US2494863A

US2494863A - Calibration of thermal relays

Info

Publication number: US2494863A
Application number: US565976A
Authority: US
Inventors: John J Dietz
Original assignee: Thomas A Edison Inc
Current assignee: Thomas A Edison Inc
Priority date: 1944-11-30
Filing date: 1944-11-30
Publication date: 1950-01-17
Anticipated expiration: 1967-01-17

Description

Jan. 17, 1950 J. J. DIETZ 2,494 863 CALIBRATION 0F THERMAL RELAYS Filed Nov. 30, 1944 11a .1? ll j INVENTOR. 39 John J. Dia

Patented Jan. 17, 1950 CALIBRATION F THERMAL RELAYS John J. Diets, New York, N. Y.,' sssignor to Thomas A. Edison, Incorporated ,WestOrsnge,

N. 1., a corporation of New Jersey Application November 30, 1944 Serial No. 565.976

1 "Claims.

This invention relates to improved methods of and apparatus for setting the timing of thermal relays of the sealed-in type.

This type of relay comprises a thermallyresponsive switch and an associated heater element for operating the switch, both of which are enclosed in an airtight casing that is substantially evacuated of gas. When the heater element is energized the switch is operated to make or break its contacts as the case may be. There occurs however a time interval between the instant energization of the heater element is begun and the instant the switch is operated. In different uses of these relays, diilerent time intervals are required. Typically, the timing may vary from a fraction of a second to a minute or more.

The setting of the timing of these relays has been diilicult because the relays have to be sealed in before they can be checked. Heretofore, it has been a common practice to predetermine by experience the spacing between the switch contacts and the degree of evacuation necessary to meet a given timing and to construct the relays accordingly. This method of setting the timing of the relays is however quite inaccurate and is moreover inflexible in that it does not lend itself readily to setting the timing to diilerent prescribed values. According to my invention, however, thermal relays of the character described may be set accurately to diflerent timing specifications after they are sealed in, and this is done reliably and by a very simple method and apparatus.

Among other things, the gaseous pressure in the relay is important in determining its timing. By my invention 1 provide a novel and effective method and apparatus for externally controlling the pressure after the relays are sealed in.

It is a known principle that the heat conductivity between two spaced members in a gaseous ambient varies sharply with the gas pressure at pressures where the mean free length of path of the gas molecules is at least of the order of the distance of spacing between the two members. By my invention, a novel and practical application is made of this principle for controlling externally the heat conductivity between different components of sealed-in thermal relays, and it is an object of my invention to employ this principle in setting the timing of such relays.

It is a further object of my invention to employ an efiective spacing between the heater element and associated switch which permits eflective control of the conductivity therebetween at ranges of evacuation of the casing which are.

readily attained in practice.

It is another object to provide a novel method of calibrating sealed-in thermal relays, and novel timing of the relay to a value greater than that desired, and then release a gas in the casing to lower the timing to that desired value. Preferably, but without limitation thereto, this gas is retained in the casing in an occluded state, and is released by thermal excitation of the member in which the gas is occluded.

It is an object 0! my invention to provide a novel and efiective means and method for releasing controlled quantities of such gas.

It is another object to control remotely the release of such gas by electrical means.

It is another object to retain the gas in a member which serves other useful functions in the relay.

Still other objects and features of my invention will more fully appearfrom the following description and the appended claims.

In the description of my invention reference is had to the accompanying figures, of which:

Figure 1 is an axially sectional view of a sealed-in thermal relay taken on the line l--l of Figure 2; and

Figure 2 is a partially axial sectional view of the same relay, taken in a plane at right angles to the view of Figure 1, and showing a circuit system connected to the relay for automatically calibrating the samen The thermal relay shown in the accompanying figures and referred to as R comprises a cylindrical casing I, typically of glass, having astem 2 at the bottom. Sealed into this stem and passing therethrough are semi-rigid lead wires 3. These lead wires are connected electrically to, and serve in part to support, a thermal switch unit 4 within the casing I.

The switch unit 4 comprises two

thermallyresponsive contact members

5 and 8 supported in cantilever fashion in a stack I. The stack 1 includes a series of terminals 8 between the two contact members, which are insulated from each other and from the contact members by intervening insulating blocks 9 and Ill. At the ends of the stack there are. metal plates II which are insulated from the contact members by insulating strips. l2 and clamped together by bolts ll that pass insulatedly through the stack. These plates H have wings Ha at the top and bottom which bear resiliently against the wall of the casing I to hold the switch unit 4 in a central position within the casing I.

The contact members and 6 respectively include the contacts 5a and 6a which, for example, may be normally open as shown. Contact 5a is fixedly secured to the free end portion of the member 5,but the contact 6a is screw-threaded to the free end portion of the member 6 so that it may be adjusted to set the gap spacing between the contact members when they are in their equilibrium positions. The arm portions of these contact members comprise identical bimetals of which each is made typically of a strip of Invar applied to a strip of stainless steel. On the bimetal of the contact member 5 there is a heater winding I4. This winding is wound tightly onto mica insulating strips l5 which are interposed between it and the opposite sides of the contact member, the mica strips being somewhat wider than the contact member to space the winding from the edges thereof. The winding H has circuit connections leading out of the casing l by way of two of the terminals 8 and two of the lead wires 3. Upon applying a rated voltagetypically 115 voltsto this heater winding for a period representing the timing of the relay, the contact member 6 is deflected sidewise into contact with the semi-stationary contact member 5 to close the thermal relay.

In order that the thermal relay will be compensated for changing ambient temperature, the two

contact members

5 and 6 are polarized in the same direction and designed to have the same thermal characteristics. To this end the contact member 5 comprises a bimetal identical with that of the member 6 and has applied thereto insulating mica strips l6 and a surrounding winding I1, the same as the strips i5 and the winding H which are applied to the member 8. This winding I1 is idle in the functioning of the relay, and has been heretofore provided, together with the mica strips [6, solely for the purpose of giving the semistationary contact member 5 the same resilience as has the contact member '6 with its associated heater winding H.

There are several heat-flow paths in the abovedescribed relay which affect its timing. These are from the heater winding H to (1) the wall of the casing I, (2) the bimetal of the contact member 6, and (3) the bimetal of the contact member 5. Path 1 represents a heat loss and has the efi'ect of decreasing the timing-4. e.,-of making the relay work faster as the heat conductivity of this path is decreased; path 2, to the contrary, has the effect of increasing the timing-i. e., of making the relay work slower-as the heat conductivity is decreased; and path 3 is significant principally in that it causes a heating of the semistationary contact memberi and thereby decreases the timing as the heat conductivity of this path is decreased. It is found that with an increasing degree of evacuation of the casing I, the timing is not affected significantly until the gas pressure is reduced within a wide-pressure range starting at a maximum value of the order of .2 mm. of mercury. Within this range the timing increases as the gas pressure is reduced, and vice versa. In other words, seemingly, as the heat conductivity within the casing is reduced, the timing is increased,

The heat-flow paths 1 and 3 abovementioned however produce the opposite effect. It appears accordingly that in the pressure range just mentioned only the heat-flow path between the heater winding l4 and its associated contact member 6 4 is significant in determining the timing of the relay.

It is believed the heat-conduction path between the heater winding H and associated contact member 6 significantly controls the timing of the relay within the pressure range mentioned because the mean free length of path of the gaseous molecules is, in this pressure range, equal to the effective length of gaseous spacing of this path, the term gaseous spacing" being herein em ployed to mean the effective distance of spacing which is free of solid material. The other two paths are relatively long--being of the order of thirty times as great. In order for the mean free length of path of the gaseous molecules to become equal to the length of these pathsi. e., to the distance of spacing'from the heater winding I4 to the casing I or contact member 5-a very much higher degree of vacuum would be required in the casing. It is accordingly an advantageous feature that one of the significant heat-conduction paths in the relay above described has a widely different length than have the other paths or, in other words, that the paths which have the same direction of influence on the timing in response to a given change in gaseous pressure shall have a widely different length from those paths which 1 ave the opposite direction of influence on the timing. Also, it is important that one of these paths shall be relatively short so that the timing will be controlled without having to obtain a high degree of vacuum.

The regions of gaseous spacing between the heater winding [4 and contact member 6 are localized between the heater winding and the narrower sides of the contact members and between the mica strips l5 and the broader sides of the contact members... This latter spacing regionwhich is believed to be the main one in importance-arises because the heater winding is in direct contact with the mica strips along the width thereof but the central portions of the mica stripsare bowed outwardly away from the contact members by the tensioning of the heater winding on the portions of the mica strips which overhang the edges of the contact members. The spacing accordingly varies at different points, and the effective spacing is a mean value of the limits between which the spacing varies. It is considered beneficial that the spacing is not well defined-4. e., not uniform-because this has the effect of giving a more gradual control over the timing with change in the gas pressure.

As a typical example, the bimetal of the contact member 6 may be approximately .930" thick and .250" wide, the mica strips [5 may be approximately .0O8" thick and .375" wide, and the winding I! may have a tensioning o! the order of .75 lbs. For these values, I find the conductivity between the heater winding l4 and the contact member 6 is influenced significantly by'the gas pressure at pressures of the order of .2 mm. of mercury and less. Pressures of such low order are herein. referred to generally as representing a substantially evacuated condition.

Since the heat conductivity between the heater winding i4 and the contact member 6 varies sharply with variation in gaseous pressure within the range abovementioned, it follows that small variations in the quantity of gas within the casing I are eflective to produce wide changes in the timing of the relay. In carrying out my invention, I initially over-exhaust the casing so that the timing of'the relay will ticularly as they affect the spacing between. the

heater winding l4 and the contact member 6. cause the initial timing to vary within a range of values. Purely by way of example, for a given set of relays this range may be from 45 to 60 seconds, and the margin may therefore be seconds.

My invention employs a means within the casing i which will produce'or release an amount of free gas-typically hydrogen-according to the degree to which, and the length of time, that means is heated. Preferably, the gas is made available by being stored within the casing in anoccluded state. For this purpose, I preferably use the normally idle winding H as the gasretaining means. In order that this winding may be heated by external means to drive off portionsof the gas therefrom, lead connections. are brought out therefrom through the casing l.

p of circuit 20, and the tap 21a is connected through

relays

23 and 29 to the high-potential side 2 la of circuit 2 l. Thus the voltages supplied to the respective circuits 2!! and 2! may be adjusted independently of each other to desired values as indicated by respective voltmeters Vi and V2; also, it will be seen that the relay 2% serves as an on-ofi control not only for the circults 20 and M but for the entire system as will hereinafter appear.

The circuit 20 has lead connections 39 and to those lead wires 3 of the relay R, which con nect to the heater winding is and the circuit it has

lead connections

32 and 33 to those lead wires 3 which connect to the gas-retaining wind ing i1, one of these lead wires being to the corn tact member 5 as above explained. Also, in order that the energization of the winding it will beterminated when the relay R is closed, the high-potential side 2| a of the circuit 2! has a lead connection 34 to the contact member ii, the action of .this connection being to provide a short circuit around the winding I! when the relay R closes. However, in order that there However, only one extra lead connection is: re- 25, will not be 8 direct Short Placed 011 e quiredwhich is the lead connection by way of when the relay R 610588, there s s y one of the terminals 8 and lead wires 3 aforementionedsince the winding i1 is connected at I 8 to the contact member 5 and the lead connection to this contact member serves also as a lead connection to the winding.

The method of charging the winding IT with occluded gas is as follows: The casing i is first connected to a vacuum line for a period of approximately 10 minutes during which time voltage ls applied to the winding I! to cause it to glow a bright red. Hydrogen gas to approximately 760 mm. pressure is next introduced into the casing, and the voltage to thewinding i1 is V adjusted so that it will again glow a bright red. After a few minutes of this excitation, the voltage is removed from the winding and the winding is left to cool in the hydrogen gas. The casing is then again connected to a vacuum line to reduce the pressure to a value of the order of .001 mm. of mercury. At the same time voltage is applied to the heater winding 14 to cause it to glow a bright red so as to drive out the occluded gas therefrom. This evacuating of the casing I and energizing of the heater winding I4 arez'continued for approximately 30 minutes, after which the voltage is removed and the casing l is sealed.

The initial timing of the relay R may be assumed to be 45 seconds and the final desired timing to be 30 seconds. The reduction in the timing to this desired value is carried out aut'o matically by the timing system shown inFigure 2. This timing system includes two ener'- gizing circuits 20 and. 2! having a common return lead 22 which may be considered as the low or ground side of the system. The high potential side of the circuit 2|] is accordingly the lead 20:: and that of circuit 2| is the lead 2m. These circuits derive their voltages from a common source (not shown) by way of a sup ly line23 and a manual on-off switch 24 for the system. One side of the supply line is connected through a relay 25 to the ground side 22 of the"

circuits

20 and 2! and the other side of the supply line is connected through respective voltage-adjust-'

ing devices

25 and 21 to the ground side 22, the

devices

26 and 21 being auto transformers having variable taps 26a and 21a, these devices being known commercially as Variacs. The tap 26a is connected to the high-potential side 20::

eluded a resistor 35 in the lead connection 32 from the low-potential side 22 to the contact member 5. The resistance 35 is small in relation to the resistance of ,the winding ll, a typical value being 100 ohms for a winding of 2000 ohms.

The relay 25 is normally closed, and this being the only relay in the circuit 20, this circuit is energized the instant current is applied to the system. The relay 2!! is also normally closed, but rela 231s normally openit being understood that the term "normally is here applied to mean the condition of the relays when they are unenergized and at ambient temperature. Thus ethc circuit 2i is initially unenergized when current is applied to the system. Each of the relays 25, 28 and 28 have stationary contact arms and tomperature-responsive contact arms designated respectively by the reference numerals of the relay with the sufllx letters a and b. Associated with the movable arms of the relays are heater windings designated by the reference numerals of the relays with the suffix letter 0. The heater of relay 28 is connected across circuit 20 by leads 3'5 and 31 and is therefore energized the instant current is initially supplied to the system. This relay has a, make-contact timing period equal to that desired for the relay B. bein calibrated, this being 30 seconds in the present instance as above mentioned. Thus, it will be understood that the timing system operates to energize the heater winding It the instant current is supplied to the system and that when the winding l 4 is energized for a. period equal to the timing period desired for the relay R, circuit 2| is closed by relay 28 to supply energizing current to the gas-retaining wind- 7 ing i1 and to start the release of gas therefrom.

When the relay 28 closes, energ'zing current is supplied to the heater windings of the

relays

25 and 29 from the circuit 2|. The energizing circuit for the relay 25 is from the movable contact 28b of relay 28 through a lead 38, heater winding 250, a connection 25d of this winding to the contact 251), and the contacts of the relay 25 to the low side of the supply line 23, and the energiz-' 7 to relay 2 and through this relay to the low side of the supply line 28. Thus. the instant relay II closes to start the energization of the gas-retaining winding l1, relays I! and II are energized and begin moving to open positions. Relay 29 controls the length of time the gas-retaining winding is energized, and may suitably have a-timing of 8 seconds. It is desired that relay II shall have a timing Just longer than that of relay 2!, a suitable value being 10 seconds. Accordingly, it will be understood the gas-retaining winding I1 is energized for a period of 8 seconds, beginning when the heater winding H has been energized for the timing period desired for the relay R; after the lapse of an additional 2 seconds following the termination of energization of the winding l1,-

during which the heater winding It continues to be energized, relay 2! opens to shut off the current supply to the entire system. Relay 2! has a long contact-make timing period to allow the relay R to cool and return substantially to ambient temperature, a suitable timing for this p pose being 3 minutes. The contact-make period of the relay 29 and the contact-break period of relay 28 are each relatively short and accordingly these relays are respectively in closed and open conditions at the time the relay 2! recloses.

The closing of relay 25 represents the completion of a cycle of operations of the automatic timing system, or in other words an energizing cycle for the relay R being calibrated, it being noted that this cycle comprises energizing the heater winding I for the desired timing period (30 seconds) of the relay R, a continuing energization of this heater winding together with a concurrent energization of the gas-retainingwinding I! for an additional period (8 seconds), and thereafter a deenergization of the timing system and the relay R for a period (3 minutes) to allow the relay R and the relays of the timing system to return to their initial positions. Thereupon, with the reclosing of relay 25, a second energizing cycle is started, and so on.

Each 8-second period of energization-of the gasretaining winding II will cause a quantity of free gas to be released in the casing l and will cause the timing of the relay R to be reduced by a deflnite period, typically 3 seconds. Thus, assuming the relay R has an initial timing of 45 seconds, it will have a timing of 42 seconds at the completion of the first energizing cycle.- At the completion of a second energizing cycle, the timing of the relay will be reduced approximately another 3 seconds to 39 seconds.

the contact member I is negligible. We may assume the deflection of the contact member I to be sufficiently large during the third cycle here considered that the relay R will not close before the relay 28 opens. As a result, the timing of the relay R is reduced during this third energizing cycle by another 3 seconds, bringing the timing of the relay to approximately 36 seconds.

During the fourth and succeeding energizing 10 cycles, the period of energization of the gas-retaining winding ll becomes shorter and shorter, the amount of gas released during each cycle is less and less, and accordingly the timing of the relay R is reduced by smaller steps, the timing 5 approaching asymptotically the predetermined 30-second period as a limit. (In view of the inherent thermal delay in the winding ll reaching a temperature whereat it will release gas when it-is energized, the timing of the relay 28 is desirably set to a value somewhat less than that desired for the relays being calibrated.) In no case, however, can this system operate to reduce the timing of the relay R below this limit. Accordingly, it will be understood that a series of relays to be calibrated to the same timing may be connected in parallel to the

circuits

20 and 2| and that even though they have different initial timing periods varying over a wide range, all will be brought to the same timing by the 30 present calibrating system.

While I have herein particularly shown and described my invention in terms of a preferred embodiment thereof, it will be understood that this embodiment is illustrative and not limitative of my invention as the same is subject to changes and modifications without departure from the scope of my invention, which I endeavor to express according to the following claims.

. I claim:

1. The method of reducing the timing of a sealed-in thermal relay to a predetermined value, said relay including a substantially evacuated casing and a temperature-responsive contact member, which comprises storing a gas in said casing out of contact with said members, and releasing a quantity of said gas according to the differential between the initial timing of the re-- lay and said predetermined value.

2. The method of reducing the timing of a sealed-in thermal relay, said relay including a substantially evacuated casing and a temperature-responsive contact member, which comprises occluding a gas within said casing, and driving of! portions of said occluded gas into the Durin the third energizing cycle, the timing free Spa within d 0 8- of the relay R is reduced to less than 38 seconds and the relay R may close before the B-second timing period of the relay 2! elapses to stop energization of the gas-retaining .winding ll. If this should happen, the energizing period of the gasretaining winding is cut short, since the winding I1 is effectively shorted through resistor 35 when relay R closes as hereinbefore described. However, the temporary energization of the gasre- 3. The method of reducing the timing of a sealed-in thermal relay, said relay including a substantially evacuated casing and a temperature-responsive contact member, which comto prises occluding gas within said casing, and thermally releasing portions of said gas into the free space within said casing.

4. The method of reducing the timing of a sealed-in thermal relay, said relay including a taining winding I1 produces some deflection of substantially evacuated casing and a temperathe contact member 5 away from the other contact member 6, and tends therefore to delay the closing of the relay R. The deflection of the contact member 5 has the veffect therefore of hastenture-responsive contact member, which comprises occluding a thermally releasable gas within said casing, and electrically heating said gas to release portions thereof into the free space within ing the action of the timing system, but this holds said casing. a

5. The method-of setting the timing of a sealed-in thermal relay having a casing, a tem-.

perature-responsive contact member and an associated heating member having a. predetermined log are gradually .shorterand the deflection of eilective distance of spacing from said contact member, which comprises evacuating said casing to a pressure whereat the mean free length of path of the gaseous molecules in said casing is substantially greater than said etlective distance of spacing between said contact and heating members, and producing a free gas within said casing to reduce said mean free length of path to the order of magnitude of said distance of spacing.

d. The method of setting the timing of a sealed-in thermal relay having a casing, a temperature-responsive contact member and an associated heating member having a predetermined eiiective distance of spacing from said contact member, which comprises storing occluded gas in a member within said casing, evacuating said casing to a pressure whereat the mean free length of path of the gaseous molecules in said casing is substantially greater than said efiective distance of spacing, and heating said storing membe: to release a portion of said occluded gas whereby said mean free length of path is reduced to the order of magnitude of said distance of spacing. I1

7. In a thermal relay including a substantially evacuated casing, atemperature-responsive contact member and an electrically-energizable heating member associated with said contact member for operating said relay: the combination of an electrically-energizable heating member within said casing, saidv latter heating member having occluded gas stored therewithin; and circuit element leading out of said casing from said latter heating member whereby the heating member is electrically energizable to cause portions of said gas to be released therefrom.

8. In a thermal relay including a substantially evacuated casing, a pair of similar contact members within said casing, said contact members including bimetals polarized in the same direction, and a heating winding on one of said members for operating said relay: the combination of a second heating winding provided on the other of said contact members for imparting thereto a resilience equal substantially to that of said one contact member, said second heating winding having occluded gas stored therewithin; and circuit means leading out of said casing for electrically energizing said second heating winding to release portions of said occluded gas therefrom.

9. The method of reducing the timing of a thermal relay to a predetermined value, said relay including a substantially evacuated casing, a thermally-responsive switch, and a gas-retaining means electrically energizable to cause a free gas to be released within said casing, which comprises subjecting said relay to successive energizing cycles of which each cycle is for a time period greater than said predetermined value, energizing said gas-retaining means for a time interval within said period beginning when said relay has been energized for a time duration equal to said predetermined value, and thereafter deenergizing said relay and gas-retaining means.

10. The method of reducing the timing of a thermal relay to a predetermined value, said relay including a substantially evacuated casing, a thermally-responsive switch, and a gas-retaining means electrically energizable to cause a free gas to be released within said casing, which comprises subjecting said relay to successive energizing cycles of which each cycle is for a time period greater than said predetermined value, energizing said gas-retaining means when said relay is energized for a time duration equal to said predetermined value, stopping energize,- tion of said gas-retaining means during said period at a fixed time interval following the expiration of said predetermined value or upon operation oi said thermal switch in response to energization of said relay depending upon which occurs the sooner, and subsequently stopping energization of said relay to allow the same to cool.

11. The method of reducing the timing of a thermal relay to a predetermined value, said relay including a, substantially evacuated casing, a thermally-responsive switch, and a gas-retaining means electrically energizable to cause a free gas to. be released within said casing, which comprises subjecting said relay to successive energizing cycles or which each cycle includes simul-- taneously energizing said gas-retaining means upon said relay being energized for a time duration equal to said predetermined value, stopping energization of said gas retained means upon operation of said thermal switch, and subsequently stopping energization of said relay until the same and said gas-retaining means are restored substantially to ambient temperature.

- ,12. The method of reducing the timing of a thermal relay to a predetermined value, said relay including a substantially evacuated casing, athermally-responsive switch, and a gas-retaining means electrically energizable to cause a free gas to be released within said casing, which comprises subjecting said relay to successive cnergizing cycles of which each cycle includes simultaneously energizing said gas-retaining means for a fixed time interval beginning when said relay is energized for a time duration equal to said predetermined value, and subsequently stopping energization of said relay to allow the relay to cool and return substantially to ambient temperature.

13. A method of reducing the timing of a thermal relay to a predetermined value, said relay including a substantially evacuated casing, a switch including a pair of temperature-responsive contact members, and a gas-retaining means electrically energizable to release free gas within said casing which comprises subjecting said re-= lay to successive energizing cycles of which each cycle comprises applying a given voltage to said relay, simultaneously applying a predetermined energizing voltage to said gas-retaining means, the energization of said gas-retaining means being started upon said relay being energized for a time duration equal to said predetermined value, stopping energization of said gas-retaining means upon operation of said switch, and thereafter deenergizing said relay.

14. A thermal relay comprising a movable temperature-responsive contact member, a relatively stationary cooperating contact member, an electrically-energizable heating member thermally associated with said movable contact member for operating said relay, an enclosing casing for said contact members and heating member, and a gas medium in said casing, said relay being characterized as having heat-flow paths of different lengths from said heating member which respectively influence the timing of the relay in different directions, the pressure of said gas medium in said casing being set to a value whereat the mean free length of path of the gaseous molecules is of the order of the effective length of the gaseous spacing in that of said heat-flow paths having unidirectional timing influence on said relay.

15. In a system for automatically timing a assures ll sealed-in thermal relay, said relay including a substantially-evacuated casing, a thermally-responsive switch, a heater for operating said switch and gas-retaining means electrically energizable to release free gas within said casing: the combination of a first circuit for energizing said heater; a second circuit for energizing said gasretaining means; means for connecting said circuits to a source of electric current; a first control switch in said second circuit having a normally-open position; means, connected to said first circuit and operated by current therefrom, to close said first control switch at the expiration of a preset interval; a second control switch serially included in said second'circuit and having a normally-closed position; means, rendered operative by said first switch as the same is closed, to

open said second switch at the expiration of a preset interval following the' closing of said first switch; and means, timed with said last-stated means, to deenergize said system following the opening of said second switch.

18. In a system for automatically timing a sealed-in thermal relay, said relay including a substantially-evacuated casing. a thermally-responsive switch, a heater for operating said switch and gas-retaining means electrically energizable to release free gas within said casing: the cofiiks bination of a first circuit for energizing said heater; a second circuit for energizing said gasretaining means, a first thermal-type control relay having normally-open contacts in said second circuit and having a heater winding energized by said first circuit, said relay being adapted to close to supply energizing current to said gas-retaining means when the heater winding thereof has been energized for a preset interval; means for connecting said circuits to a source circuit by said second nd circuit when of electric current; a second thermal-type control relay having normally-closed switch contacts in said second circuit and having a heater winding energized by way of said connecting means and said contacts of said first control relay when said connecting means and first control relay are closed. said second control relay being adapted to open said second circuit to cut oil the current supply to said gas-retaining means at the expiration of a preset interval following the closing of said first control relay; and a master control thermal-type relay for said system having normally-closed switch contacts serially included in said first and second circuits and having a heater winding energized by way of said connecting means and said contacts of said first control relay when said connecting means and first control relay are closed. said master relay having a contact-break period at least greater than that of said second control relay whereby to deenergize said system following the opening of said second control relay.

17. The combination set forth in claim 16 comprising a circuit serially including said thermany-responsive switch for disabling said secsaid thermally-responsive switch is closed.

JOHN J. DIETZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,245,168 Suits June 10, 1941 2,310,747 Payne Feb. 9, 1943 2,369,619 Stibitz Feb. 13, 1945 Certificate of Correction Patent No. 2,494,863 January 17 1950 JOHN J. DIETZ It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as r'ollows:

Column 9, line 33, for element read elements; column 10', line 21, for gas-retained read gas-retaining;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 23rd day of October, A. D 1951.

[SEAL] THOMAS F. MURPHY,

Assistant Uom'mz'ssioner of Patents.

Certificate of Correction January 17, 1950 Patent No. 2,494,863

JOHN J. DIETZ It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 9, line 33, for element read elements; column 10", line 21, for gas-retained read gas-retaining; and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Otflice.

Signed and sealed this 23rd day of October, A. D. 1951.

[SEAL] THOMAS F. MURPHY,

Assistant Commissioner of Patents.