US3909569A

US3909569A - Inertia switch having movable liquid contact medium retained in reciprocating actuator and engaging helical fixed contact array

Info

Publication number: US3909569A
Application number: US490200A
Authority: US
Inventors: W Dale Jones
Original assignee: Individual
Current assignee: Individual
Priority date: 1974-07-19
Filing date: 1974-07-19
Publication date: 1975-09-30
Anticipated expiration: 1992-09-30

Abstract

A hermetically sealed piston in cylinder type inertia switch employing an electrically insulative piston with mercury seal rings which serve both as bearings and as movable electric switch contacts to engage helically arranged and very closely spaced mercury wetted contacts embedded in and flush with the inner surface of a close fitting electrically insulative cylinder case. Response of the piston to acceleration is spring controlled in acceleration switching applications and is gas metering controlled in velocity change amount switching applications. The fluid engagement between the movable and fixed contacts and the very close axial spacing of the fixed contacts enables the invention to avoid switching error commonly resulting from engagements of compliant type contacts, to virtually avoid calibration error and to be programmed to switch within broad acceleration and velocity change amount ranges.

Description

United States Patent 91 Jones 1 INERTIA SWITCH HAVING MOVABLE LIQUID CONTACT MEDIUM RETAINED IN RECIPROCATING ACTUATOR AND ENGAGING HELICAL FIXED CONTACT ARRAY {76] Inventor: W. Dale Jones, 7020 Aztec Rd.

N.E.. Albuquerque, N. Mex. 87110 122] Filed: July 19, 1974 l21| Appl. No; 490.200

[52] US. Cl. 200/6I.S3; 200/6147; 200/209; 200/210 [51] Int. Cl. H01H 35/18; H01H 29/00 [58] Field of Search ZOO/61.45 R, 61.45 M, 61.52, 200/6147. 61.53. 209. 210, 214. 233. 234; 73/1 D; 335/47, 48

1 1 Sept. 30, 1975 3,786.217 1/1974 Bitko 200/210 X Primary E.\'amiuer-.lames R. Scott 15 7 1 ABSTRACT A hermetically sealed piston in cylinder type inertia switch employing an electrically insulative piston with mercury seal rings which serve both as bearings and as movable electric switch contacts to engage helically arranged and very closely spaced mercury wetted contacts embedded in and flush with the inner surface of a close fitting electrically insulative cylinder case. Response of the piston to acceleration is spring controlled in acceleration switching applications and is gas metering controlled in velocity change amount switching applications. The fluid engagement between the movable and fixed contacts and the very close axial spacing of the fixed contacts enables the invention to avoid switching error commonly resulting from engagements of compliant type contacts. to virtually avoid calibration error and to be programmed to switch Within broad acceleration and velocity change amount rangesv 12 Claims, 5 Drawing Figures U.S. Patnt Sept. 30,1975 Sheet 2 of2 3,909,569

INERTIA SWITCH HAVING MOVABLE LIQUID CONTACT MEDIUM RETAINED IN RECII'ROCATING ACTUATOR AND ENGAGING IIELICAL FIXED CONTACT ARRAY BACKGROUND OF THE INVENTION Inertia switches are commonly used to initiate functions in vehicles, to control speed of machinery, to record shock forces and the like. In many of these applications the utility of prior art switches is limited because only one or two acceleration levels can be switched. This limitation arises mainly from the inertia sensing error which can occur when the movable switching member successively engages compliant type contacts, the mechanical forces involved in deflecting the compliant contact introducing a restraint on movement of the movable switching member which produces an unacceptable error, from a practical standpoint, when three or more linerally-spaced compliant contacts are embodied in the switch. Also, the use of such contacts requires a relatively great displacement distance per contacting function of the movable switching member, and hence great switch volume in breakbefore-make switching applications, while the expected inertia sensing error effects of closely spaced makebefore-break compliant fixed contacts would be unacceptable in most applications.

In many vehicular applications requiring inertia switching as means for initiating control functions the effectiveness of the functions could be substantially improved by having means for selecting and programming the threshold acceleration level at which the function is to be initiated, depending on the unexpected acceleration experience of particular vehicle trips. These improvements cannot be effected with required switching accuracy and space, weight and cost parameters through prior art switching technology, and thus have seldom been attempted. One such attempt is represented in the two level acceleration switch invented by R. Urenda and disclosed in US. Pat. No. 3,715,535. Calibration of the axial positions of the fixed contacts of this switch is made possible by screw type adjustment means. However, because of the relatively great switch length required for this adjustment feature and for the break-before-make switching requirement, acceleration switches thus designed require at least one cubic inch of volume per acceleration level to be switched.

The present invention is specifically designed to overcome the limitations cited above.

SUMMARY OF THE INVENTION The problems referred to above are overcome in the present invention by providing a piston in cylinder switching geometry which enables the inertia sensing piston to employ deposits of mercury both as electrical contacts and as bearings, which makes it possible to use very closely spaced and fluid frictioning fixed (noncompliant) wall embedded contacts, and which makes it possible to originally locate or calibrate multiple iner tia switching level contacts with great accuracy under equivalent end use environment conditions and recalibration of contact locations at any time without breaking the hermetic seal.

A method developed for completely filling the pistons grooves with mercury in a manner which avoids the entrapment of bubbles is disclosed.

These objectives and others, such as compactness and minimization of extraneous shock effects, will become apparent to those skilled in the art when considered in reference to the examples of the inventions embodiment forms and applications described herein.

IN THE DRAWINGS FIG. I is a side elevational view, partially broken away, showing one form of the invention;

FIG. 2 is a detailed cross-sectional view of the FIG. I embodiment;

FIG. 3 is a detailed cross-sectional view showing a modified form of piston;

FIG. 4 is a detailed cross-sectional view showing still another form of piston assembly; and

FIG. 5 is a detailed cross-sectional view of another form of the invention.

Referring first to FIGS. 1 and 2, an acceleration responsive switch embodying the present invention is shown as including a hollow, cylindrical casing designated generally I0 having its opposite ends closed by

end caps

12 and 14 to define a hermetically sealed chamber I6. A piston designated generally I8 is slidably received within chamber 16 and is mechanically coupled to end cap 12 by a spring 20 which normally acts in tension so that the displacement of piston 18 from end cap 12 in response to an acceleration of the switch casing axially to the left as viewed in FIG. I is proportional to the magnitude of applied acceleration.

Casing 10, end caps 12 and I4, and at least the exposed exterior surfaces of piston 18, are formed from an electrically non-conductive material which preferably, for reasons set forth below, is dimensionally stable in the face of temperature variations and which is capable of being formed and machined to close tolerances with an extremely smooth surface. A preferred material for this purpose is a high glass content diallyphthalate.

Referring now particularly to FIG. 2, piston 18 may take the form ofa cylindrical mass or slug 22 of metal or other electrically conductive material, upon whose periphery is fitted a cylindrical sleeve 24 and a cap 26 of electrically non-conductive material, both the sleeve and cap being mounted upon slug 22 with a firm press fit. The facing ends of sleeve 24 and cap 26 are axially spaced from each other to define a circumferentially extending annular groove 28 in the periphery of piston I8, the bottom or radially inner wall of groove 28 being defined by an exposed surface of the metal slug 22. Sleeve 24 and cap 26 are both formed of diallyphthalate and the outer diameter of the sleeve and cap and the opposed surface of chamber 16 are machined and formed to a high finish with dimensions such that a clearance of about 0.0002 to 0.0005 inches is provided between the opposed surfaces of piston I8 and chamber I6. This clearance has been somewhat exaggerated in FIG. 2 for purposes of explanation.

Groove 28 is filled with mercury. after the piston is located in chamber 16, in a manner described below, the ring of mercury 30 contacting the wall of chamber In to support piston I8 for sliding movement relative to casing I0. A plurality ofcircumferentially spaced axially extending grooves 32 in sleeve 24 retain individual pools of mercury 34, the ring 30 and pools 32 functioning as a slide hearing which slidably supports piston I8 for movement within chamber 16. Ring 30, in addition to functioning as a slide bearing, also serves as a peripheral gas or fluid seal isolating those portions of chamber 16 opposite sides of the piston from each other. In addition, ring 30 functions as the movable electric contact of the acceleration responsive switch.

Because those portions of chamber 16 on opposite sides of piston 18 are sealed from each other by mercury ring 30, one or more orifices such as 36 are formed to pass through piston 18 to accommodate movement of the piston with a controlled degree of damping. Chamber 16 may be filled with a chemically inert damping fluid such as silicone oil or nitrogen, depending on desired piston response.

Spring 20 is of electrically conductive material and has its right-hand end fixedly secured, in any suitable manner, to piston 18, with the conductive spring electrically connected to mercury ring 30. In the form shown in FIGS. 1 and 2, the end of spring 20 is axially fixed to slug 22, as by c rings, with the electrical connection from spring 20 to ring 30 being constituted by the electrically conductive material of slug 22. The opposite end of spring 20 is mechanically anchored to the casing as at end cap 12 and is electrically connected to a terminal 38 in casing which is accessible at the exterior of the casing.

A plurality of electrical contacts 40 are embedded in the wall of casing 10, the contacts 40 extending entirely through the casing wall and having their inner surfaces finished flush with the wall of chamber 16. Each contact 40 is accessible at the exterior of the casing so that the contacts 40 may be individually connected into appropriate electrical circuitry.

Contacts 40 are arranged in a series of axially aligned rows, the axially extending rows being uniformly spaced from each other circumferentially of casing 10, and the contacts in adjacent rows being uniformly axially offset from each other so that the contacts are not only aligned on axially extending center lines, but are also centered on a helical line concentric with the axis of casing 10.

The helical contact pattern enables an extremely precise determination of the axial position of piston 18 within the casing because the contacts in adjacent rows may axially overlap each other to present a different set of contacts engaged with ring 30 in response to a very slight axial displacement of piston 18 by the use of appropriate external electrical circuitry, connected to ring 30 and to the individual contacts 40. The precision of piston position measurement is effectively inversely proportional to the pitch of the helical pattern upon which the contacts are centered. For example, if the axial offset between contacts in adjacent rows such as contacts 400 and 40b (FIG. 2), is 0.002 inches, over a range of travel of piston 18 of 5 inches some 2500 switching combinations would be provided, giving the possibility of measuring increments of acceleration of 0.04 g over an acceleration range of lUO g.

The mercury bearing support provided piston 18 by ring 30 and pools 34 combined with the high degree of finish of the wall of chamber to achieves a relatively low friction mounting of the piston which is substantially uniform throughout the range of piston movement because any variation in sliding friction between ring 30 and the casing wall and between ring 30 and the flush surfaces of contacts 40 is averaged out. The stationary fixed contacts 40 avoid the intermittent resistance to piston movement inherently present in systems employing compliant or spring biased contacts. Where chamber 16 is filled with inert fluid, the surfaces of contacts 40 exposed on the wall of chamber 16 are preferably mercury wetted.

It is believed apparent from the foregoing description that in use casing 10 would be fixedly secured upon a test structure with the axis of the casing extending parallel to the direction of acceleration of the test structure. The various contacts 40 and spring terminal 38 are connected in appropriate external electrical circuitry, not shown, designed to indicate or to record the position or displacement of piston 18 within the casing in response to accelerations applied to the test structure by the various combination of contacts 40 engaged by ring 30. The general range of accelerations to be measured by the instrument is established by the stiffness or spring constant of spring 20, the mass of piston 18 and the damping factor applied by the restricted orifices 36 through the piston, the acceleration a being approximately equal to the spring constant k multiplied by the displacement x of the piston from its normal position divided by the piston mass m.

Although the embodiment disclosed in FIG. 1 discloses a single spring 20 connected to act in tension to constrain movement of piston 18 away from end cap 12 in response to axially directed acceleration of casing 10 to the left as viewed in FIG. 1, it is believed apparent that other spring suspension arrangements may be employed, as, for example, a spring connection between piston 18 and both of

end caps

12 and 14 with piston 18 being located centrally of the axial extent of contacts 40 when in its rest position so that both positive and negative accelerations could be sensed. Where spring 20 has the possibility of engaging contacts 40. the exposed portions of the spring are preferably coated with an electrical insulating material.

The dimensions and circumferential locations of mercury pools 34, electrically insulated from each other, are selected so that pools 34 cannot serve as switching members. Piston 18 is provided with a suitable means (not shown) for preventing its rotation within casing 10.

Mercury is introduced into groove 28 and recesses 32 with the piston located within casing 10 in the following manner.

Groove 28 is filled with mercury by inserting the piston into the left-hand end of casing 10 as viewed in F IG. 1 and locating groove 28 in alignment with two diametrically opposed bores 42 and 44 which extend through the wall of casing 10. The casing is positioned with

bores

42 and 44 on a true vertical line and mercury is poured into the upper bore 42. From bore 42 the mercury passes downwardly along both of the opposite sides of groove 28 to push air ahead of the mercury outwardly through the bottom bore 44. Mercury is passed through bore 42 and permitted to drain through the relatively restricted bore 44 until the groove system is filled, the 0.0002 to 0.0005 inch clearance between piston 18 and the wall of chamber 16 being small enough so that the surface tension of the mercury prevents the mercury from flowing along the external sides of piston 18.

When groove 28 is filled, opening 44 is plugged and the piston is slightly advanced to locate one of the axially extending recesses 32 with its right-hand end, as viewed in FIG. I, underlying bore 42 and its opposite end underlying a second bore 46. Mercury is then passed into one of the

bores

42 and 46 to fill the recess while the recess is vented through the other bore and the process is repeated for the remaining grooves 32.

A modified form of piston 18 is shown in FIG. 3 wherein instead of a continuous annular mercury ring 30, two or more circumferentially segmental pools of

mercury

30a and 30b are employed. This arrangement presents the possibility of using the switch to control two independent circuits, by connecting spring 20' electrically to only pool 30 while employing pool 30b as a bridging contact to complete external circuits connected between various fixed contacts such as 400 and 40b (FIG. 1), in accordance with the piston position. It can be appreciated that the pattern or arrangement of circuit connected fixed contacts 40 will be varied and designed in accordance with acceleration responsive switching requirements.

In FIG. 4 there is disclosed a form of piston especially designed to sense or signal post peak accelerations where it is desired to initiate a function only after the acceleration of a carrier vehicle has fallen off from a peak acceleration.

In the FIG. 4 embodiment, a piston is constructed with a cup-shaped hollow shell of an electrical insulating material 60 having a complete ring of mercury 30 and additional mercury pools 34" generally corresponding to the ring and

pool

30 and 34 of the FIG. I embodiment. At the rearward or lefthand end of the piston cup 60. an annular metal switching ring 62 is fixedly secured, as by adhesive bonding or other suitable method. A puddle of mercury is deposited on the circumferential periphery of switching ring 62. A metallic slug 66 is loosely slidably received within the interior of cup 60 and is coupled to an end cap (not shown) by a spring 20" which passes freely through the opening of the switching ring 62. The weights and support frictions of cup 60 and slug 66 are chosen such that the cup can move axially under normal conditions only when its end wall or conductive ring 62 is engaged by the moving slug 66.

Assuming the casing is subjected to an acceleration (tending to move casing 10" to the left as viewed in FIG. 4), the inertia of slug 66 will cause the slug to move relatively to the right as viewed in FIG. 4, and the slug will come into contact with the end wall of piston cup 60 as shown in FIG. 4 to drive piston cup 60 to the right relatively as viewed in FIG. 4, as long as the acceleration is increasing. As slug 66 retracts to the left following peak acceleration, cup 60 will remain at its peak acceleration position until pulled therefrom by the slug. When slug 66 contacts conductive switching ring 62, an electrical circuit will he completed from spring thus conditioning the system to switch programmed circuits connected to contacts 40' embedded in the casing wall. By this arrangement, the FIG. 4 embodiment operates as an acceleration responsive switch which is responsive only to decreasing acceleration. The distance necessary for slug 66 to travel from contact with the end wall of piston cup 60 to contact the conductive ring 62 can represent a displacement representative of a decrease in acceleration of a selected magnitude below maximum of peak acceleration.

While the use of pools of mercury as piston support and switching means is assumed in the FIG. 1 illustrated accelerometer embodiment of the invention, it can be appreciated that the piston support and switching means can be effected entirely with metal rings such as 62 wetted with mercury puddles. Advantages of use of mercury puddles rather than pools as piston support and switching means are two-fold.

First, because a close fit between the piston s circumferential periphery and the inner surface of the cylinder case is not a condition for keeping the mercury puddles intact, the use of mercury puddles makes it possible for the fit clearance to be 0.001 to 0.003 inch as compared with 0.0002 to 0.0005 inch which is felt necessary to insure that mercury pools such as 30 will remain intact in the face of severe shock forces. Second, the circumferential periphery of a ring may be relieved to the extent that only 0.0l inch or less effective contact width is obtained, an advantage in invention applications warranting break-before-make switching of axially closespaced circuit connected fixed contacts 40. While the use of mercury puddles as the piston support and the switching means would be acceptable in certain applications utilizing the invention embodiment disclosed below; where extraneous shock forces are not experienced and/or where great switching accuracy is not required, the preferred piston support and switching means of this embodiment are pools, rather than puddles, of mercury. The preferred electrically insulative material for the piston, cylinder and end caps is, as stated, diallyphthalate with high glass content. This very dense and dimensionally stable material can, when molded, be machined to very exacting tolerances and can be given a virtual mirror finish. In order to further minimize friction the mating surfaces of invention pistons and cylinders made of diallyphthlate should be treated with hydrofluoric acid to dissolve minute asperities of glass.

A velocity change switching or velocimeter embodiment of the invention is illustrated in FIG. 5, in which a piston 76 is slidably disposed within a close fitting cylinder case 78 by deposits of mercury in the form of pools or rings on

mercury

80 and 80a contained in annular grooves 82 and 8211, respectively, of the piston's circumferential periphery, and the case is filled with a chemically inert gas such as nitrogen and is hermetically sealed by end caps 84 and 86. Case 78 contains wall embedded contacts 85 arranged, spaced and mercury wetted in the manner specified with reference to the FIG. 1 embodiment of the invention. Also, like the accelerometer embodiment, the preferred material of the piston, case and end caps is diallyphthalate of high glass content.

Two means for controlling response of the inertia sensing piston to acceleration of the case, and hence the carrier, are disclosed. The first of these two constraints is the attraction between permanent magnet 88 molded in or otherwise fixedly attached to end cap 84, and a soft iron member 90 molded in or otherwise fixedly fastened to piston 76, with which magnet 88 abuts when piston 76 is at its illustrated rest position. The piston will be thus constrained until it senses the calibrated acceleration level required to overcome the magnetic attraction, at which moment it will begin displacing in response to increasing acceleration. This delay of the pistons response to acceleration would be warranted when the carrier's function initiated by the invention could be effected more effectively or with acceptable effectiveness with reference to the required length limit of the invention. It would also be warranted in applications in which the inventions carrier might experience sudden t; i severe extraneous shock forces during its normally low acceleration experience. Under this abnormal condition the magnetic constraint of the pistons movement and the damping of the pistons displacement afforded by the fill gas would cause the piston to return to its rest position following its minimal displacements in response to the abnormal shock forces.

The second means for controlling the axial displacement of piston 76 is the combination of the annular gas seal provided by mercury rings 80 and 80a and the restricted passageway of small bore orifice tube 92, which provides the only avenue of communication of the fill gas, from one side of the piston to the other. Orifice 92, preferably made of stainless steel hypodermic needle tubing, connects the opposite end interiors of case 78 at

holes

94 and 96, where the junctures are sealed with suitable epoxy. Filters 100, preferably made of fine mesh stainless steel, are located at the entrance of each of

holes

94 and 96 to prevent orifice 92 from becoming clogged by particulate matter. Through proper choice of the diameter and weight of piston 76 and the bore size and length of orifice 92, the pistons rate of displacement in its sensing axis can be directly proportional to its sensed components of the invention carriers acceleration, in accordance with the Hagen Poiseuille law of flow of fluid through round tubes having bore sizes which are very small relative to tube length, As thus designed the piston, in displacing in response to acceleration, integrates sensed acceleration with respect to time, to thereby measure the invention carriers velocity change. Since the carriers velocity change can be predicted with relatively great accuracy in many applications, it follows that the inventions measurement of carrier velocity change as a carrier function initiation delay function makes it possible for the invention to replace timers in certain applications.

It is noted that certain prior art piston in cylinder type velocity change switches or velocimeters depend on the fluid flow constraint of a small clearance between the pistons circumferential periphery and the inner wall of cylinder case as the piston displacement control means. Because of the varying degree of symmetry of this clearance it can be appreciated that the resultant velocity change measurement precision of such devices is relatively low. Also, because of the relatively great volume of gas which flows annularly past the piston in a given period ofa given level of acceleration, only a relatively small amount of velocity change can be measured with a given amount of switch volume. A comparison of the velocity change measurement capacity of the present invention and a welldesigned piston in cylinder and annular gas metering type of velocimeter showed that the invention having the volume of the referenced velocimeter can measure fifty times as much velocity change as the referenced velocimeter. Again assuming that the axial offset distance between helically adjacent contacts 85 is 0.002 inches, the invention can resolve its carriers velocity change in increments as small as 10 feet/second.

Since piston 76 of the velocimeter embodiment of the invention does not have a direct electrical connection, the circuit equivalent of the FIG. 1 illustrated electrically qualified spring is effected by common switching circuit 106. This circuit 106 is produced by series connecting a sufficient number of contacts 850 spaced so that the pistons mercury ring 80 will always engage at least one of contacts 85a when switching is required. In addition to the carrier function switching, the velocimeter embodiment of the invention provides a means for cooperating with other devices, such as the accelerometer embodiment of the invention. For example, as shown in FIG. 5, when piston 76 is magnetically constrained at its rest position its mercury ring 80a may bridge a contact pair to thereby close a circuit which may control another device. Also, it can be seen that such a circuit could be used to monitor the presence of piston 76 at its rest position. Similarly, when at its righthand displacement limit position the pistons mercury ring 80 could bridge a pair of normally open contacts to thereby close a fail-safe circuit of another device. If a separate embodiment of the invention is needed for fail-safing other devices it can be appreciated that it could be quite small and could be used redundantly (two modules in parallel) to both provide great failsafing reliability and to provide great assurance that the fail-safed circuits would be closed as required. The volume of a two module present invention thus designed could be held to 0.5 cubic inch.

While various embodiments of the invention have been disclosed, it will be apparent to those skilled in the art that the disclosed embodiments may be modified. Therefore, the foregoing description is to be considered exemplary rather than limiting and the true scope of the invention is that defined in the following claims.

I claim:

1. An inertia switch comprising a closed casing of electrical insulating material having a hermetically sealed non-oxidizing cylindrical internal chamber, a close fitting piston mounted in said chamber for axial movement in response to acceleration of said case, means defining a circumferentially extending deposit of liquid metal on a restricted portion on the periphery of said piston to constitute an annular bearing surface slidably supporting said piston, and a plurality of axially extending rows of axially spaced electrical contacts fixedly embedded in said casing and having inner surfaces flush with the inner wall of said chamber in axial alignment with said deposit of liquid metal to be slidably engaged and electrically contacted by said deposit during movement of said piston along said chamber, said rows of said contacts being circumferentially spaced from each other with said contacts of any one said row being uniformly axially offset from said contacts of adjacent said rows to produce a generally helical arrangement of said contacts about the axis of said casing.

2. The invention defined in claim 1 wherein said close fitting surface is electrically insulative and extends around the entire circumferential periphery of said piston, and said liquid metal deposit constitutes a pool of said liquid metal confined in an endless annular groove in said close fitting surface, meniscally sealing the clearance between said close fitting surface and the inner diameter of said casing, said clearance limited to 0.0002 to 0.0005 inch.

3. The invention defined in claim 2 further comprising passage means including a restricted orifice for controlling the flow of inert fluid from one side of said piston to the other induced by movement of said piston in said chamber to thereby regulate the rate of response of said piston to acceleration of said case.

4. The invention of claim 3 further comprising cooperating magnetic means mounted on said case adjacent one end of said chamber for magnetically attracting said piston toward said end of said chamber to establish a minimum acceleration of said case necessary to cause movement of said piston away from said one end of said chamber.

5. The invention defined in claim 1 further comprising spring means coupled between said piston and an end of said case for controlling movement of said piston relative to acceleration of said case.

6. A switch as defined in claim 1 further comprising damping means for damping the response of said piston to acceleration of said casing in a direction axially of said chamber.

7. The invention defined in claim 5 wherein said spring is electrically conductive, with means for connecting a first end of said spring to said deposit and means for connecting the second end of said spring to an electric terminal accessible at the exterior of said casing.

8. The invention defined in claim 7 wherein said deposit extends only a portion of the entire circumference.

9. The invention defined in claim 7 wherein at least a portion of said close fitting surface is solid metal and said liquid metal deposit constitutes a puddle of said liquid metal wetting said solid metal close fitting surface.

10. The invention defined in claim 7 wherein said piston comprises a hollow cup-shaped member of electrical insulating material having said annular bearing surface on its periphery, an electrical conductive member mounted on one end of said cup-shaped member to define an enclosed chamber. and an electrically conductive inertial mass mounted in said enclosed chamber and connected to said first end of said spring. for axial sliding movement between opposite end limits defined by an end wall of said cup-shaped member and an op posite end wall constituted by said conductive member, where said first means constitutes engagement of said mass with said conductive member.

11. An inertia switch comprising a closed casing of electrical insulating material having a hermetically sealed non-oxidizing cylindrical internal chamber, a plurality of electrical contacts embedded in said casing and having exposed inner surfaces flush with the wall of said chamber and exterior surfaces accessible at the exterior of said casing, an inertial piston of electrical insulating material slidably received in said chamber, said piston having a circumferentially extending groove on its periphery, a pool of mercury confined in said groove and contacting the wall of said chamber to slidably support said piston and to electrically contact said contacts as said piston is displaced along said chamber, electrically conductive spring means coupled between said piston and said casing at one end of said chamber, an electric terminal on said casing, and means electrically connecting said spring to said terminal and to said pool of mercury.

12. A switch as defined in claim 11 further compris ing damping means for damping the response of said piston to acceleration of said casing in a direction axially of said chamber.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,909 569 Dated September 30, 1975 lnventorsz W. Dale Jones It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 26, change "expected" to -compounding.

Column 1, line 35, change "unexpected" to -expected.

Signed and Scaled this twenty-third Day of December 1975 [SEAL] Attest:

C. MARSHALL DANN Arresting Officer

Claims

10. The invention defined in claim 7 wherein said piston comprises a hollow cup-shaped member of electrical insulating material having said annular bearing surface on its periphery, an electrical conductive member mounted on one end of said cup-shaped member to define an enclosed chamber, and an electrically conductive inertial mass mounted in said enclosed chamber and connected to said first end of said spring, for axial sliding movement between opposite end limits defined by an end wall of said cup-shaped member and an opposite end wall constituted by said conductive member, where said first means constitutes engagement of said mass with said conductive member.

12. A switch as defined in claim 11 further comprising damping means for damping the response of said piston to acceleration of said casing in a direction axially of said chamber.