Mercury containing reed switches

Description

Field of the Invention

[0001] The present invention relates to a method of preparing mercury containing reed switch blades for use in such switches.

Background of the Invention

[0002] It is important that reed switches used in automatic test equipment, process control equipment and data processing terminals should have very stable and consistent contact resistance over many millions of operations.

[0003] Although in most of the above applications 'the power handled by these switches is very low (less than 100 milliwatts) the very high number of operations during the expected life of the switches brings about gradual deterioration of the contact surface caused by minute electrical transients, mechanical erosion and electrochemical disturbances.

[0004] Dry reed switches are known for such applications which incorporate switch blades normally made of nickel/iron plated with a thin layer, about 2 µm of a suitable noble metal such as gold, palladium, rhodium or ruthenium.

[0005] During operation of these switches the plated noble metal is eroded by the arc formed on make and break which causes a steady deterioration of contact resistance and limits the life of the switch.

[0006] In general only reed switches containing mercury are able to satisfy fully the requirements of contact resistance stability.

[0007] A known mercury-wetted reed switch comprises a pair of switch blades extending within a glass envelope and hermetically sealed therewith. · Each switch blade has a spaded end portion the free ends of which overlap to form the switch contact. One of the spaded portions is either wholly or partially plated with a substance wettable by mercury, for example gold, the other blade being non-wettable by mercury except for a very small contact button welded to the blade.

[0008] To ensure that the mercury-wettable switch blade is provided with sufficient mercury to produce the contact resistance required, especially under high power applications, the mercury dosage is such as to provide a reservoir of mercury within the envelope.

[0009] The presence of such an excessive amount of mercury in the envelope has the disadvantage that the reed switch must be operated in a vertical position such that the mercury reservoir is located at the bottom of the capsule and therefore does not swamp the contact area between the blades thus interfering with make and break.

[0010] Moreover the presence of mercury in the envelope means that during the metal-to-glass sealing procedure, the mercury must be held at an extremely low temperature down to around minus 50° centigrade so as to obviate the production of mercury vapour which would affect the seal.

[0011] GB-A-778229 discloses a method of making electric contacts including a gold amalgam. A pasty gold amalgam is transferred to the contact surfaces by rubbing over the contacts a gold plated pencil tip which has been disposed in the mercury gold amalgam.

[0012] US-A-3576415 discloses an electrical contact surface of gold which is wetted with mercury and then tumbled or agitated to achieve uniform coating with mercury, followed by heating to cause amalgamation of the gold and mercury.

Summary of the Invention

[0013] It is the aim of the present invention therefore to obviate these problems and to this end, attempts have been made to control the amount of mercury introduced into the envelope so that an extremely thin layer of mercury is present at the contact area sufficient to provide the stability of contact resistance required.

[0014] This objective has been achieved by the discovery that such switches may be manufactured by introducing mercury to the contact area of the switch blades in amalgam form. This has considerable advantages in that firstly the introduced mercury is, at the glass-to-metal sealing stage, remote from the sealing area thus obviating the necessity of sealing under low temperature conditions. Secondly the amount of mercury introduced is controlled thus obviating mercury wastage and reducing the adhesion, due to surface tension forces, between the mercury at the contact area.

[0015] All these advantages lead to reduction in cost of manufacture and considerable improvement in performance enabling switches to be produced having stable contact resistance over at least fifty million operations.

[0016] According to the invention there is provided a method of forming mercury containing reed switch blades suitable for being mounted in an hermetically sealed envelope with their free ends gold plated in overlapping relationship to form thereat a make and break contact of the switch, a gold amalgam being formed by rubbing the tip portion of the gold plated free ends in mercury for a predetermined time to form a gold amalgam, characterised in that the gold amalgam is formed by loosely packing a batch of said blades in a container having a mercury-wetted base with which said tips make contact, said packed container being eccentrically rotated so that the tips move correspondingly over the base of the container to form the amalgam.

[0017] Mercury containing reed switches formed in this way are extremely suitable for use in low level, low power applications. In contradistinction to presently known reed switches which are heavily dosed with mercury after the switch blades have been encapsulated in the switch envelope, the present invention provides a more simple method of forming the contact area on the switch blades so enabling the reed switches to be more cheaply constructed for low power applications in the computer industry.

[0018] Moreover the formation of the amalgam on the switch blades reduces the amount of expended mercury in any given switch which is an important cost reducing factor, improves the life of the switch and reduces the deleterious effects of, amongst other things, mechanical erosion.

Best Modes of Carrying Out the

Invention

[0019] Since the contact area of interest between the switch blades of a reed switch is extremely small only a fraction of the tip portion of the spaded portion of each of the switch blades need be provided with mercury and this is where the method of the invention has considerable advantages over the prior art.

[0020] In a preferred form of carrying out the invention a batch of switch blades comprised of nickel/iron having spaded portions plated with gold, are placed in a container such that the tips of the spaded portions are in contact with a thin film of mercury which has been deposited on a mercury-wettable substrate at the base of the container.

[0021] A number of such containers are then eccentrically rotated together for a time sufficient to form a mercury gold amalgam at the tips of the spaded portions effected by rubbing contact between the gold and mercury during the rotation.

[0022] Typically to adequately amalgamate the tip portions of 3000 blades, a 2 mg amount of mercury is deposited onto the base of a container of sufficient size to accommodate 300 loosely packed switch blades, and ten such filled containers are together rotated eccentrically for a period of two minutes. In this way the tips of the blades are maintained in eccentric rotational rubbing contact with the mercury and this has been found to be sufficient to amalgamate the mercury with the gold to provide the mercury/amalgam contact area required.

[0023] Subsequent to this treatment of the blades, a pair of such blades are mounted in position in a glass envelope and a metal-to-glass seal is effected between the shanks of the switch blades and the glass envelope.

[0024] Apart from the layer of metal which forms amalgam with mercury, the performance of the switches described can be further enhanced by introducing additional metals such as tin, copper and silver to the material of the switch blades. This is done to ensure that the formation of nickel- mercury compounds does not occur on the blades which could cause sticking at the contact area.

[0025] Other features and advantages of the present invention will be readily apparent to those skilled in the art without departing from the scope of the invention as defined in the appended claims.

[0026] In particular other methods of forming the mercury amalgam at the tips of the switch blades can be readily contemplated.

[0027] The reed switches provided by the invention have been shown, under test, to have considerable operational improvements over mercury-wetted reed switches which are doped with mercury by injection techniques, in low power applications.

[0028] Moreover the introduction of mercury in controlled quantities by means of the invention, helps to control mercury vapour pressure in the switch envelope, this being a contributory factor, at least, to improved performance.

Claims

1. A method of forming mercury containing reed switch blades suitable for being mounted in an hermetically sealed envelope with their free ends gold plated in overlapping relationship to form thereat a make and break contact of the switch, a gold amalgam being formed by rubbing the tip portion of the gold plated free ends in mercury for a predetermined time to form a gold amalgam, characterised in that the gold amalgam is formed by loosely packing a batch of said blades in a container having a mercury-wetted base with which said tips make contact, said packed container being eccentrically rotated so that the tips move correspondingly over the base- of the container to form the amalgam.

2. A method as claimed in claim 1 wherein the material of the switch blades is nickel/iron with the addition of a predetermined amount of any one of the materials tin, copper or silver.

3. A method as claimed in claim 1 or claim 2 wherein the container has a base mercury-wetted with 2 mg of Hg and 300 of said blades are loosely packed in said container.

Ansprüche

1. Verfahren zur Herstellung von Quecksilber enthaltenden Reedschalter-Zungen, die zur Montage in einer hermetisch abgeschlossenen Umhüllung vorgesehen sind und mit ihren freien, goldbeschichteten Enden in überlappender Anordnung zueinander sind, um dort einen Wechselkontakt des Schalters zu bilden, wobei eine Gold-Quecksilberlegierung gebildet wird, indem der Spitzenbereich der goldbeschichteten freien Enden eine bestimmte Zeit lang in Quecksilber gerieben wird, um eine Gold-Quecksilberlegierung zu bilden, dadurch gekennzeichnet, daß die Gold-Quecksilberlegierung durch lockeres Einfüllen einer Charge der Zungen in einen Behäl- . ter gebildet wird, der einen mit Quecksilber benetzten Boden aufweist, mit dem die Spitzen in Kontakt stehen, wobei der gefüllte Behälter exzentrisch in Rotation versetzt wird, so daß sich die Spitzen entsprechend über den Boden des Behälters bewegen, um die Quecksilberlegierung zu bilden.

2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß das Material der Schalterzungen Nickel/Eisen mit Zusatz eines bestimmten Betrages eines der Materialien Zinn, Kupfer oder Silber ist.

3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß der Behälter einen mit 2 mg Quecksilber benetzten Boden aufweist, und daß 300 Zungen locker in den Behälter eingefüllt werden.

Revendications

1. Un procédé de réalisation de lames pour relais à lames de contact sous enveloppe scellée contenant du mercure et appropriées pour être montées dans une enveloppe scellée hermétiquement avec leurs extrémités libres plaquées d'or en relation de recouvrement pour former, à cet endroit, un contact ouvert-fermé pour le relais, un amalgame d'or étant formé en frottant le bout des extrémités libres plaquées d'or dans du mercure pendant une durée déterminée à l'avance afin de former un amalgame d'or, caractérisé en ce que l'amalgame d'or est formé en plaçant, sans les serrer, un lot desdites lames dans un récipient présentant une base mouillée au mercure et avec laquelle lesdits bouts viennent en contact, ledit récipient ainsi rempli étant amené à tourner de manière excentrée pour que les bouts se déplacent de manière correspondante sur la base du récipient afin de former l'amalgame.

2. Un procédé selon la revendication 1, dans lequel la matière des lames du relais est du nickel/ fer avec l'addition d'une quantité prédéterminée de l'une quelconque des matières étain, cuivre ou argent.

3. Un procédé selon la revendication 1 ou la revendication 2, dans lequel le récipient a une base mouillée par du mercure avec 2 mg de Hg et 300 desdites lames sont placées dans ledit récipient sans être serrées.