MERCURY CONTAINING REED SWITCHES

Abstract: The present disclosure relates to mercury containing reed switches and to methods of preparing switch blades for use in such switches. Heretofore such reed switches have been doped with mercury after the reed switch blades have been mounted in the envelope, the mercury migrating to the contact area of the switch blades, made of a mercury-wettable material, after sealing. The present invention avoids the ex­ pense and complexity of this procedure by improving the con­ trol of mercury dosing to enable improved performance of such reed switches in low power applications, for example in automatic test equipment, process control equipment and da­ ta processing terminals. This is achieved by forming a mercury amalgam on the contact area of the switch blades prior to in­ ertion in the glass envelope and subsequent sealing of the switch blades to the glass envelope.

Description: "Mercury Contains g iseed Switches" FIELD OF THE INVENTION The present invention relates to mercury containing reed switches and to a method of preparing switch blades for use in such r#itchcs. BACKGROUND OF THE INVENTION It is important that reed switches used in automatic test eqlJipment, process control equipment and data processing terminals should have very stable and consistent contact resistance over many millions of operations. Although in most of the above applications the power handled by these switches is very low (less than 100 milliwÅatts) 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. Dry reed switches are known for such applications which incorporate switch blades normally made of nickel/iron plated with a thin layer, about 2 Fm of a suitable noble metal such as gold, palladium, rhodium or ruthenium. During operation of these switches the plated noble metal is eroded by the arc formed on make and break which causes a steasy deterioration of contact resistance and limits the life of the switch. In general only reed switches containing mercury are able to satisfy fully the requirements of contact resistance stability. 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. 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 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. 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 thesseal. SUMMARY OF THE INVENTION 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. 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. 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. According to the invention there is provided a method of forming a mercury containing reed switch having a pair of switch blades mounted in an hermetically sealed envelope with their free ends in overlapping relationship to form thereat a make and break contact of the switch, characterised in that prior to mounting the switch blades in the envelope a mercury amalgam is formed on a part of at least one of the switch blades which is in contact with the other blade during the make condition of the switch. 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. 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 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. 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 A number of such containers are t}ien eccentrically rotated ttL ther 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. 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. 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. 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. 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. In particular other methods of forming the mercury amalgam at the tips of the switch blades can be readily contemplated. 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. 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: CLAIMS 1. A method of forming a mercury containing reed switch having a pair of switch blades mounted in an hermetically sealed envelope with their free ends in overlapping relationship to form thereat a make and break contact of the switch, characterised in that prior to mounting the switch blades in the envelope a mercury amalgam is formed on a part of at least one of the switch blades which is in contact with the other blade during the make condition of the switch. 2. A method as claimed in Claim 1 wherein at least the tips of the switch blades are gold plated, and the amalgam is a mercury/gold amalgam. 3. 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. 4. A method as claimed in Claim 1 wherein each of the switch blades has a gold plated spaded portion the tip of which is rubbed in mercury for a predetermined time period to form a mercury/gold amalgam. 5. A method as claimed in Claim 4 wherein a batch of 300 of said blades is loosely packed in a container having a base mercury-wetted with 2 mg. of Hg. with which said tips make contact, one or more of such packed containers being eccentrically rotated such that the tips move correspondingly over the base of the or each container to form the amalgam.

Inventors: COOTE, CYRIL EDWARD, HILL, JOHN, TURCZANSKI, HENRYK
Applicants: STANDEX ELECTRONICS (UK) LIMITED
Application year: 1982
Main classification: H01H11
IPC codes: H01H1