US3351726A

US3351726A - Low power mercury switch

Info

Publication number: US3351726A
Application number: US533105A
Authority: US
Inventors: Leonard W Cook
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1966-01-20
Filing date: 1966-01-20
Publication date: 1967-11-07
Anticipated expiration: 1984-11-07

Description

Nov. 7, 1967 1.. w. COOK 3,351,726

LOW POWER MERCURY SWITCH Original Filed March'l9, 1964 Inventor= Leonard W. Cook b mm a. mwci His Attorney United States Patent 3,351,726 LOW POWER MERCURY SWlTCH Leonard W. Cook, Warwick, R1,, assignor to General Electric Company, a corporation oi New York Continuation of application Ser. No. 353,658, Mar. 19, 1964. This application Jan. 20, 1966, Ser. No. 533,105 4 Claims. (Cl. 200-452) This application is a continuation of application Ser. No. 353,658, filed Mar. 19, 1964-, now abandoned, which is a continuation-in-part of application Ser. No. 113,177, filed May 29, 1961, now abandoned.

This invention relates to a low-power mercury switch; more particularly, the invention relates to a metal-envelope mercury switch of low resistance and high sensitivity.

In my patent US. 3,098,137, there is disclosed a glow switch consisting of a cylindrical metal housing into which an axially positioned terminal rod insulated from the housing extends a short distance. A bimetal element mounted on the portion of the terminal rod extending into the cylinder is distorted upon heating so as to make contact with the sides of the cylinder. The present invention is an outgrowth of the invention of US. 3,098,137, and can utilize the metal cylinder and terminal rod construction of that invention. However, in addition to a number of other modifications which can be made to provide novel articles and performance capabilities in general, a quantity of mercury is substituted for the bimetal element so that it is the orientation or movement of the cylinder rather than the temperature alone which effects operation of the switch. A mercury switch constructed in this manner possesses a number of valuable properties as will be outlined hereinafter.

One of the objects of the present invention is to provide a durable, metal-enclosed, mercury switch.

Another object of the present invention is to provide a mercury switch having a very small operating angle.

Another object of the invention is to provide a small, lightweight mercury switch.

Another object of the invention is to provide a durable, metal-enclosed switch which is unaffected by soldering or welding as an attaching means.

A further object of the invention is to provide a mercury switch which operates reliably at very low operating potentials.

The above and other objects are accomplished in accordance with the following specification taken in conjunction with the attached drawing in which FIG. 1 is a sectional view of a mercury switch constructed in accordance with this invention;

FIG. 2 is a perspective view of the metal elements of the invention prior to assembly;

FIG. 3 shows the construction of the cylindrical portion of the switch as viewed from the flanged end; and

FIG. 4 is an enlarged sectional view across the cylinder taken along the line 4-4- in FIG. 1 showing the shape assumed by the mercury in closed position. Briefly stated, in accordance with the present invention in one of its broader aspects, the invention is directed to a mercury switch comprising a hollow gas-tight cylindrical metal housing, at least one terminal rod extending through one end of the housing and being insulatingly sealed from the housing, and a quantity of mercury in the housing positionable to bridge a gap between a terminal rod and the housing and to open the gap according to the orientation of the cylinder in a mass-attracting field.

Referring to the drawing, one form of a switch provided by the present invention consists of a cylindrical housing ltl having a closed end 11 and a flanged end 12. Because of its low cost and because it can be treated in accordance with the present invention to resist the corro- 'ice sive action of mercury, cold rolled steel is a particularly satisfactory material for constructing the housing 10 as well as other metal parts of the switch used in contact with mercury. More expensive material such as stainless steel and surface coatings of refractory metal carbides, nitrides and borides would also be satisfactory in numerous such constructions as will be discussed more fully below.

It is sometimes desirable that the interior, or a portion of the interior of the housing It) be surface roughened as by sand blasting, acid etching, or by grooving as by the longitudinal grooves 13 spaced around the wall as shown in FIGS. 1 and 3. Regarding such surface roughening it has been found to reduce friction or the impediment to flow or other movement between the solid surface in contact with mercury and the mercury itself, while preserving low resistance electrical contact. However, to be effective the dimensions of grooving or other surface roughening must be substantially smaller than the dimensions of the mercury globule bearing on the roughened surface. The method of forming ultrasensitive switches which includes the steps of surface roughening by particle impingement or etching, and the products formed from such methods, including a pointed axial contact terminal, are described and claimed in my copending application S.N. 229,527, filed Oct. 10, 1962.

To continue with the description of the device of FIG. 1, there is welded to the flanged portion of housing 10 an assembly composed of a flange ring 14- and axially positioned terminal rod 15 sealed by means of an inorganic insulating material 16. Terminal rod 15 extends a short distance into the housing 10 where it may be brought into contact with a pool of mercury 1'7 in accordance with the angle of tilt of the housing with respect to a gravity or other field which attracts the mass of the mercury 17.

For the variety of switch illustrated in FIGS. 1-4, the

preferred amount of mercury in the switch is that quantity which, as it forms into a sphere, would have a diameter slightly less than the inside diameter of the housing 10. Mercury naturally tends to assume a globular shape and it is desirable for some applications of the switch to reduce the quantity of mercury used to a minimum in order to hold to a minimum the friction of the mercury surface with the switch elements in contacts. Other applications or mercury switches such as in thermostats operated by bimetal elements involves a contrasting use of larger amounts of mercury and also is described in my copendmg application S.N. 229,527, referred to above, where the object is to increase the switch sensitivity, i.e., to reduce its operating angle. This is done in part by increasing the contained mercury; in part by reducing flow resistance or friction at the side walls, and in part by using a pointed center contact element. Where such high sensitivity and small operating angle is not essential, smaller quantities of mercury may be employed even though the operating angle of the switch is thus increased. Reliable switches may be formed with smaller amounts of mercury because it is in part the roughened surface on the interior of the housing, such as the grooves 13- which reduces the friction of the mercury. It is also in part the cleanliness of the mercury surface, and absence from the surfaces contacted by the mercury of elements which tend to contaminate or amalgamate with the mercury. Since mercury tends to assume a globular shape, the bottom portions of the grooves 13 (or the valleys of any other type of roughened surface) do not make contact with the mercury thereby decreasing the friction of mercury with interior surfaces. Since the cylindrical surface is much greater than the surface ofthe terminal rod 15, this decrease does not significantly affect the electrical resistance through the switch.

Pursuant to the present invention, a switch is provided containing a half gram or less of mercury, having an overall weight of less than 2.5 grams, a durable steel shell, low internal resistance and great reliability of operation. Such a switch has numerous applications as will be pointed out hereinafter.

The inorganic insulating seal 16 is composed of glass or a ceramic material. A performed disk with a central aperture for the terminal rod 15 is a convenient form for the seal. The flange 14 is made quite thick in order to withstand stresses which might be established during fabrication. Where a preform is used for the seal 16, the preform is mounted on the terminal rod 15 and positioned in the central aperture of the flange 14. The assembly is then heated in a jig to a temperature above the softening point of the seal 16 to effect a gas-tight seal which insulates and positions the terminal rod with respect to the flange 14.

The foregoing describes some of the elements, ingredients and assemblies of the switch and from this description it may have a superficial appearance of being quite similar to switches taught in the art. However, the novelty and distinctness from the devices taught in the art will become additionally clear from the descriptions which follow of the methods by which they are prepared.

In essence, the improved method of the present invention is thought to reside in one of its broader aspects in the selective removal of those materials which are responsible for the higher surface contact resistance and contamination of mercury switches formed with metal casings. Stated in another way, the improved method resides in the formation on the interior of a metal casing of an irregular surface, the irregularities of which are of dimensions capable of supporting a liquid metal of high surface tension therebetween and out of contact with a substantial portion (greater than 50%) of the total surface; and wherein the upper portions of the irregularities of the surface are highly conductive; are in intimate electrical contact with the surface substrate, and are incapable of being dissolved by liquid metals brought into contact therewith.

While this describes in general terms the nature of the improved contact surface, its nature will become somewhat clearer by a further description of the several methods by which such a surface may be formed pursuant to the present invention.

A first method involves the selective removal of the more highly resistant portions of a composite surface to expose the more highly conducting, or potentially more highly conducting components of a multi-component solid material, from which an irregular surface of the present invention may be formed. This selective exposure method can be illustrated by reference to the method disclosed in my copending application, S.N. 229,527, referred to above. As is pointed out therein, extreme care must be exercised in the manufacture of the mercury switch of this invention if it is to have a small operating angle. The interior wall of the cylindrical housing is roughened by sand blasting or alternatively ultrasonically cleaned in detergent to remove any oils picked up in processing and it is then subjected to a further acid etching process. The acid etching may be achieved by dipping three times in 50% nitric acid for about 10 seconds each dip, with a distilled water rinse after each dip. The housing is then ultrasonically cleaned in detergent after which it is cleaned in a hydrogen furnace at approximately 1,000 F. The assembly formed as the closure member 14, terminal rod 15, and seal 16 is ultrasonically cleaned in detergent, rinsed in distilled water, and oven dried.

A measured quantity of ultra pure mercury is then metered into the cylindrical housing 10 and immediately thereafter a closure member-terminal rod assembly is placed in position on the flange 12. The housing 1% is then evacuated to about 10 micron pressure. The housing 10 is then filled with pure hydrogen or a rare gas such as argon at about 50 pounds absolute pressure and the closure member 13 is sealed to the flange 12 as by welding.

To understand the significance of the foregoing procedure in forming irregular surfaces contemplated by the present invention, one must briefly review the chemistry which takes place during the process. Chemically, a mild steel contains a small percentage of manganese, sulfur, phosphorous, and carbon and the remainder iron. The sand blasting may be useful in increasing the surface area of the metal surface which is to undergo treatment or which has already undergone treatment. The nitric acid which then is used to etch the surface makes a more rapid attack thereon. An important factor in the results produced by this etching is that although the iron is soluble in and dissolved by the nitric acid, the manganese contained in the iron is not as readily dissolved, but is rather oxidized to a manganese oxide insoluble in nitric acid. This is particularly likely to result where at least a portion of the manganese is initially present as the oxide. The probability of at least some of manganese being present as the oxide is quite high inasmuch as the manganese is included in the iron primarily as a deoxidizing and desulfurizing agent.

As is pointed out in my copending application, S.N. 229,527, as a preliminary step of the treatment procedure given above, the interior wall of the cylinder 10 was acid etched by contact with nitric acid. A comparison based on carrying out this step with sulfuric, hydrochloric and nitric acids showed that of these three acids, the nitric gives superior end product. This superior end product is due in part to the selective removal of the iron and the attendant exposure of the manganese oxide which remains at the etched surface due to its insolubility in the nitric acid.

Regarding the remainder of the process, it is also known that the manganese oxide is not reduced by the furnace hydrogenation treatment, but that it is in fact reducible by hydrogenation only with hydrogen gas at elevated pressure. Accordingly, it is only after the switch has been sealed by welding to contain a 50 pound absolute pressure of pure hydrogen that the conditions are provided for the reduction of the crests of manganese oxide which have been exposed through the selective leaching and removal of the surface layer of iron with the nitric acid.

The foregoing method is one for use of liquid leaching or etching agents to selectively expose an irregular pattern of a material incorporated within a metal matrix and to form thereby an irregular surface, the peaks or crests of which are both insoluble in mercury and either are or can be rendered highly conductive to a liquid metal brought into contact therewith.

However, the method of the present invention is not limited to the use of liquid leaching or etching agents to accomplish the selective exposure of a mercury-insoluble highly conductive material. This selective exposure also may be accomplished by means of gas and the following procedure illustrates this alternative method of the present invention.

By this method a mixture of carbon dioxide, carbon monoxide and nitrogen is employed containing, in general, a minor proportion of the carbon oxide. In a particular treatment of mild steel parts, the parts were furnaced on a travelling belt in an atmosphere containing 4.8% CO 10.6% C0; and the balance nitrogen. The furnacing was carried out at a temperature of about 1,800 F. and the belt traveled at a rate to keep the parts in the hot zone of the furnace for about 15 minutes. The parts were then cooled in this same atmosphere, but outside the hot zone for 10 minutes. This was followed by 5 minutes additional cooling at the discharge end of the furnace gas flow tube through which the belt traveled. The cooled parts were then washed in denatured ethanol for 10 minutes before being air dried with a hot air blower. To avoid contamination following furnacing and prior to assembly, the parts were stored in dust-free containers such as clean hot covered jars.

This example illustrates the selective removal of iron from the manganese oxide of the mild steel partially as the volatile iron carbonyl. In addition, due to the solubility of iron carbonyl in alcohol, the foregoing procedure may also, in part, provide for the removal of the iron carbonyl with a liquid leaching agent.

While hydrogen is the principal atmosphere on the interior of the switch resulting from the above-recited method of manufacture, other nonoxidizing atmospheres are satisfactory. The rare gases, and particularly argon, and mixtures of such gases with hydrogen, are entirely satisfactory. The pressure of 20-30 pounds per square inch absolute creates reducing conditions on the interior which promote long life. However, it is obvious that satisfactory results for many applications could be obtained from wider pressure limits.

If desired, a protective outer coating may be applied as by electroplating of copper or dipping in molten zinc. The switch of this invention lends itself very well to miniaturization. The actual size of a production model has an interior cylinder diameter of less than one-quarter inch and a length, excluding the terminal rod portion which protrudes, of less than one inch. Such a switch uses approximately a half-gram of mercury. This switch lasted for over 2,000,000 on off cycles with a 100- watt lamp on a 110 volt A.C. circuit. The minimum operation angle of the switch is about 4 degrees, and it is responsive to 1 /2 volts or less. The switch is very rugged and has applications in missiles where a gravity-responsive switch is desired as well as more prosiac applications such as automobile trunk lights, refrigerator lights, etc.

It may be incorporated in a circuit by soldering or welding the switch structure without damage to the device.

It is to be noted that the switch is symmetrical about the axis of the cylindrical housing and terminal rod 15. In applications where the switch can rotate about this axis during use, this feature extends the life of the switch since the contacting surfaces constantly change thereby preventing undue corrosive wear at any one point. By incorporating in the switch that quantity of mercury whose spherical diameter is only slightly less than the interior diameter of the cylindrical housing, the mercury tends to roll into contact position between the housing wall and terminal rod to assume the shape illustrated in FIG. 4an action which appears to improve the contact between the mercury and the rod housing.

From the foregoing it will be appreciated that although the roughening of a metal surface can improve the flowability of mercury in contact therewith, the lowering of the flow resistance or friction may be due primarily to a mechanical phenomena inasmuch as the amountof surface contacted is reduced. However, this surface roughening by itself is not sufficient to be the source of an increase of more than one thousandfold which has been found in the conductivity between the metal surface and theliquid mercury in contact therewith for switches prepared by the improved methods of this invention.

Alternative methods of forming highly conductive mercury insoluble irregular surfaces is by a deposit on ,a receiving surface of a layer of a particulate material having the desired properties. For this purpose generally, the refractory metals and compounds of refractory metals which have relatively low resistance and relatively low solubility in liquid metals may be employed. For examr ple, tungsten, tantalum, molybdenum and similar metals having melting points near or above 2,000 C. are useful either by themselves or in the form of compounds which have high conductivity and low solubility in liquid N metals. For example, titanium forms a highly conductive nitride as well as a carbide which is insoluble in liquid mercury even at elevated temperatures. Carbides, borides and nitrides of refractory metals are known and may be employed in themethod of the present invention or incorporated in articles hereof where the appropriate properties of high conductivity and low solubility in liquid metals are both present. Methods are known in the art for forming deposits of such materials on steel surfaces. For example, a metal powder may be dusted on a steel surface and then pressed into contact with the surface and then bonded there employing powder metallurgy techniques. The use of powders and powder metallurgy techniques is particularly useful where formation of an irregular surface layer is desired as the powder itself becomes the irregular surface when bonded to the metal substrate.

One procedure which effects both good bonding and good electrical contact between powder and metal substrate is a pressing of the deposited powder into contact with the surface followed by a heating to form a sinter bond therebetween.

While the foregoing procedures yield highly satisfactory switches, it is within the scope of the present invention to employ alternative materials, steps, and treatments in preparing switches which emphasize the generation of internal surfaces having very low resistance coupled with very low friction. It must be emphasized that these two properties are normally antagonistic in that in this art one normally associates low resistance with a wetting contact between the mercury and internal metal container surfaces but such Wetting severely restricts the flow of mercury along the internal surfaces.

The importance of mercury flowability or low mercury flow resistance in the switch is that it is this property which makes the switch motion sensitive, that is, highly responsive to changes in its position relative to a field attracting force such as gravity. As indicated above, the switch of FIGS. 1-4 containing about one-half gram of mercury operates when axially displaced through a minimum angle from its on position to its off position of only four degrees. However, for other switch configurations, some of which are illustratively discussed below, reliance is not placed solely on. angular displacement of a switch axis about which the switch is otherwise symmetrical.

The modifications of the switch design as well as of switch fabrication methods disclosed herein permit use in low power circuits such as those operating at voltages and amperages of the order of less than 0.001, volt and 0.001 ampere respectively without sacrifice in the motion or position sensitivity.

This is more than a thousandfold lower useful power level than the level given for the switch described in the predecessor application of this continuation-in-part.

For many applications, particularly those using low power sources, it is most desirable that the switch operate at low resistance. The modified mild steel mercury switches as described herein have been made to operate at a switch resistance of the order of 0.01 ohm and less.

While a low operating resistance may be due in part to the design of the switch shown in FIGS. 1 and 4 in providing a high level of contact between the central contact rod 15 and the shell 10, it can also be provided in switches of other design due to the improved nature of the electrical contact possible at the surface of the mild steel elements.

Because of the improved characteristics of the switch of the present invention, numerous modifications of structure and function can be accomplished.

A number of these modifications involve a change in the structure or location or content of the contact post such as 15 of FIG. 1.

In one modification, for example, the post may be placed eccentrically with respect to the casing 10 so that it will not be contacted uniformly by any movement of mercury within the switch. Where the amount of mercury is relatively smaller and the extent of eccentricity or axial displacement of the post 15 is larger, a

rotation of the switch about its axis when the axis is horizontal to a field of gravity, will produce a single signal corresponding to the flow of electricity from the post 15 to the shell 10 for each revolution of the switch. In this modification the switch becomes a rotational counting device. For example, where this modified form of the switch is mounted on a rotating member, the number of rotations of the member may be determined by means of the signals produced from the switch.

This type of counting application of the modified switch is useful at lower speed where the globule of mercury will not be broken up by the force of the post coming into contact with it. For rotary counting applications it is important that the globule maintain its integrity as otherwise spurious signals may be produced.

Where higher speeds are to be employed, modifications of the switch may include the incorporation of a ceramic spacer element in the container 10 so that a moving mercury globule can pass over two or more electrical contacts embedded in the ceramic spacer. The method of attaching a spacer having sections designed to regulate mercury flow within a sealed metal casing is disclosed in copending application, S.N. 104,939, filed Apr. 24, 1961 and assigned to the same assignee as the subject application. It is evident that in a switch such as that described therein, having a ceramic closure element similar to 16 of the subject application, but where the ceramic closure has a diameter of approximately /2 inch, that a substantial number of electrical contact pins can be sealed into the ceramic window. Where the contact pins are distributed at their internal extensions into various sections of the space enclosed by the switch, it is possible to produce a great variety of switch constructions adapted to be used in numerous specific applications. It is because the assembly of ceramic closure element 16, contact post 15 and metal ring are formed as a separate assembly to be later welded to the vessel 10, that accurate spacing of the several contact posts can be accomplished to form a large variety of switching elements.

One such application is in use of the switch as a distributor such as is employed with internal combustion engines to supply the electrical energy to the spark plugs. Another application for the switch element is as a liquid brush in distributing electrical energy to the segments of an electric motor. At the present time, both of these electrical distribution functions are performed by mechanical contact. In the case of motors, this is done by a contact of carbon with a metal surface in air and in the case of internal combustion motors, by the contact of metal conductor tips also in air. Because of the need for the electrical switching elements to be used in air, their life expectancy is not as great as that of the switching element described herein inasmuch as it can be used in a sealed container having a protective atmossphere.

While it has been the practice heretofore to employ mercury switches where the electrical contact is between a solid surface and a mercury globule only for applications where the amount of electrical energy transmitted through the switch is relatively small, a distinct advantage is made possible by the present invention is that it permits mercury switches which rely on liquid-solid contact for electrical transmission to be used for switching much higher levels of electrical energy. Evidence of the improved capability of the switch of the present invention is given above wherein it is noted that a small switch containing 'but /2 gram of mercury was employed in switching on the 100-watt bulb through 2,000,000 cycles. By increasing the manganese content of the steel from which the switch is formed so that a more highly refractory conductor surface is in contact with the mercury, the load carrying and load switching capacity of the switches is increased.

Materials which are used as coatings or liners for the switches of the present invention may be described generally as ceramic materials as this term is generally used meaning that the materials are useful at temperatures above about 1,000 C. and which have vaporization temperatures which are substantially higher.

The value of the higher vaporization temperature lies in the resistance which this imparts to the surface layer being affected by electrical discharges generated when an electrical contact is made or broken, Because the refractory material may comprise only a very thin layer on the electrical contact surfaces within a switch, the need for high conductivity in the refractory material is not as great as where the ceramic materials are employed in depth. Also, particularly where the thickness of the ceramic surface layer need not be substantial, switches can be formed pursuant to the present invention which employ ceramic materials which are more costly, without substantially increasing in the cost of the switch. For this same reason, more unfamiliar material may be employed where they have the combination of properties which render them suitable for use pursuant to the present invention. In general, the refractory metals such as tungsten, molybdenum and tantalum are useful as metals to form electrical contact surfaces for liquid metals within metal encased switches. However, several other metals which are not normally considered refractory metals, but which may be combined with other elements to form layers having high melting points and high vaporization points may be employed. For example, carbides, nitrides, borides, silicides and sulfides of various sorts had been considered as refractory materials and uses can be made of such metal compounds which possess the set of properties discussed above.

For example, a material which has a useful combination of properties is silicon carbide. This has been used in many refractory shapes and has an outstandingly good combination of thermal and electrical conductivity. The properties, in addition to good heat shock resistance, strengths at high temperatures and abrasion resistance. Pursuant to the present invention, a deposit of particular silicon carbides may be formed at the surface of metal casing of a switch element and will provide both low flow resistance to contained mercury as well as high conductivity from the mercury through the carbide layer to the metal surface.

Another group of materials which may form surface liners for metal switch housings are the so-called cermet materials which are intimate mixtures of metals and nonmetals. Illustration of such a material is a cermet containing aluminum oxide, Al O and chromium. Although some cerinets are formed with oxides such as that indicated above is more common to incorporate a carbide or nitride as in cemented tungsten carbide. The surface formed on the switch of FIG. 1 in the manner described above may be thought of as closely analogous to a cermet prior to the reduction of the manganese compound by high pressure hydrogen.

While particular embodiments of this invention have been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects and, therefore, it is intended that the appended claims cover all such changes and modifications as fall within the true spirit and scope of this invention.

I claim:

1. In a metal enclosed mercury switch the improvement which comprises a low resistance at an internal surface, said surface being formed as an internal surface of a steel casing containing at least 2.0% manganese in the steel thereof, and said surface having a higher manganese content than that of the steel due to the preferential depletion of iron from said surface.

2. The switch of claim 1 in which the manganese of said iron depleted surface is at least partially present as manganese oxide, and said metal enclosure contains hydrogen under pressure.

3. The switch of claim 1 wherein the switch contains less than one-half gram of mercury, the internal resistance of said switch through said mercury is less than 0.01 ohm and wherein the switch is operable in low power circuits at less than 0.001 volt and 0.001 ampere.

4. A mercury switch comprising a hollow gas tight metal housing,

a quantity of mercury in said housing said metal housing being provided with a roughened interior surface,

said roughened surface being formed of particles deposited on and bonded to said metal housing,

said particles being chosen from the group consisting of refractory metals and compounds of refractory metals which are of high conductivity and low solubility in mercury at least one terminal rod extending through one end of said housing,

References Cited UNITED STATES PATENTS Warren 200-152 XR Bear et a1 200-152 Harrison 200152 Bucklen et al 200-152 Engel et a1 200-152 ROBERT S. MASON, Primary Examiner.

ROBERT K. SCHAEFER, Examiner. 20 H. J. HOHAUSER, Assistant Examiner;