US3145280A

US3145280A - Glow switch having a cadmium covered electrode

Info

Publication number: US3145280A
Application number: US832230A
Authority: US
Inventors: Waigh Douglas Thomas
Original assignee: Thorn Electrical Industries Ltd
Current assignee: Thorn Electrical Industries Ltd
Priority date: 1958-08-19
Filing date: 1959-08-07
Publication date: 1964-08-18
Anticipated expiration: 1981-08-18

Description

1964 D. T. WAlGH 3,145,280

01.0w SWITCH HAVING A CADMIUM COVERED ELECTRODE Filed Aug. 7, 1959 I Fl, 1:- g1 1% w INVENTUR DOUGLAS THQMAS WMG 1+ A TTORNE y United States Patent :ULOW swiron HAVING A cAnMiUM covnnnn ELEQTPDEBE Douglas Thomas Waigh, London, England, assignor to Thorn Electrical industries Limited, London, England Filed Aug. 7, 1959, Ser. No. 832,230 Claims priority, application Great Britain Aug. 19, 1958 1 Claim. (Cl. ZOO-113.5)

The present invention relates to glow switches and methods for their manufacture.

Such switches are manufactured in large numbers for use as starter switches for electric discharge tubes, particularly fluorescent lamps. They consist of two electrodes, one or both of which are bi-metallic elements, enclosed in an envelope which is filled with a suitable gas. In operation a glow discharge is established between the electrodes and the resultant heating causes the electrodes to come into contact with one another.

It is desirable that the operating characteristics of the switch, the glow voltage, glow current, and operating time, should be predictable and should not vary widely during the life of the switch. To this end known methods of manufacture of glow switches include an ageing process to stabilise the operating characteristics. It is known to plate the electrodes of the switches with zinc, so as to reduce the time for which the switches must be subjected to the ageing process in order to produce acceptable characteristics, and stability of those characteristics, in a high proportion of the switches made.

According to the present invention there is provided a method of making a glow switch, including providing a layer of cadmium on the bi-metallic electrode or electrodes of the switch, removing most of the air from the switch, and thereafter evaporating oif some of the cadmium under reduced pressure.

A preferred method of putting the method of the invention into effect will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a view of a glow switch in elevation, and

FIG. 2 is a partial section on the line Il--II of FIG. 1 showing some parts on an exaggerated scale for greater clarity.

The two bimetallic strips forming the electrodes 1 and 2 are first plated with a cadmium layer 3 to a thickness of 110% of the thickness of the strip. The plated electrodes 1 and 2, which each have the outline of a parallelogram, are then mounted on

respective leads

4 and 5 within a glass envelope 6 so as to lie in parallel spaced planes with their upper ends overlapping.

The envelope 6 is evacuated in conventional manner by withdrawing air from it and the temperature is simultaneously raised, reaching at least 290 C. when the pressure drops to its minimum value. This temperature ensures the removal of water vapour and occluded gases and 3,145,280 Patented Aug. 18, 1964 causes evaporation of some of the cadmium layer 3 from the electrodes 1 and 2, cadmium vaporising at a lower temperature than would zinc in similar circumstances. The clean cadmium surface thus produced shows less tendency to form an oxide film than a zinc surface would in the same conditions and consequently, even though some air remains in the envelope after evacuation, the ageing time for glow switches produced in the manner described can be reduced to one minute, as compared with five minutes for zinc-plated electrodes, and greater stability in operation throughout the working life of the switch can be achieved.

After evacuation of the envelope and heating to cause evaporation of some cadmium, the envelope is filled with argon and sealed off in conventional manner before the ageing process is carried out.

The range of thicknesses of the cadmium layer given in the example described is controlled by two factors. As is well known in the art, too great a thickness of a thermally inert material such as cadmium will impair the sensitivity of a bi-metallic strip. While the initial thickness of cadmium can be reduced by evaporation it is not normally convenient to have an initial thickness for the cadmium layer greater than 10% of the thickness of the bi-metallic strip. The minimum initial thickness is determined by the requirement that some cadmium shall be left on the electrodes after evaporation, and may be as low as .0005 inch Without requiring abnormally strict control of the conditions during manufacture.

Where only one electrode is a bi-metallic element it is not necessary to provide a layer of cadmium on the other, non-bimetallic, electrode. While plating with cadmium offers economic advantages, it is also possible to apply the layer of cadmium by other methods, for example vacuumdeposition.

I claim:

A glow switch comprising a sealed envelope, said envelope being evacuated of air and filled with an inert gas, two electrodes contained within the envelope, and leads for the two electrodes emerging from the envelope, at least one of the electrodes being a bi-metallic element having a thin layer of cadmium on at least one surface thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,449,236 Lamont Sept. 14, 1948 2,457,487 Peacock et a1. Dec. 28, 1948 2,488,701 Blood Nov. 22, 1949 2,536,280 Haegele Jan. 2, 1951 2,596,469 Cooper May 13, 1952 2,677,071 Carne Apr. 27, 1954 2,740,861 Lake Apr. 3, 1956 2,870,524 Kinnear Jan. 27, 1959 2,907,100 Lindblad Oct. 6, 1959 2,909,777 Germashausen Oct. 20, 1959