US3833084A

US3833084A - Attitude responsive switch for vehicle safety system

Info

Publication number: US3833084A
Application number: US00323380A
Authority: US
Inventors: K Henderson; L Smith
Original assignee: TILT SAFE Inc
Current assignee: TILT SAFE Inc
Priority date: 1973-01-12
Filing date: 1973-01-12
Publication date: 1974-09-03
Anticipated expiration: 1991-09-03

Abstract

Safety apparatus for a self-propelled vehicle disables the engine of the vehicle if the critical angle of pitch or roll is approached during vehicle operation. An attitude responsive switch senses the position of the vehicle, and employs a reservoir containing a small quantity of mercury. The reservoir is formed in a body of insulating material, and may be of an inverted, irregular cone shape presenting a sidewall that diverges from a vertical axis to a degree depending upon the critical angles of pitch and roll. When a critical angle (less a safety factor) is reached, the mercury gravitates along the sidewall to the edge of the reservoir and bridges a pair of contact elements that define a narrow, annular space therebetween extending completely around the edge. In diesel engine applications, closing of the switch effected by bridging the elements energizes a solenoid operated fuel shut-off valve to kill the engine.

Description

United States Patent 11 1 Henderson et al.

[ Sept. 3, 1974 ATTITUDE RESPONSIVE SWITCH FOR VEHICLE SAFETY SYSTEM [75] Inventors: Kent DoudI-Ienderson; Kenneth T.

Henderson, both of Filer; Leon E. Smith, Jr., Twin Falls, all of Idaho [52] US. Cl. 180/104, 200/6147 [51] Int. Cl B60k 27/08 [58] Field of Search 180/104; 200/6147, 61.45

[56] References Cited UNITED STATES PATENTS 2,535,207 12/1950 Hook et al. 180/104 X 2,692,652 10/1954 Wilson .4 180/104 2,774,835 12/1956 Staley 180/104 X 2,782,276 2/1957 Woods 200/6147 2,812,399 11/1957 Schroeder.... ZOO/61.47 2,823,367 2/1958 Huron ZOO/61.47 X 3,034,097 5/1962v English et al. 180/104 X 3,367,439 2/1968 Zweben 200/6147 X FORElGN PATENTS OR APPLICATIONS 729,387 4/1932 France 180/104 Primary Examiner-Kenneth H. Betts Attorney, Agent, or Firm-D. A. N. Chase 5 7 ABSTRACT Safety apparatus for a self-propelled vehicle disables the engine of the vehicle if the critical angle of pitch or roll is approached during vehicle operation. An attitude responsive switch senses the position of the vehicle, and employs a reservoir containing a small quantity of mercury. The reservoir is formed in a body of insulating material, and may be of an inverted, ir-

regular cone shape presenting a sidewall that diverges from a vertical axis to a degree depending upon the critical angles of pitch and roll. When a critical angle (less a safety factor) is reached, the mercury gravitates along the sidewall to the edge of the reservoir and bridges a pair of contact elements that define a narrow, annular space therebetween extending completely around the edge. In diesel engine applications,

closing of the switch effected by bridging the elements energizes a solenoid operated fuel shut-off valve to kill the engine.

7 Claims, 12 Drawing Figures snmzur PATENIEUSEP 31914 Fri/5.12,.

ATTITUDE RESPONSIVE SWITCH FOR VEHICLE SAFETY SYSTEM This invention relates to improvements in safety systems for self-propelled vehicles subject to operation over terrain that may cause a dangerous pitch or roll condition and, in particular, to an attitude responsive switch for sensing suchcondition and automatically killing the engine of the vehicle.

Self-propelled vehicles such as tractors used in farming must oftentimes traverse rough terrain and be maneuvered on banks and hills which may cause the vehicle to assume a severe angle of pitch or roll (side tilt).

In farm tractors, rearward pitch (elevation of the front axle) and side tilt must especially be guarded against by the operator in order toprevent the tractor from reaching a critical angle from which it can not return to a normal position. It has been estimated that overturning of tractors due to excessive side tilt is the cause of over half of the deaths and serious injuries incurred in the operation of tractors, particularly inturning maneuvers on banks and hills.

A safety system may be employed with such a selfpropelled vehicle to disable the engine if the vehicle approaches a critical angle, thereby preventing the operator from inadvertently permitting the vehicle to reach the criticalangle without his knowledge. Such systems have been associated with the ignition circuit of a gasoline powered engine utilized to propel the vehicle, as

disclosed in the US. Pat. of English et al., No. 3,034,097. However, normally the critical angle of pitch and the critical angle of roll are different in a given vehicle, and such angles commonly vary from vehicle to vehicle thereby requiring that a practical system possess inherent flexibility to meet the requirements of a number of vehicles. In the English et al patent, an attitude-sensitive electrical switch is employed to sense the position of the vehicle, but no provision is made for accommodating differing critical angles of pitch and roll. Furthermore, most modern tractors are diesel powered, thus an ignition system is not available for interruption or grounding to kill the engine.

It is, therefore, an important object of the present invention to provide an improved attitude-responsive electrical switch particularly adapted for use in safety systems as discussed above, wherein a reservoir containing an electrically conductive liquid is employed to continuously monitor the position of the vehicle.

As a corrollary to the foregoing object, it is animportant aim of this invention to provide a reservoir as aforesaid which is defined by a body of insulating material, wherein the reservoir is of generally cup-shaped configuration'and has a bottom and a sidewall extending upwardly therefrom that diverges from a vertical axis in preselected angular relationship thereto, the switch being operated by tilting the reservoir to an extent to cause the conductive liquid to gravitate along the sidewall to the edge of the reservoir where it bridges a pair of electrically conductive contact elements connected to the respective terminals of the switch.

Another important object of this invention is to provide a switch as aforesaid wherein the different critical angles of pitch and roll of a given vehicle may be accommodated by forming the reservoir in the body with portions of the sidewall diverging from a vertical axis to a different degree in accordance with the differing critical angles.

Still another important object of this invention is to provide a reservoir in a switch of this type which is formed in a body of insulating material, wherein the reservoir may be readily adapted to various vehicles by changing the shape of the recess that is formed in the body to present the reservoir configuration, the body being preferably composed of a suitable plastic which is structurally strong and has good insulating properties.

Furthermore, it is an important object of this invention to provide a safety system of the type discussed above capable of accommodating different critical angles of pitch and roll of a given vehicle, and adapted to vehicles utilizing diesel engines which must be substantially instantaneously disabled should the vehicle assume a dangerous position.

In the drawings:

FIG. I is a perspective view of the attitude responsive switch of the present invention;

FIG. 2 is a side elevational view of the switch, a portion of the body thereof being shown in section to reveal details of construction;

FIG. 3 is a top plan view of the switch with components thereof being broken away to reveal the interior construction;

FIG. 4 is an end view of the switch, the cover member and a portion of the body being revealed in an irregular section to show a transverse cross-section of the reservolr;

FIG. 5 is a view similar to FIG. 2, but with the cover member and a portion of the body being in crosssection to show a longitudinal section through the reservoir;

FIG. 6 is a detail of the switch terminal formed by one of the machine screws extending through the switch housing;

FIG. 7 is a similar detail, but showing the arrangement of a screw through the housing to which no electrical connection is made;

FIG. 8 is a diagrammatic side view of a tractor illustrating the preferred mounting location of the switch of the present invention;

FIG. 9 is a diagrammatic view illustrating the critical angle of pitch of a tractor;

FIG. 10 is a diagrammatic view illustrating the critical angle of roll of a tractor;

FIG. 11 is an electrical schematic diagramshowing the safety system used in conjunction with a diesel engine; and

FIG. 12 is an electrical schematic diagram showing the safety system used in conjunction with a gasoline powered engine.

Referring initially to FIGS. 1-5 in particular, the housing of the switch of the present invention is broadly denoted by the numeral 20 and includes a main body 22 and an overlying, square cover member 24. The body 22 is a block of insulating material and may be a suitable plastic which is structurally strong and has good insulating properties. Nylon, for example, may be used as the body material. It is important that such material be chemically inert to mercury, as will be evident as this description proceeds.

The body 22 is of generally square configuration except for a pair of opposed mounting flanges or wings 26 having holes 28 therethrough for receiving suitable fasteners (not shown) to secure the switch to the vehicle. Accordingly, the body 22 presents a square upper surface 30, a thin, electrically conductive plate 32 being interposed between the surface and the overlying cover member 24. The plate 32 may be made of any suitable metallic material and is of the same square shape as the surface 30 and cover member 24. As will be discussed, the plate 32 comprises one of two fixed contact elements of the switch of the present invention.

A recess 34 of an inverted, irregular cone shape is formed in the body 22 at an oval shaped opening in the upper surface 30. This recess 34 presents a reservoir having a bottom 36 and a sidewall 38 extending upwardly from the bottom 36 to the oval opening. The plate 32 has an oval aperture 40 therein registering with the underlying opening in surface 30, as is clear from a comparison of FIGS. 3, 4 and 5. The bottom 36 of the reservoir or recess 34 is the vertex of the irregular cone described by the sidewall 38.

A quantity of mercury 42 may be seen in FIGS. 4 and 5 at the bottom 36 of the reservoir 34, an umbrella shaped baffle 44 being secured (such as by cement) to the sidewall 38 directly over the mercury 42. The switch is illustrated in its normal position with the reservoir 34 in a level attitude, thus the mercury 42 remains beneath the baffle 44 with no tendency to flow upwardly along the sidewall 38. The cover member 24 is also composed of an insulating material such as Nylon. A threaded shank 46 extends through the center of the member 24 and is rigid with a thin, oval shaped element 48 that is the second fixed contact element of the switch configuration. A nut 50 on the shank 46 holds fines the oval opening with which the aperture 40 in plate 32 is in register. Being of the same oval configuration but slightly smaller, the outer periphery of the element 48 and the inner margin of the plate 32 at the aperture 40 define a narrow, annular space 52 between the two

fixed contact elements

32 and 48. I

The manner of making an electrical connection to the plate element 32 is shown in FIG. 6. A machine screw extends completely through the body 22 and the cover member 24 and is threaded through the plate 32, thereby making electrical contact therewith. Accordingly, the screw 54 presents one of the terminals of the switch as illustrated in FIG. 6, the other terminal thereof being provided by the threaded shank 46 used to mount the oval element 48 on the undersurface of the cover member 24. The three other machine screws 56 employed to hold the body 22 and the member 24 tightly together in clamping relationship to the plate 32 do not make contact with the plate, as is illustrated in FIG. 7. I

In FIG. 8, the preferred location of the switch on a farm tractor is illustrated. Note the total length represented at 58 and 60 which is the distance between the axes of the front and rear axles of the trailer. The segment 60 is one-third of this length and it is in this rear segment that the switch housing 20 is located. The housing 20 in FIG. 8 is shown in its most forward location, it being preferred that the switch be mounted as close to the rear axle as is conveniently possible. This precludes excessive bouncing or splashing of the mercury 42 which would otherwise be caused by the inherent tendency of the front axle of the trailer to move up and down during normal operation.

In FIG. 9 the critical angle of rearward pitch of a tractor is represented by the angle a. In FIG. 10 the critical angle of roll or side tilt is represented by the angle b. It may be appreciated that angle b is significantly greater than angle a, which is commonly the case in tractor configurations of the type employed for agricultural purposes. (It should be understood that angles a and b are somewhat smaller than the true critical angles in order to allow a safety factor, thus they represent the maximum inclination that should be permitted without endangering the operator or the equipment.)

Now referring to FIG. 5, the angle with the horizontal formed by the portion of the sidewall 38 to the right of the vertex 36 is equal to the angle a. Similarly, viewing FIG. 4, the angle with the horizontal formed by each portion of the sidewall 38 at the plane of the section is equal to the angle b.

FIG. 11 illustrates a safety system'incorporating the switch of the present invention to disable a diesel engine when the vehicle propelled by such engine approaches a critical angle. The fuel line of a diesel engine is shown at 62, and a shut-off valve 64 is interposed in the line 62 and operated by a solenoid 66. The valve 64 would be located close to the fuel intake of the engine so that, upon closure thereof, operation of the engine would cease nearly instantaneously due to the sudden drop in fuel pressure. Preferably, the solenoid operated valve is of the type employing an internal spring (not shown) biasing the valve to the open position, the solenoid 66 effecting closure of the valve upon energization thereof. 1

A lead 68 extends from the vehicle power source to the terminal 54 of the switch, and a second lead 70 extends from the terminal 46 to the solenoid 66, the other electrical side of the solenoid being returned to chassis ground as indicated by the symbol. Accordingly, the switch of the present invention being normally open, the solenoid 66 is energized whenever the vehicle reaches the maximum permissible angle of pitch or roll. A normally closed, push button override switch 72 is interposed in series with lead 68 and enables the operator to momentarily disable the safety system to return the engine to operation if it is necessary to drive the vehicle out of the potentially dangerous condition.

FIG. 12 illustrates the safety system as it may be employed with gasoline powered engines. The high voltage lead 74 from the ignition coil 72 is connected to the distributor 76 in the usual manner. A lead 77 is illus-. trated extending from one of the low voltage terminals of the coil 72 to the breaker points (not shown). The switch of the present invention is connected between the lead 77 and chassis ground so that the ignition voltage will not be delivered by the coil 72 whenever the vehicle reaches a dangerous position and lead 77 is grounded. A normally closed, push button override switch 78 is interposed in series with the attitude responsive switch for the same purpose described above with respect to the diesel system.

OPERATION The switch housing 20 is mounted on the vehicle with the longitudinal center line of the reservoir 34 parallel to the fore and aft axis of the vehicle. Being oval shaped as viewed in plan, the longitudinal center line of the reservoir 34 may be visualized in FIG. 3, the right mounting flange 26 being toward the rear of the vehicle. This places the more gradually inclined portion of the reservoir sidewall 38 toward the rear of the vehicle so that the switch will respond if the rearward pitch reaches the angle a. Since the particular system illustrated is for a farm tractor, there is less concern about the forward pitch condition and hence the opposite portion of the sidewall 38 is inclined at a significantly steeper angle, reflecting the fact that the tractor may assume a rather steep nose-down position with safety. It may be appreciated, therefore, that the oval shape of the opening in surface30 and the irregular configuration of the conical sidewall 38 are dictated by the particular characteristics of the tractor, i.e., the critical angles of pitch and roll are different, and the critical angle of rearward pitch is different from the critical angle of forward pitch. Therefore, depending upon the particular application, the reservoir sidewall may assume other conical configurations or a hemispherical configuration or combination thereof, and the shape of the openingin surface 30 will conform accordingly.

Due to the variations in the critical angles just discussed, and the fact that these angles will vary depending upon the vehicle, it is particularly advantageous in the present invention that the reservoir 34 is formed as a recess in a body of insulating material that may be conveniently molded with the recess shaped as dictated by a particular application.

When the reservoir 34 is in the level attitude best illustrated in FIGS. 2, 4 and 5, it is evident that the switch may be rotated about a vertical axis 80 (FIG. 2) without causing the mercury 42 to flow along the sidewall 38. Bouncing or splashing of the mercury due to vibrations or bumps is inhibited by the baffle 44. Accordingly, activation of either of the safety systems illustrated in FIGS. 11 or 12 is precluded, and the engine of the vehicle is permitted to operate in the normal manner.

Assuming, however, that the tractor pitches rearwardly to the angle a, this causes the switch to close by the action of the mercury 42 bridging the two

conductive plate elements

32 and 48. Viewing FIG. 5, a degree of rearward pitch equal to the angle a would result in the less inclined portion of the sidewall 38 shifting to a horizontal position, or slightly past horizontal for sure actuation depending upon the safety factor (difference between angle a and the truecritical angle which, if exceeded, would cause overturning of the tractor). As the mercury 42 flowsto the edge of the reservoir, it enters the narrow annular space 52 and easily fills the space atthe lowest point thereof to bridge the

elements

32 and 48 and close the circuit between the

terminals

46 and 54.

It may be appreciated that this same action is obtained regardless of the direction of tilt of the reservoir once a portion of the sidewall 38 passes a horizontal position and gravity flow of the mercury 42 commences. Since the periphery of the element 48 is closely spaced from the edge of the reservoir completely therearound, and the aperture 40 in plate element 32 is in register with the opening defined by such edge, positive bridging of the elements will occur whenever the mercury gravitates from the vertex 36.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

about a vertical axis without causing said liquid to flow along said sidewall,

said body presenting a continuous edge at said opening circumscribing said reservoir; and

cover means on said body closing said opening and provided with a pair of spaced, normally electrically isolated contact elements adjacent said edge,

said sidewall diverging from said axis in preselected angular relationship thereto as the sidewall extends from said bottom to said opening, whereby said reservoir may be tilted in a predetermined direction to an extent to cause said liquid, at a selected degree of tilt, to gravitate along said sidewall to said edge and bridge said elements, thereby closing the switch,

one of said elements extending completely around said opening substantially at said edge, the other of said elements being disposed in said opening and having a peripheral portion closely spaced from said edge inwardly therefrom completely around said opening, whereby tilting of the reservoir in any direction to a sufficient extent will cause the liquid to bridge the elements,

said one element overlying said body and having an aperture therein registering with said opening, said other element having a shape conforming to the configuration of said opening. 2. The switch as claimed in claim 1, wherein said body is provided with an upper surface having said opening therein, and wherein said body has a recess therein at said opening presenting said reservoir. 3. In safety apparatus for a self-propelled vehicle where electrically responsive means is coupled with the engine of the vehicle for disabling the engine when the vehicle reaches a dangerous position, the improvement in said apparatus comprising:

an attitude responsive electrical switch for actuating said electrically responsive means,

said switch including a pair of terminals, a body of insulating material defining a generally cup-shaped reservoir having a bottom and a sidewall extending upwardly therefrom to an opening in the body, an electrically conductive liquid in said reservoir only partially filling the same, said body having a normal position in which the reservoir is in a level attitude and may be rotated about a vertical axis without causing said liquid to flow along said sidewall, said body presenting a continuous edge at said opening circumscribing said reservoir, and cover means on said body closing said opening and provided with a pair of spaced, normally electrically isolated contact elements adjacent said edge and electrically connected with corresponding terminals;

circuit means connecting said terminals to said electrically responsive means; and

means for mounting said switch on said vehicle with said reservoir in said level attitude when the vehicle is in its normal operating position,

said sidewall of the reservoir diverging from said axis in preselected angular relationship thereto as the sidewall extends from said bottom to said opening, whereby said reservoir may be tilted by the vehicle in a predetermined direction to an extent to cause said liquid, at a critical degree of tilt, to gravitate along said sidewall to said edge and bridge said elements, thereby closing the switch to actuate said electrically responsive means to disable the engine,

one of said elements extending completely around said opening substantially at said edge, the other of said elements being disposed in said opening and having a peripheral portion closely spaced from said edge inwardly therefrom completely around said opening, said elements defining a narrow, annular space therebetween adjacent said edge, whereby tilting of the reservoir in any direction to a sufficient extent will cause the liquid to bridge the elements,

said one element overlying said body and having an aperture therein registering with said opening, said other element having a shape conforming to the configuration of said opening.

4. The improvement as claimed in claim 3, wherein said sidewall has a first portion diverging from said axis to a degree dependent upon the critical angle of pitch of the vehicle, and a second portion diverging from said axis to a different degree dependent upon the critical angle of roll of the vehicle.

5. The improvement as claimed in claim 3, wherein said electrically responsive means includes an electrically operated shut-off valve adapted to be interposed in the fuel line to said engine.

' 6. An attitude responsive electrical switch comprisa body of insulating material defining a generally cup-shaped reservoir having a bottom and a sidewall extending upwardly therefrom to an opening in the body;

an electrically conductive liquid in said reservoir only partially filling the same, said body having a normal position in which the reservoir is in a level attitude and may be rotated about a vertical axis without causing said liquid to flow along said sidewall, said body presenting a continuous edge at said opening circumscribing said reservoir; and cover means on said body closing said opening and provided with a pair of spaced, normally electrically isolated contact elements adjacent said edge,

said sidewall diverging from said axis in preselected angular relationship thereto, as the sidewall extends from said bottom to said opening, whereby said reservoir may be tilted in a predetermined direction to an extent to cause said liquid, at a selected degree of tilt, to gravitate along said sidewall to said edge and bridge said elements, thereby closing the switch,

said one element comprising an electrically conductive plate overlying said body and having an aperture therein registering with said opening, said other element having a shape conforming to the configuration of said opening, said elements defining a narrow, annular space therebetween adjacent said edge.

7. The switch as claimed in claim 6, wherein said cover means includes a cover member overlying said plate and secured to said body, and wherein electrically conductive means is provided mounting said other element on said member therebeneath and extending upwardly through said member to present a first switch terminal, there being a second switch terminal on said member connected with said plate.

Claims

1. An attitude responsive electrical switch comprising: a body of insulating material defining a generally cup-shaped reservoir having a bottom and a sidewall extending upwardly therefrom to an opening in the body; an electrically conductive liquid in said reservoir only partially filling the same, said body having a normal position in which the reservoir is in a level attitude and may be rotated about a vertical axis without causing said liquid to flow along said sidewall, said body presenting a continuous edge at said opening circumscribing said reservoir; and cover means on said body closing said opening and provided with a pair of spaced, normally electrically isolated contact elements adjacent said edge, said sidewall diverging from said axis in preselected angular relationship thereto as the sidewall extends from said bottom to said opening, whereby said reservoir may be tilted in a predetermined direction to an extent to cause said liquid, at a selected degree of tilt, to gravitate along said sidewall to said edge and bridge said elements, thereby closing the switch, one of said elements extending completely around said opening substantially at said edge, the other of said elements being disposed in said opening aNd having a peripheral portion closely spaced from said edge inwardly therefrom completely around said opening, whereby tilting of the reservoir in any direction to a sufficient extent will cause the liquid to bridge the elements, said one element overlying said body and having an aperture therein registering with said opening, said other element having a shape conforming to the configuration of said opening.

2. The switch as claimed in claim 1, wherein said body is provided with an upper surface having said opening therein, and wherein said body has a recess therein at said opening presenting said reservoir.

3. In safety apparatus for a self-propelled vehicle where electrically responsive means is coupled with the engine of the vehicle for disabling the engine when the vehicle reaches a dangerous position, the improvement in said apparatus comprising: an attitude responsive electrical switch for actuating said electrically responsive means, said switch including a pair of terminals, a body of insulating material defining a generally cup-shaped reservoir having a bottom and a sidewall extending upwardly therefrom to an opening in the body, an electrically conductive liquid in said reservoir only partially filling the same, said body having a normal position in which the reservoir is in a level attitude and may be rotated about a vertical axis without causing said liquid to flow along said sidewall, said body presenting a continuous edge at said opening circumscribing said reservoir, and cover means on said body closing said opening and provided with a pair of spaced, normally electrically isolated contact elements adjacent said edge and electrically connected with corresponding terminals; circuit means connecting said terminals to said electrically responsive means; and means for mounting said switch on said vehicle with said reservoir in said level attitude when the vehicle is in its normal operating position, said sidewall of the reservoir diverging from said axis in preselected angular relationship thereto as the sidewall extends from said bottom to said opening, whereby said reservoir may be tilted by the vehicle in a predetermined direction to an extent to cause said liquid, at a critical degree of tilt, to gravitate along said sidewall to said edge and bridge said elements, thereby closing the switch to actuate said electrically responsive means to disable the engine, one of said elements extending completely around said opening substantially at said edge, the other of said elements being disposed in said opening and having a peripheral portion closely spaced from said edge inwardly therefrom completely around said opening, said elements defining a narrow, annular space therebetween adjacent said edge, whereby tilting of the reservoir in any direction to a sufficient extent will cause the liquid to bridge the elements, said one element overlying said body and having an aperture therein registering with said opening, said other element having a shape conforming to the configuration of said opening.

6. An attitude responsive electrical switch comprising: a body of insulating material defining a generally cup-shaped reservoir having a bottom and a sidewall extending upwardly therefrom to an opening in the body; an electrically conductive liquid in said reservoir only partially filling the same, said body having a normal position in which the reservoir is in a level attitude and may be rotated about a Vertical axis without causing said liquid to flow along said sidewall, said body presenting a continuous edge at said opening circumscribing said reservoir; and cover means on said body closing said opening and provided with a pair of spaced, normally electrically isolated contact elements adjacent said edge, said sidewall diverging from said axis in preselected angular relationship thereto as the sidewall extends from said bottom to said opening, whereby said reservoir may be tilted in a predetermined direction to an extent to cause said liquid, at a selected degree of tilt, to gravitate along said sidewall to said edge and bridge said elements, thereby closing the switch, one of said elements extending completely around said opening substantially at said edge, the other of said elements being disposed in said opening and having a peripheral portion closely spaced from said edge inwardly therefrom completely around said opening, whereby tilting of the reservoir in any direction to a sufficient extent will cause the liquid to bridge the elements, said one element comprising an electrically conductive plate overlying said body and having an aperture therein registering with said opening, said other element having a shape conforming to the configuration of said opening, said elements defining a narrow, annular space therebetween adjacent said edge.