage of a tube, excessive amounts of harmful mercury may escape into the environment.
One prior proposal to overcome this drawback is to mount an intermetallic mercury compound around the cathode, on an anti-sputtering cathode shield, before tipping off the exhaust tube. After tipping-off the mercury is liberated from the compound when the latter irreversably breaks down under externally applied heating. While this method allows the dosage of mercury to be controlled better and reduced in magnitude, production is rendered more difficult and also more expensive.
In another prior proposal (U.S. Pat. No. 3,764,842) the required amount of mercury is sealed into a glass capsule in heat-conducting contact with an outer heater wire. A current is generated in the wire to melt and cut through the glass wall, whereby to release the mercury. The capsule and wire are mounted on a shield, known as the anti-sputtering or disintegration shield, (hereafter: disintegration shield) disposed about the cathode. The drawbacks are that the assembly and mounting of the capsule and heater wire are rather intricate and special measures have to be taken to prevent pieces of broken glass from falling off. The disintegration shield requires special shaping.
In yet other prior proposals, e.g. U.S. Pat. Nos. 3,794,402 and 4,182,971, a glass or metal capsule containing mercury has a sealed-in heating filament extending longitudinally through its interior. It is either connected to an external current source by way of current supply conductors passing through a wall of the tube, or a current is induced in it from a radio-frequency (R.F.) source. The heating current vaporises the mercury and the capsule cracks under the effect of the increased vapour pressure. The capsule may or may not be mounted about a disintegration shield but it has the disadvantage that it requires additional lead-in wire(s) through the wall of the tube or an R.F. heater. Also, the preparation of the capsule with a metal wire sealed in it is cumbersome and expensive.
In still another prior proposal (GB-PS No. 1,475,458) the mercury dispenser is located in the exhaust tube of the discharge lamp. The dispenser consists of two juxtaposed platelets of preferably dissimilar metal welded together and defining a depression therebetween to accommodate liquid mercury. On heating the vapour pressure of mercury forces the platel
REFERENCES:
patent: 3722976 (1973-03-01), Porta et al.
patent: 3764842 (1973-09-01), Ridders et al.
patent: 3794402 (1974-02-01), Ridders et al.
patent: 4056750 (1977-11-01), Latassa
patent: 4182971 (1980-01-01), Cassidy et al.
patent: 4282455 (1981-08-01), Latassa et al.
Grenfell Julian P.
Stephens Stanley W.
Moore David K.
Sale Tilney Technology PLC
Wieder K.
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