US2232875A - Mercury switch relay - Google Patents

Description

5.1 c. H. CARTWRIGHT ET AL .8

MERCURY SWITCH RELAY Filed .Oct. 7, 1940 3 Sheets-Sheet 1 Patented Feb. 25, 1941 UNITED STATES.

PATENT oFFIcs 2,232,875 Msaormr swrrcn RELAY Charles H. Cartwright and Carl B. Larson, Elke, hart, Iml, asslgnors to The Adlake Company, a

co poration of Illinois V Application ma 7, 1940, Serial No. 360,184

This invention, relates broadly to mercury switches, for certain features or partsoi it can be used to advantage with almost any type of trols the movement of the displacer within the envelope to change the mercury level and thereby' cause the electrical circuit between the spaced electrodes to be closed or opened according to the particular construction. of the switch.

In general, the objects of the invention are to provide a time delay switch which, by slight changes in physical structure and/or apportion- 20 ment of parts during fabrication, can be made to fulfill widely varying performance specifications; one which is relatively cheap to manufacture, particularly because it can be easily tested for its actual time delay characteristics before the parts are hermetically sealed. within the switch envelope; and one which is reliable in performance over long periods of time.

These are but a few of the objects and advantages of the switch which will hereinafter be described, reference being had to the accompanying drawings, in which 7 Fig. 1 is a perspective view showing a novel and preferred mounting for the switch;

Fig. 2 is a vertical sectional view taken on the line 2-2 of Fig. 1, the switch, however, being shown in elevation;

Fig. 3 is a plan view of the switch with its mounting;

Fig; 4 is an enlarged vertical sectional view of a back contact switch embodying some of-the principles of this invention, the switch parts being shown in their normal position of rest when the associated relay coil is deenergized';

Fig. 5 is a view showing the relationship oi the switch parts at the instant when the displacer is raised by theassooiated coil:

Fig. 6 is a horizontal sectional view taken on thelineL-SofFlsJ;

Fig. 'l is a side elevational view of the time delay cup;

rm. 8isavertlcal sectionalviewofairontoontact switch em ying some .of the principles of this invention, the switch parts being shown in 55 the position which they take at the instant when 18 Claims. c1. zoo-an the associated coil is deenergized and the displacer rises, because of its buoyance, to the top 01' the switch envelope;

Fig. 9 illustrates the manner in which the time delay cup is instantly filled. whenever the displacer is lowered by the action of the coil;

Fig. 10 is a horizontal sectional view taken on the line ill-l0 of Fig. 8; and

Fig. 11 shows a modified form of the invention in which the switch parts are constructed and ar- 10 ranged so that there is 'a-relatively short time interval when the circuit through the switch is being closed and a relatively long time interval when the circuit through the switch is be opened. 15

Certain preferred forms of the invention have been chosen for illustration and description, in compliance with Section 4888 of the Revised Statutes, and the appended claims should be construed as broadly as the prior art will permit.

The switch mounting The switch mounting shown in Figs. 1 to 3 is a simple and inexpensive construction which not only provides an eiiicient iron circuit for the magnetic flux, but also afiords physical protection for the switch.

The mounting for the switch, generally designated 20, comprises an L-shaped plate 28 connecting top and bottom pole plates 22 and 23, re-

- spectively, the latter having a downwardly extending flange 24 by which the relay may be mounted on the wall by screws 25. The switchoperating coil 20 is telescoped over the switch 20 and over top and bottom p'ole sleeves 21 and 28, which are rigidly secured to the top and bottom pole plates 22, 23, respectively. The plates 22 and 23 are removably secured to the L-shaped plate 2| by screws 29, sothat the switch may be readily inserted in the coil 26 and assembled with the associated iron circuit, and when so assembled the bottom oithe switch is Supported on the horizontal flange 30 oi the plate 2|, and the top of the switch is slightly below the top of the sleeve 21, so that it is amply protected from accidental blows.

Preferably .the iron circuit is made or cold rolled steel, annealed, or any high grade soft iron.

Back cont ct switch (quick make, slow brcalcl The switch shownlin Figs. 4 1 inclusive is a quick make, slow break, back contact switch, comprising a switch envelope ll (preferably of 733A glass), through the base of which molybdenum electrodes 32, II, are pinch-sealed. The electrode 82' is surrounded by a glass insulating sleeve 34 to apoint II, where it is fused with a short sleeve ll, of refractory tubing, preferably made of ceramic material, suchas Alundum RAM or RA360, inasmuch as this material is readily fusible with Corning 733A glass. The end of the electrode 32 is leaving a contact portion 31 which extends a substantial distance above the top of the sleeve 8!, and is adapted to be intermittently placed in'electrical contact with the mercury flil ll of the switch.

The electrode ll may, if desired, be surrounded by a glass sleeve for a. portion of, its length, but a substantial part oi the'electrode is exposed to contact with the mercury iill below the time delay cup, generally d 3|, forming a contact II. The bared end oi the electrode 23 extends into the time delay cup 30, forming a contact ll which is constantly immersed in the mercury within the cup.

The time delay cup It consists of an elongated ceramic tube 42 (preferably made from Alundum RAM or RA360),thebaseoiwhichisfusedto a glass enlargement or head I! associated with r the electrodes II, 33. The cup ll is telescoped over both of the contacts 31, II, and at some point below the contact 31; the side wall of the cup is reduced in thickness, as indicated at N, by means of agrinding wheel or flle, and through this thinned portion of the -cup wall a small mercury passage 4! is provided which constitutes the timing element for the switch.

Themercuryiillisadaptedtoberaisedand lowered between high and low levels by means of a magnetically responsive displacer ll, the

top of which is slightly above the lower pole sleeve 28 when the coil II is deenergised. Springs 6 41, ll, are secured to the top and bottom. re-

spectively, oi the displacer II to-cushion the movement of the displacer within the envelope, and toclamp guide washers ll in place on the displacer. The upper portion of the displacer is enlarged to provide greater mass for the displacer, but the interior. of the displacer is in free communication with the space on the out side of the displacer through an opening II, so that the gas fill of the envelope, which preferably includes both helium 1 and hydrogen, can readily pass through the top of the dlspiacer without impedance.

Theswitchpartsinl'iaiareshownin'the position which they take when the coil 2| is deenergized, and it should be noted that the level- ,0! the mercury, when the displaccr is in its lower position (the high mercury level), is above the lower-edge II of the cup 3!. The upper margin of the cup is inclined, so that when mercury is displaced to close the circuit through the electrodes I2 and II, the mercury can enter the cup on the side adjacent to the low point ll, while the gas in the cup will escape along the'o'pposite side of the cup. Obviously when the cup II is filled with mercury the circuit through the contacts I1 and it is closed.

When'the displacer I is raised to the position shown in Pig. 5, the mercury level on the outside of the cup 3! falls immediately to a position slightly above the passage 4', but the column of mercury within the cup ll recedes gradually as mercury passes'slowly through the restricted passageway 4! into the main body of mercury. As soon a the mercury level within assasvs is, the circuit through the contacts 31 and n is broken. 7

Subsequent deenergization of the coil 26 will result in the displacer falling to the position in' which it is shown in Fig. 4, and the immediate Front contact srm'tch (quick males, slow break) The front contact switch shown in Figs. 8-10 inclusive differs but slightly from the back contact switch of Figs. 4-7 inclusive, the principal diii'erence being that the time delay cup it of the front contact switch is positioned somewhat higher within the envelope and the displacer 52 is of lighter construction. To the extent that the switch parts are the same, or substantially alike, no description is necessary, and these parts are indicated by like numbers on the drawings.

The displacer 52 comprises an outer sleeve 53 of magnetic material, interlorly fitted with a refractory sleeve 54, preferably of glass or ceramic material, which projects from both ends of the sleeve 51. The two sleeves are held in desired relative position by the top and bottom springs 41, ll, which are tightly fitted over the projecting ends of the sleeve 54, as clearly shown in the drawings.

.The air gap ll between the top and bottom pole pieces 21, 28, is somewhat lower than the air gap in the back contact switch of Figs. 4-7 inclusive, and when the cell It is energized the displacer is pulled down to the bottom of the switch envelope, as shown in Fig. 9. to immediately flood the clip I! with mercury and sub-.

stantially instantaneously establish the electrical circuit between the electrodes 32 and 31. 'Upon deenergization of the coil 28 the displacer rises by its buoyancy to the position in which it is shown in Fig. 8, and the mercury level outside of the cup I! immediately drops to the level in which it is shown in Fig. 8. As the mercury gradually escapes from the cup I! through the opening II, the mercury level within the cup 39 recedes until it falls belowthe edge of 9. ccramic cup I'l associated with the contact 31, and as the mercury passes the edge 56 the circuit through the contacts 31 and it is broken. The time which it takes for the mercury to recede from the top of the time delay cup 3! to the time delay factor of the switch. When the mercury has leveled oil with the displacer in raised position it is at its low level, as indicated by the line It.

Front or back contact switch (slow make and slow break) tity of mercury in the switch, so that when the displacer l! of the back contact switch and the displacer 2 of the front contact switch are .lowered to the positions in which they are shown in Figs. 4 and 9, respectively, the mercury fill fails to rise above the edge SI of the cup 66, although it must rise above the top of the sleeve 36 (Fig. 4) and above the edge 66 of the-electhe contact 31 (the top of the sleeve" in Fig.

4, and the edge 56 of the cup 61 inFlg. 8), determines the time delay characteristics of the switch for the making of the'circuit.

Since the high mercury level, indicated by the line 59 in Figs. and 8, in aslow make, slow break switch of this type (as well as the slightly higher mercury level which is reached at the instant the displacer is lowered) is above the top of the sleeve 36 and the edge 56 of the electrode cup 5?, the breaking of the circuit through the electrodes will be delayed by the retarded drop of the mercury column within the time delay cup 33. In some cases it may be desirable to increase the height of the cup 39 in order to make certain that mercury will not overflow the top of the cup and destroy the timing action of the restricted mercury passage 5 when closing the circuit between the electrodes 32 and 66.

Front or bdciccontact switch with retarded make and slow break In Fig. 11 there is shown a switch which has a relatively short time delay when the circuitis being closed, and a relatively 'long time delay,

when the circuit is being opened. By way of example, the switch may have a six second time delay when making the circuit, and a three minute time delay when breaking the circuit,

and this particular relationship or time delays is extremely diflicult, to obtain in switches of this type. i

The switch shown in Fig. 11 is of the" front contact type, but it is obvious that the principles of the'switch could easily be incorporated in a basic contact switch. To-the extent that the switch corresponds with thefront contact switch in Figs. 33-10 inclusive, the description will not be repeated, and like reference characters-are used on the drawings. The principal difference between the two switches is that a gas impervlous thimble 66, preferably ogjglass, is sealed to the electrodes 32,33, in place of the cup 39, and the thimble is provided in its. top wall with a mercury valve, generally designated 6|, comprising a porous seat 62, preferably of coarse ceramic material, such as Alundum, grade RA98,

and a telescoping s1eeve 63 ofrelatively dense' ceramic material, such as Aiundum, grade M84. The sleeve 63 and seat 62 are iused into the top wall of the thimble to form a mercury valve cup, and the parts of the switch are so proportioned that when the displacer 52 is lowered the mercury rises above the top of the thimble 66' a suificient distance to make certain that a quantity of mercury 66 is in the valve 'cup 6!. The lower portion of the thimble 66 is provided with a restricted mercury passage 45, which retards the flow of mercury betweenthe interior and. exterior of the thimble.

When the switch parts are in the position in which they are shown in Fig. 11, with the mercury at its low level, the circuit through the electrodes is open. Upon lowering the displacer 52 by the action of the coil 26 (or by deenergizing the coil in the case of a back contact switch) themercury level onthe outside of the thimble immediately rises to a level indicated by the line 65, which preferably is above the edge 66 of the sleeve 63 to insure that the mercury valve cup 6| will be filled with mercury. The mercury within the thimble 60 gradually rises as mercury is permitted to flow into the thimble through the restricted passageway 45, and as soon as the level reaches the edge 56 01' the cup 51 the circuit through the electrodes 32, 33, will be closed. The time for the mercury to reach this height depends, of course, upon the size of the opening 45, the diameter of the thimble 60,

and certain. other factors. The gas entrapped within the thimble 60 does not in any way retard the movement of mercury to circuit closing position, because the seat 62 is sufficiently porous to readily pass the gas through the seat and mercury, the thimble 60 is completely filled with mercury and subsequent deenergization of the coil, which causes the displacer 52 to rise to its uppermost position, as shown in Figs. 8 and 11, has the result-of lowering the outside mercury level to the position indicated by the line 68; but the mercury within the thimble 60 is restrained in its downward movement by the slow passageof gas through the dense porous sleeve 63 and seat. 62, the mercury 64 in the valve 6| preventing gas from passing through the seat 62 into the thimble 66 without first traversing the dense time delay sleeve 63. The time that it takes for the mercury to fall from the top of the mercury chamber in the thimble 60 to the edge 56 of the electrode cup 51 constitutes the time delay interval which characterizes the breaking of the circuit, and'thls interval is dependent for its value on the density of the sleeve 63,,the diameter and thickness of the walls of that sleeve, the diameter of the thimble 60, the diil'erence in height between the top of the mercury chamberin the thimble 60 and the edge 66 of the mercury cup, and various other factors. It is not difilcult with this arrangement to obtain time delays of several minutes on the breaking of the 'circuit through the electrodes of the switch.

,It should be noted that the restricted mercury passage 55 does not interfere with the tim-' I ing action of the sleeve63, because the gas time stances, by placing the contacts of both electrodes within the time delay cup. Referring to Fig. 1 1, it may be seen that in this type of switch the circuit through the electrodes cannot be completed until the mercury within the time delay body has risen above the level of the electrode cup 56, and that the circuit, when completed,

instantduringthe cycleoftheswitch cup, and the contact 81, totally excluding the restricted mercury passage-ll.

In other types of switches, however, such as thetypeillustratedinmgailandlwhere top of the time delay cup is open, there the circuit between the electrodes could be mpletcd through the mercury metering orifice I, if it were not for the provisionpf the contact 3! without the cup. Assuming that the electrode ll is completely insulated, and that no contact 38 without the cup is provided, it may be seen that when the circuit through the electrodes is open and the coil is suddenly energised, the mercuryilllwillberaisedbythedisplaceril and will flow over the edge ll of the time delay cup 3!. During this portion of the cycle the mercury may come in contact with the contact I! or the mercury within the electrode cup 51, thereby completing a circuit through the contact II, the mercury metering oriii'ce u, the mercury fill and the contact 31.

However, when the electrode 88 is provided with a contact 30, without the time delay cup 8! (as shown in the drawings), the mercury of current will pass from the contact ll through spilling overthe edge ll of the cup ll will complete the circuit primarily between the contacts 31 and II, as this is the circuit of least-resistance.

It is, of course, obvious that a certain amount the mercury metering oriflce ll to the contact II, but this current will be negligible, for, dueto the small diameter of the orifice II, the resistance in the circuit between contacts II and II will be much greater than the resistance in the circuit between contacts 31 and II, and the current flowing in each of the circuits will,'therefore, be inversely proportional to the resistances in the circuits. Actually the current flowing through the orifice ll will be' so small that it cannot damage the walls of the orifice. Once the time delay cup I! is filled the normal path of the current through the electrodes will be between the contacts ll and II, which does not, of course, include the restricted mercury passage 4i.

It may be seen, therefore, that in 'a switch of the type illustrated in Fig. 11 it is not necessary to provide a continuously exposed heel contact It, but in switches of the type illustrated in Figs. 8 and 9 the provision of such a contact is highly desirable as it greatly increases the accurate life of the time delay element.

- Summary of advantages II and the mercury orifice l8, and by properly positioning the intermittently immersed contact within the cup 38.

2. Since the functioning otthe time delay element is not dependent upon gas densities (with the exception of the modified form shown in Fig. 11), the switch can be ,readily tested during fabrication for its actual time delay values with the assurance that the same time delay values will obtain when the switch is hermetically sealed off at the top, as indicated at ll. example, it

is known that a .027 drill will give a six second time delay for a switch of given dimensions.

Much time and labor is saved in this way and I the switch can be produced with a corresponding saving in cost.

3. Making the time delay cup 39 of ceramic material and placing the mercury orifice below the low mercury level in a relatively thin wall of the cup greatly increases the accuracy of the time delay functions of the switch, since the short passage through the side wall of the cup is too small to house a bubble which might interfere with the smooth flow of mercury between the interior and error introduced by the erratic effect of surface tension is avoided.

5. The extended contact 31 in Figs. 4-10 in-' elusive serves a dual function. Although it is impossible to determine for a certainty that the 'exceptionally quick and clean closing of the circuit through the electrodes 32 and II (clean in the sense that cscillograph records show that there is no chatter when the circuit is closed) is obtained in a particular way, it seemed probable that the explanation for this desirable action is that the mercury, in spilling over the edge of the cup 41, first strikes the contact 81, thereby immediately closing the circuit through the electrodes, and the continued rush of mercury into the cup prevents any separation of the mercury column from the contact which could produce a chatter. In addition the extended contact 31 decreases the contact resistance of the switch when the circuit is closed.

6. It has been found by extensive tests that the time for closing the circuit is decreased and made more uniform when the top of the cup it is slanted, as shown in the drawings, presumably because the mercury tends to enter the cup on V life of the switch. The increased contact area obtained through' the use of contacts 38 and ll exterior of the cup, and any bubble which ilnds on a heel electrode cooperates with the extended contact 31 to further decrease the contact resistance of the switch.

8. A switch of the type disclosed can be used very successfullyon structures which are subjected to much jostling or vibration, as, for example, automobiles, buses,'railway cars, and the like.

We claim as our invention:

1. In a mercury switch, a switch envelope a mercury fill, electrodes projecting into the envelope, one of the electrodes having a contact adapted to be intermittently placed in electrical contact withthe mercury fill, a cup telescoped over said contact and being provided with a restricted mercury passage below the level of said contact, and a displacer telescoped over the cup for raising the mercury fill within the cup to a point above the contact. said cup having a gas 1 contact, and a displacer telescoped over the cup column passing through the restricted mercury pervious wall in the region of the restricted mercury passage and the passage being normally below the mercury fill.

2. In a mercury switch, a switch envelope, a mercury fill, electrodes projecting into the envelope, one of the electrodes having a contact adapted to be intermittently placed in electrical contact with the mercury fill, a cuptelescoped over said contact and being provided with a restricted mercury passage below the level of said for raising the mercury fill within the cup to a point above the contact, one edge of said cup being cut away to facilitate filling'of the cup with mercury.

3. In a mercury switch, a switch envelope, a mercury fill, electrodes projecting into the envelope, one of the electrodes having a contact adapted to be intermittently placed in electrical contact with the mercury fill, a cup telescoped over said contact and being provided with a restricted mercury passage below the level of said contact, and a displacer telescoped over the cup for raising the mercury fill within the cup to a point above the contact, the other of said spaced electrodes being positioned so that the mercury passage is never required to-conduct more than a small fraction of the entire current.

a. In a mercury switch, a switch envelope, a

' mercury fill, means for shifting the fill between high and low levels including a tubular displacer open at the lower end to the free passage of mercury, spaced electrodes in the envelope, one of which is adapted to be intermittently placed in contact with the mercury fill, a cup-shaped body immovably'mounted within the envelope enclosing said one electrode and having its open end above the low mercury level, and a restricted mercury orifice below the low mercury level at all times submerged in the mercury fill, said orifice timing the flow of mercury as it passes from the contact, a cup-shaped body supported at least in part by said one electrode and enclosing the contact, said body having a restricted orifice below the low level of the mercury whereby the orifice is at all times submerged in the mercury fill, said orifice serving to time the fiow of mercury from the body when the mercury is shifted from high level to low level. by said mercury shifting means.

6. In a mercury switch, a switch envelope, a mercury fill, means for shifting the fill between high and low levels, spaced electrodes in the envelope one of which projects upwardly through the mercury fill and is insulated from the mercury fill for a portion of its length leaving an exposed contact, the other electrode also having an exposed cdntact, a cup-shaped body supported at least in part by said one electrode and enclosing both contacts, said body having a restricted orifice below the low level of the mercury whereby the orifice is at all times submerged in the mercury fill, said orifice serving to time the fiow of mercury fromthe body when the mercury fill is shifted from high level to low level by said mercury shifting means.

'7. In a mercury switch, a switch envelope, a mercury fill, means for shifting the fill between high and low levels, spaced electrodes in the envelope, one of whichis adapted to be intermittently placedin contact with the fill, said one electrode projecting upwardlythrough the fill and being insulated for a portion or its length leaving an exposed contact, a cup-shaped body of insulating material mounted on said one electrode and enclosing the contact portion thereof, said cup-shaped body having its open top positioned above the contact and having a restricted orifice below the contact for restraining the fiow of mercury from the cup-shaped body to the main body of the mercury fill as the mercury fill is shifted from high level to low level.

8. In a mercury switch, a switch envelope, a mercury fill, means for shifting the fill between high and low levels, spaced electrodes inthe envelope, one of which is adapted to be intermittently placed in contact with the fill, said one electrode projecting upwardly through the fill and being insulated for a portion of its length leaving an exposed contact, the other electrode also having an exposed contact, a cup-shaped body of insulating material mounted on said one electrode and enclosing both contacts, said cup-- shaped body having its open top positioned above the contact which is intermittently placed in electrical connection with the main body of the mercury fill and having'a restricted orifice below said intermittently immersed contact for timing the fiow of mercury from the cup-shaped body as the mercury fill is shifted from high level to low level.

a. In a mercury switch, a switch envelope, a mercury fill, means for shifting the. fill between high and low levels including a tubular dis-=- placer open at its lower end to the free passage of.mercury, spaced electrodes in the envelope one of which is adapted to be intermittently placed in contact with the mercury fill, a cupshaped body immovably mounted within the envelope enclosing said one electrode, and having body of the mercury fill as the latter is shifted from high to low level.

10. In a mercury switch, a switch envelope, a mercury fill, electrodes within the envelope having contacts at difi'erent heights, a ceramic cup rigidly telescoped over two of the contacts including a mercury. metering orifice below the low level of the mercury fill to time the passage of mercury out of the cup, and meansvto raise the mercury fill above the level of both contacts.

11. In a mercury switch, a switch envelope, a mercury fill, spaced electrodes extending upwardly through the fill and insulated therefrom by sleeves defining contacts at difierent heights, one of said contacts being adapted to be intermittently immersed in the mercury fill, a cupshaped body supported at least in part by one of said sleeves and enclosing two of the contacts, said body having a restricted mercury metering orifice in its wall below the upper contact and at all times submerged in the mercury fill, and means for shifting the mercury fill to above both contact-s, said cup-shaped body also enclosing a portion of one of said insulating sleeves.

12. In a mercury switch, a switch envelope, a

' tioned that the mercury column in the restricted mercury passage is at no time required to con-V duct more than a fraction of the entire current flowing through the electrodes and the mercury fill.

13. In a mercury switch, a, switch envelope, a mercury fill, electrodes having contacts within the envelope, one of the electrodes having a contact adapted to be intermittently placed in electrical contact with the mercury fill, a ceramic cup having a restricted mercury e,

in its side wall rigidly telescoped over said contact, and a magnetically responsive displacer for shiiting the level or the mercury fill, the other oi said electrodes having contacts so positioned that the mercury column in the restricted mercury is at no time required to conduct more than a fraction oi the entire current flowing through the electrodes and the mercury 14. In a mercury switch, a switch envelope. a mercury fill, electrodes having contacts within the envelope, a cup telescoped over at least part of said electrodes, said cup being open at its upper end and having a restricted mercury passage at all times immersed in the mercury fill, and means for shitting the level of the mercury fill. said electrodes having contacts 'at difierent heights within the cup and one 0! said electrodes also having a contact without the cup whereby the mercury column in the restricted mercury passage is at no time required to conduct more than a fraction of the entire'current flowing through the electrodes and the mercury fill.

15. In a mercury switch, 'a switch envelope,

'amercuryfilLmeansror-shiitlngthefillbetween high and low levels including a tubular displaceropenatitslcwerendtotheireem sage of mercury, spaced electrodes in the enout of the cup-shape'dbody'responsive to changes 10 in the level of the mercury fill.

16. In a mercury switch, a switch envelope, a mercury fill, means for shitting the level 0! the mercury fill, electrodes having contacts at diiierent heights within the envelope, a cup asso- 1 ciated with said contacts having a restricted mercury orifice below the level of the upper contact and at all times immersed in the mercury fill, said cup being fixed with respect to the switch envelope and means for preventing the passage of a substantial current through the restricted mercury orifice.

17. In a mercury switch, a switch envelope, a mercury fill, means ior shifting the level of the mercury fill, electrodes having spaced contacts at 'difi'erent heights within the envelope, a cup associated with said contacts having a restricted mercury orifice below the level 0! the upper contact and at all times immersed in the mercury fill, said cup being fixed with respect to the switch envelope and means for preventing the e of a substantial current through the restricted mercury orifice, said means comprising telescoping the cup over said contacts.

18. In a mercury switch, a switch envelope, 9.

I mercury fill, means for shifting the level of the mercury fill, electrodes having contacts at difi'erent heights within the envelope, a cup tele-' scoped over said contacts and having a restricted mercury orifice below the level of the upper contact at all times immersed in the mercury fill. one oi said electrodes having contacts within and without said cup in order to prevent the e of a substantial current through the restricted I mercury orifice.

CHARLES H. CARTWRIGHT.

CARL H. LARSON.

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