US2324907A - Thermal switch - Google Patents

Description

y 20, 1943. B. N. CLACK 2,324,907

THERMAL SWITCH Filed July 22, 1941 lnvervbor't Ba iL Noel CLack, b ,MM

His Af'tor-neg.

Patented July 20, 1943 li silfloe Cla k; 'Wei ;;E lawl a sisnq t8 -General Electric Company, a corporation of New York entire-lemmin 1e41 seeratha ie JnrGrcat Britain AugustZZ, 194i) "roman. (01.250-275) I i tention r lates t therma l ppei'a ed swi ch e 'thwtym whe ei aitherme y s s tiv leme t, o example :a bimetal, is heated b e low dis har Suchswi c sarenow se o con ol n th -s.tar i g re fluorescent dis-- ha e amps ihavin el ctrodes which ,require 'h t eabeio s a tina d s harg :th re ee term n l of a fiurr u s u fo th i h Attenajimesuihcient toraisethe lamplelec'trcdes to their operating temperature, the series connection therebetweenisprolien by the opening of the switch contacts, due to the cooling and consequent flexing of the bimetal, but the glow discharge does not start again in the switch if the lamp has started because the voltage then across the switch is not the full supply voltage, but the operating voltage of the lamp. It is necessary, therefore, that the starting potential of the switch should be less than the voltage of the current source, but greater than the operating voltage of the lamp.

I have observed that switches of the kind de scribed above, which are perfectly satisfactory when exposed to light, have higher starting potentials than the voltage of the current source when they have been in the dark for some time and, thus, are useless for starting discharge lamps when the switch is inthe dark.

The object of my invention is to provide a glowtype thermal switch having approximately the same starting potential in the dark and in the light. Other objects and advantages of my invention will be apparent to those skilled in the art from the following description and from the appended claim.

I have discovered that an exceedingly small quantity of radioactive material capable of emitting alpha-rays is effective for maintaining the starting potential of thermal switches of the glow discharge type substantially the same in the dark and in the light when said material is mounted in such position within the switch envelope that the alpha-rays can reach the discharge path between the electrodes.

While I prefer to use radioactive uranium and thorium compounds for the purposes of my invention, this preference is based largely on the availability and comparatively low cost of these maerials and I r q emnle e th t the za ph tav emittin ma erial :s ch as med um chloride, 1 0, oth rsui abl eom eun .9 r dium-m h se I9 attainin vthedes red r sult If th envelene ma o uranium slasaw ueh a 'the id sqlc d n m impendin e alicatieaserie N 13 19.03 filed'Ma e fi... .9 1 ,o wh ht is a l cation is a c inua ipnii zpa th uranium i th a e t e g ss next to t nte io of the en el pe is s ficient Accordingl heireii acti mateia i t-sa d ib c, in di t en e o fit sls -l cated thati t i. StQRGe-fE- m th d s har pa i essithanith i an of theelnh -r s n the interven nemat a :lip e e zt ipla ea f w milli ra s o uranium o id :in: .enve1Ci e i u m itien thatth -,a1nh -,r ys emitted t ereb here t h d sen age Pa h wi hou pes net pugh;a ei su st ne n; essa n ee eieee n en qfo e ha t of this specification, a species of thermal switch of the glow discharge type is shown in a side elevational view.

Referring to the drawing, the glow-type thermal switch illustrated comprises a sealed, tubular envelope ID of glass having a glass stem is fused thereto. Said envelope [0 is about 50 mm. in length and about 18 mm. in internal diameter and has a gaseous atmosphere therein consisting of helium mixed with approximately 0.15 per cent of argon at a pressure of approximately 45 mm. mercury column. Two bimetallic strips I2 and [3, having contacts l4 and 15 attached to the end thereof remote from the stem l6, are mounted in parallel positions in said envelope [0. Said strip [2 is fastened, as by welding, to the current inlead wire I! and the strip [3 i similarly mounted on the current inlead wire :8. Said wires 11 and I8 pass through and make a hermetic joint with the stem IS. The contacts l4 and l 5 are separated a distance of approximately 5 mm. when the strips 12 and 13 are cold and engage each other when the strips are heated by a glow discharge therebetween. When the contacts I4 and I5 are thus engaged, the discharge in th switch is extinguished and after a lapse or a short interval of time, depending on the thermal capacity of the bimetallic strips 12 and t3, the contacts are moved away from each other by the flexing of the strips due to the cooling thereof.

When switches having this structure contain no radioactive material, the starting potential thereof in the light is approximately 1-60 volts. The starting potential rises considerably in the dark and is frequently higher than the voltage of th current source which is usually approximately 220 volts. When a small quantity of an alpha-ray emitting material, such as approxi mately 20 milligrams of uranium oxide, is distributed on the interior surface of the envelope, the starting potential is about 160 volts in the dark even after the switch has been in complete darkness for aweek. The starting potential of l the switch in the light is the same whether it contains radioactive material or not. The switch is thus highly useful for starting discharge lamps of the type having electrodes which require preheating.

I prefer to apply the uranium oxide to the inner surface of the envelope "directly above the contacts M and I5 as indicatedxat I9 in the drawing. Known methods of applying materials to glass surfaces are used. A satisfactory method comprises the steps of mixing fine particles of uranium oxide with a solution of nitrocellulose in amyl-acetate and applying the suspension thus produced to the desired area oftheenvelope). The nitrocellulose and amyl-acetate arethen removed from the envelope l0 in the usual 'manner during the manufacture of the.

switch'to' leave the'uranium oxide on the surface of the glass above the contacts lland i5. While I have shown and described'my invention in connection with one speciesof a glowtype thermal switch, I contemplate the use thereof in switches of different structure. For

example, radioactive material may be used-for the purposes of the invention in switches of the type disclosed in the copending application of Wayne C; Smitley, Serial No; 289,397, filed August 12, 1939, comprising astationary electrode and a bimetallic electrode constructed .and arranged to extinguish the discharge therebetween when said electrodes are heated by said discharge. The above applications are-assigned to the assignee of the present application. I contemplate, further, that the invention is useful in conjunction with switches having other ionizable gases therein, such as neon, argon, xenon, or krypton, or mixtures of such'gases, at suitable pressures; and in conjunction with switches having electrodes comprising .a metalsupport and material. of lowerwork function, such as magnesium, zinc, or alkaline earth compounds,

on said support to obtain a desired starting potential.

"In accordance with my copending application Serial No. 385,003, filed March 24, 1941, the envelope] IU- may consist'of uranium glass or the envelope is made of any of the uranium-free glasses used commercially for discharge devices, when desired, and the uranium glass is mounted in said envelope It. In such a structure the uranium glass is preferably in the form of a sleeve mounted about the electrodes thereof.'

Letters Patent of the United- States is:

A glow-type thermalswitoh comprising 'a sealed envelope containinghelium mixed with 0.15 per cent argon at 'a pressure of about manln, electrical contacts and radioactive mate'- ri a 1 k I BASIL NOEL CLACK.

stein 1-6 of uranium I

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