US2236697A - Thermal switch - Google Patents

Description

April 1, 1941. g R ER 2,236,6Q7

THERMAL SWITCH Filed Sept. 3, 1938 3 Sheets-Sheet l Inventor: Leo R. Deters,

His CAttorrwey.

April 1, 1941, L R PETERS 2,236,97

THERMAL SWITCH Filed Sept. 3, 1938 3 Sheets-Sheet 2 A rifl 1, 19 M L. R. PETERS zpzggyfim THERMAL SWITCH Filed Sept. 5, 1933 3 Sheets-Sheet 3 Inventor: Leo F2 et er s,

Patented Apr. 1, 1941 THERMAL SWITCH Leo R. Peters,

East Cleveland, Ohio, assignor to General Electric Company, a corporation of New York Application September 3, 1938, Serial No. 228,365

11 Claims.

My invention relates generally to switches for making and breaking electrical circuits, and particularly to switches of the thermostatic type in which a bimetallic strip is adapted to be flexed by the heat radiated by a heater element connected in the circuit to thereby break the circuit through the bimetallic strip. Still more particularly, my invention relates to a switch of the above type for operating electric discharge lamps and the like.

In the operation of gas-filled electric discharge lamps having thermionic electrodes, it is necessary to provide a suitable starting circuit for the purpose of initiating the arc dischargebetween the lamp electrodes. Before the arc discharge can take place, the electron emissive substance with which the lamp electrodes are coated must be preheated to a temperature at which it will become sufliciently activated to readily support the arc discharge.

In one type of circuit employed for starting and operating gas-filled electric discharge lamps having thermionic electrodes, one current supply wire of each electrode is connected to one of the terminals of a source of alternating current in series with a choke coil and a main switch. The other current supply wires of the electrodes are interconnected by an auxiliary switch. When the discharge lamp is put into operation, both the main and auxiliary switches are closed and the thermionic electrodes are connected in series through the choke coil to the source of current so that they are traversed by a current by which they are preheated. When these electrodes are preheated to a temperature sufiicient to activate the electron-emissive material thereon, the auxiliary switch is opened so as to interrupt the series connection of 'the'electrodes and initiate the arc discharge therebetween. The initiation of the arc discharge is facilitated by the voltage impulse produced in the choke coil due to the opening of the auxiliary switch. During the continuance of the arc discharge, no separate heating current is passed through the electrodes inasmuch as the heating thereof is thereafter efiected by the arc discharge current alone.

Where the main switch for operating an electric discharge lamp is to be mounted remote from the lamp itself, it is highly desirable, in order to simplify the wiring arrangement, that the auxiliary switch referred to above for starting lamp operation be mounted either in the lamp fixture itself or in close proximity thereto. In addition, it is also desirable that the said auxiliary switch be automatic in operation so that the are diszatlon of the heat radiated by the said heater elecharge in the lamp will be automatically initiated upon closure of the main or line switch. Accordingly, one object of my invention is to provide a thermostatic auxiliary switch, simple in construction and positive in operation, for automatically starting and operating electric discharge lamps having thermionic electrodes therein upon closure of the main switch.

Another object of my invention is to provide an efficient bimetallic starting and operating switch for electric discharge lamps which can be manufactured inexpensively and which is extremely compact and of minimum dimensions.

Still another object of my invention is to provide a thermostatic switch of the bimetallic type in which the heat-actuated bimetallic element is disposed around the heater element so as to surround the same to thereby gain maximum utiliment and to readily obtain the desired movement of the bimetallic element.

A further object of my invention is to provide a bimetallic switch with effective temperature compensating means to maintain the switch contacts in their normal positions relative to one another for a protracted period of time following the energization of the switch heater element.

A still further object of my invention is to provide, in a bimetallic switch for starting and operating electric discharge lamps having thermionic electrodes, a positive delayed action feature for prolonging the period of initial heating of the lamp electrodes so that they become sufficiently electron-emissive to support an arc discharge therebetween.

Still another object of my invention is to provide a bimetallic switch having a bimetallic element formed with a portion substantially unaffected initially by the switch heater element whereby a time lag is produced in the movement of such portion, the subsequent movement of such portion thereafter combining with the initial deflection of the bimetallic element to thereby maintain the switch contact members in their operative or heat-actuated relation with respect to one another despite an appreciable drop in the current flow through the switch heater element.

A further object of my invention is to provide abimetallic switch which will cool rapidly upon de-energization of the switch heater element, thus permitting a recommencement of circuit operations in a relatively short period of time following such de-energization of the switch heater element.

Further objects and advantages of m: invention will appear from the following detailed de heat-actuated contact member of the switch.

scription of species thereof and from the accompanying drawings in which:

Fig. 1 is a front view, partly in section, of a bimetallic switch comprising my invention; Fig. 2 is a. side view, partly in section, of the switch shown in Fig. 1; Fig. 3 is a fragmentary isometric view, on an enlarged scale, of the switch shown in Figs. 1 and 2; Fig. 4 is a diagrammatic illustration of a wiring arrangement utilizing a bimetallic switch according to my invention for starting and operating an electric discharge lamp; Fig. 5 is a fragmentary side view, on an enlarged scale, of the bimetallic switch shown in Figs. 1 to 3 with the successive operative positions of the bimetallic switch elements shown in dottedv lines; Figs. 6 and 7 are fragmentary front and side views respectively, on an enlarged scale, of a modified form of bimetallic switch according to the invention; Figs. 8, 9 and 10 are front. side and rear views, similar to Figs. 6 and 7, of another modification of my invention; Fig. 11 is a diagrammatic illustration of a wiring arrangement utilizing a bimetallic switch according to the form shown in Figs. 8 to 10 for starting and operating an electric discharge lamp; Figs. 12 and 13 are front and side views, similar to Figs. 6 and '7. of still another modification of my invention; Fig. 14 is a front View, similar to Fig. 12, of another modification of the invention; and Figs. 1 5 and 16 are front and side views respectively, similar to Figs. 6 and '7, of a further modification of my invention.

Referring to Figs. 1 to 3, the bimetallic switch there shown comprises a sealed elongated container I 0, preferably of glass, provided at one end with an inwardly extending stem II having a press portion I2. A plurality of current supply or leading-in wires, comprising a front pair I3, I4 and a rear pair IE, IS, extend into the container I and have portions thereof sealed in the press portions I2 of the stem or supporting member II. A heater element I'I, comprising a closely coiled wire of suitable refractory material, preferably Nichrome wire, is supported by, and connected at its opposite ends to, the lead wires I3 and I6. As is evident from the drawings, theheater coil H is so mounted as to extend transversely of the envelope or container I0.

A pair of switch contact members I8, I9 are secured at one end. preferably by welding, to the leading-in wires I4 and I respectively.

Each of the said contact members I8, I 9 consists of a length of bimetallic strip, such as the commercially obtainable bimetal of chrome iron and Invar, approximately 0.008 of an inch thick, 0.05 of an inch wide, and three-quarters of an inch long. As shown, the bimetallic elements are mounted so as to extend substantially longitudinally of the container III. The bimetallic element I8 is bent intermediate its ends into a hook or half-loop portion 20, the open end of which preferably faces in a direction parallel to the length of the said element and towards the free or unsupported end thereof. This hooked portion 20 accordingly divides the bimetallldelement I8 into two separate leg portions, to wit: a lower or main leg portion 2I and an upper or free leg. portion 22. The curved portion of the hook 20 is of slightly larger diameter than that of the heater coil I1. and is so located with respect to the said heater coil as to be concentric therewith, thus partially surounding the same. The bimetallic element I8 accordingly forms the The other bimetallic element I9, hereinafter referred to as the spring contact member, is disposed so that the free end thereof engages the free end of the heat-actuable element I8 when the latter is in its normal or unheated position. The actual contact, however, between the two bimetallic elements is through a pair of contacts 23, 24 of a silver alloy, preferably coin silver, provided on the-free ends of the members I8 and I9 respectively. As clearly shown in Fig. 5, the spring contact member I9 is so mounted on the leading-in wire I5 as to be flexed from its normal position A (indicated by the dotted lines) by the heat-actuable element I8 when the latter is in its normal or unheated position. The member I 9 thus forms a temperature compensating element or device serving to maintain the ends of the two contact members I8, IS in positive engagement during ambient temperature changes. The amount of fiexure of the member I9 from its normal position by the heat-actuable element I8 is in this case so proportioned that the contact between the two members I8, I9 is broken in two to three seconds when a current of one-quarter of an ampere is passed through the heater element I1 and the said members are surrounded by an atmosphere of air.

To obtain rapid cooling of the switch parts upon deenergization of the switch heater element H, the container I0 is filled with a suitable gas of high heat conductivity, preferably hydrogen or helium, at a pressure of approximately 4 mm. of mercury or thereabouts. I have found that a pressure of 4 mm. of the gaseous filling provides'a maximum cooling effect while still affording adequate protection against arc-over. The rapid cooling provided by such a gaseous filling consequently permits operation of the switch in a very short period of time, approximating three or four seconds, following the deenergization of the heater coil ii.

Referring now Ito Fig. 4, illustrating diagrammatically the manner in which a bimetallic switch according to my invention may b used to start and operate an electric discharge lamp 25 having thermionic electrodm 2B and 21, one end of a choke coil 28 is connectedby lead 29 to one end of the heater element I! of the bimetallic swritch I0. The other end of the choke coil is connected by lead 30, a main switch 3|, and lead 32 to one side of the line. Th opposite end of the heater element I1 is connected by lead 33 to one end of the lamp electrode 28, while one end of the opposite electrode 2'! is connected by lead 34 to the other side of the line. The ends of the electrodes 26, 21 opposite to those ends connected to the heater coil I1 and current source are connected to the bimetallic contact members I 8, I 9 of the bimetallic switch III, electrode 26 being connegated by lead 35 to the bimetallic element I8 and electrode 21 being connected y lead 36 to the bimetallic element I9. The etal lic elements I8, I8 thus function as a switch for connecting the electrodes 26, 21 in series across the line. A small condenser 31, of approximately 0.05 mfd. capacity. is preferably connected across the switch contact members I-8, I9 so as to shunt the same.

As before stated, the heat-actua'ble bimetallic element I8, in its normal or inoperative position, is in electrical contact with the other or spring bimetallic element I9, thereby interconnecting the lamp electrodes in series across the line. To start lamp operation, the main switch 3| is closed.

aaaaeer bimetallic element towards the position B indi-' baited by dotted lines in Fig. 5. The contact, however, between the two bimetallic elements II, is is not broken during the initial stages this movement or the element l8 since the element is, due to the fiexure thereof from its normal position A by the element I8, tends to assume the said normal position and s0 follows the movement of the leg 22. The continued movement, however, of the upper leg portion 23 towards the position E eventually results in the separation oi the two bimetallic elements i8, i9 and the braking of the contact therebetween. As previously stated, the two bimetallic elements are so arranged with respect to one another that the period of time required for the two elemenm to actually separate is approximately three seconds when the current passing through the switch heater coil i'i amounts to a quarter of an ampere. This interval of time, during which the seriesconnected lamp electrodes 26, 21 are traversed by a current by which they are heated, is sufficiently long to permit the said electrodes to be preheated to arc-sustaining temperature. The interruption of the series connection 0! the electrodes 2S and 21 by the separation of elements l8, l-9 immediately initiates the arc discharge in the lamp. The initiation of this arc discharge is facilitated by the voltage impulse produced in th choke coil 28 by reason of the abrupt interruption of the electrode series connection.

When the arc discharge in the lamp commences, the current flow through the lamp circuit automatically drops to a lower value. Thus, in present commercial electric discharge lamps of the fluorescent type, the current flow drops from approximately 0.5 of an ampere before initiation of the arc discharge to approximately 0.27 of an ampere after the arc discharge occurs, and in some cases to as low as 0.20 of an ampere. Consequently, the intensity of the heating effect of the heater coil ll decreases a proportional amount. As a. result, the upper leg portion 22 of I i8 according to the invention. Thus, by the time the upper leg portion 22 actually separates from bimetallic element l9 during the initial movement of such leg portion, the lower or main leg portion 21 has become sufliciently heated, essentially 'by conduction of heat from the hook portion 20, to cause a. movement of the said main leg portion 2! to the position C indicated 'by dotted lines in Fig. 5. This subsequent movement of the main leg portion 2! accordingly adds to the deflection of the upper leg portion 22 and tends to neutralize the return movement of such upper leg portion towards the element l9, which return movement occurs when th arc discharge in the lamp commences. The additional movement thus provided by the main leg portion 2i consequently insures the maintenance of the separated relation of the two bimetallic elements II, I! and thereiore the continuance of the are discharge in the lamp. The particularly shaped construction of the element l8 accordingly provides a time-delayed compensating arrangement whereby the spaced relation of the switch contact points 23, 2 is maintained despite an 81D- preciable drop in the current flow through the switch heater coil upon the initiation of the arc discharge in the lamp.

When the main switch 3| is opened to discontinue further operation of the lamp, the current passing through the heater coil l'l ceases so that the bimetallic element i8 gradually cools and returns to its normal position. If, in the case of a conventional bimetallic switch, the main switch 3| is closed to restart the lamp before the element i8 has returned to the position where it re-= engages element l9, then the very instant ele= ment l8 engages element is so as to reestablish the circuit through the heater coil ii and the lamp electrodes 26, 2?, the slight reheating oil element 58 by the heater coil ll immediately separates the two bimetallic elements id, id without allowing sufllcient time for the electrodes 2d, 27; to become adequately preheated to are discharge= supporting temperature. As a result, the lamp fails to start. This same condition occurs if the main switch 3! is closed exactly at the moment the element l8 has just re-engaged element I5. If the main switch is left closed, the above described action is continually repeated again and again, the element i8 alternately making and immediately breaking the circuit through the electrodes 26, 21 before they have become sufficiently preheated to support the arc discharge in the lamp. To actually restart the lamp, it is necessary to keep the main switch open until the element !8 has returned to a position sufllciently intermediate the position at which it engages element l9 and its normal or unheated position. The con-tact between the two elements I8, I 9 is then maintained, after closure of the main switch SI, for a. sufllcient length of time to permit adequate preheating of the lamp electrodes.

With a switch constructed according to my invention, and with the starting arrangement shown in Fig. 4, the above-mentioned condition is materially rectified, so that the lamp will start even though the main switch 3| be closed immediately following the opening of the same. The elimination of such an undesirable condition is accomplished by the use of coin silver for the contact points 23, 240i the elements I8 and i9, together with the condenser 31 connected across the said elements. If, after discontinuation of lamp operation and before the element 18 has re-engaged element IS, the main switch 3! is closed, then the condenser 31 induces a spark of relatively high intensity between the approaching contact points which tends to weld the said contacts and causes them to stick together. Since the interengagement of th two contact points 23, 24 re-establishes the circuit through the heater coil H and electrodes 28, 27!, the consequent reheating of the bimetallic element l8 tends to separate the same from the other element l9. However, the tendency of the contact points to stick together resists such separation until the force tending to separate element l8 from element 09 is sufilcient to overcome the said sticking tendency. Thus, after closure of the circuit through th lamp electrodes 26, 21. a time lag occurs before the said circuit is interrupted. This time lag is of sufficient duration to permit adequate preheating of the lamp electrodes 26, 21 to are discharge-sustaining condition. Accordingly, when the contact points 23, 24 actually do separate so as to break the series-connection of the lamp electrodes, the arc discharge in the lamp will therefore commence. It is thus evident that the use of coin silver contact points together with the condenser 31 permits rapid and positive restart of lamp operation.

Referring now to the modification shown in Figs. 6 and 7, the bimetallic switch there shown is the same as that shown in Figs. 1 to 3 except for the particular construction of the heat-actuated bimetallic element It. Thus, instead of the hooked portion 28 surrounding the heater element ll, the element l8 in the modification is formed with a double coil surrounding the said heater element, the double coil consisting of an inner coil 39 and an outer coil 40. As shown, the outer coil 40 is formed as a continuation of the main leg portion 2| of the element l8, while the inner coil 39 is formed by reversely bending the element l8 back upon itself from the end of the outer coil 40, the inner coil 39 thu forming a continuation of the upper or free leg portion 22 of the element It. The two coils 39 and 4B are t so arranged as to be substantially concentric with one another, and are preferably spaced a slight amount.

The method of connecting this modified form of bi-metallic switch in the lamp circuit, and the operating characteristics of the'said switch, are the same as that described in connection with the first form of switch. When the main switch 31 is closed, the heating of the inner coil .39 of the bimetallic element It by the heater element it causes the upper leg portion 22 to bend towards the position B (Fig. 5), the other bimetallic element I 9 moving along with element l8 during the first stages of its movement to thereby maintain the contact therebetween. The continued movement, however, of the upper leg portion 22 eventually causes a separation of elements l8 and I9 and the initiation of the arc discharge in the lamp. By the time the leg portion 22 reaches the position B, the outer coil til and also the main leg portion 2!. have begun to heat up, by conduction of heat from the inner coil 39 and otherwise. Consequently, the resulting movement of the heated outer coil 60, and the main leg portion 2| to position C (Fig. 5) tends to neutralize any return movement of the upper leg portion 22 towards element It, thus maintaining the separated relation of elements l8, l9 and so insuring the continuance of the arc discharge in the lamp.

The modification illustrated in Figs. 8 to 10 difiers from the form of switch shown in Figs. 1 to 3 in the construction of the bimetallic element l9 (corresponding to element it in Figs. 1-3) and in the use of a second heater element to actuate the said element l9. Referring to Figs. 8 to 10, a heater coil il, similar to the heater coil II but of relatively lower wattage, is mounted in a like manner between the leading-in wire it and a support wire 62 extending from, and embedded in, the press portion l2 of the stem H. The bimetallic element I9 is secured to this auxiliary support wire 62 and is provided with a hook or U-shaped portion 43 similar to the hook portion of element H8. The bimetal in element I9 is reversed, however, from that in element It, so that the hooked portion 43, on heating, tends to open or uncoil instead of closing, as in the case of the hooked portion 20. In this manner, the upper leg portions 22, 22 of the two bimetallic elements It, I9 move in the same direction and in substantial unison when the said elements are heated by their respective heater coils II and ti, thus tending to maintain the interengagement of the two elements. However, because 01' the difference in wattage of the two heater, coils i1 and ll, the latter being of lower wattage than the former, a difierential movement of the upper leg portions 22, 22 of the bimetallic elements occurs. In other words, the leg portion 22 of the element I9 does not move as fast as the leg portion 22 of element I8, so that there is a tendency for the said elements to separate. This separation, however, is delayed by so mounting the elements l8, IS with respect to one another that the element i9 is under a slight spring tension similar to that described in connection with Figs. 1 to 3. The separation of the contact points 23, 24 therefore does not take place the instant current passes through the heater coils it and H, but instead is delayed until the differential of movement between the two bimetallic elements l8, l9 exceeds the amount of spring tension, when separation actually does occur.

Referring to Fig. 11, the circuit employed for starting and operating electric discharge lamps with a bimetallic switch according to the modification of Figs. 8 to 10 is very similar to the circuit shown in Fig. 4, the only difference being the interposition of the heater coil iii in the circuit connecting the lamp electrodes in series. Thus, as shown in Fig. 11, one end of the heater coil BI is connected by lead 64 to the bimetallic element l9, while the other end is connected by lead fifi to one end of electrode 21. The operation of this modified form of bimetallic switch in starting' and in restarting lamp operation is then the same as that described in connection with the first form of switch.

The modified form of bimetallic switch shown in Figs. 12 and 13 is similar to that shown in Figs. 1 to 3, the only difference being in the construction of the bimetallic element it. In Figs. 12 and 13, the element i8 is formed with a helical coil 68 surrounding the heater element ll, instead of the hooked portion 20 present in Figs. 1 to 3.

I This helical coil construction of element 18 does not provide the time-delayed compensating feature characteristic of the previous forms of the invention. Accordingly, care must be taken in adjusting the two bimetallic elements 58 and it with respect to one another so that the separation of the said elements is maintained once the arc discharge in the lamp has been initiated, at which time the current flow through the heater element ll drops to nearly half its former value and in some cases even more.

In Fig. 14, the modification there shown difiers from the switch shown in Figs. 12 and 13 only in that the heater element I! is enclosed within a mass of refractory material, preferably in the form of a cylinder 17 of porcelain or glass, which is in turn surrounded by the helical coil 45 in the bimetallic element It. The heater element I'i heats this refractory mass 41 which in turn heats the surrounding helical coil portion 66 of the bimetallic element l8. The time required for the refractory mass 61 to heat up accordingly produces a time lag in the heating of the bimetallic element 18. Consequently, the movement of element I8, and resulting separation of the same from element i9, is delayed for a definite interval of time following the closure of the circuit through the heater element ll, thus permitting the lamp electrodes to heat up to arc-supporting temperature.

The construction shown in Figs. 15 and 16 is a slight modification of the switch illustrated in Figs. 12 and 13, the only difierence being in the construction of the spring contact element H9 (corresponding to element IS in Figs. 12 and 13) which in this modification is formed with a helical coil 48 the same as the helical coil 46 in element l8. The two bimetallic elements l8, 9 are therefore exactly alike in construction, thus greatly simplifying manufacture of the switch. A further but inconsequential difierence of this form of switch from the previous forms is in the connection of one end of the switch heater element I! to leading-in wire ll instead of IS, and the connection of the bimetallic element II to leading-in wire l6 instead of i4. I

It will be noted that in all the above-described forms of bimetallic switches, the bimetal surrounds the heater coil l1 instead of the latter surrounding the bimetal, as in the case of conventional switches of this type. The disposition of the bimetal around the heater coil provides a definite control over the movement of the bimetal with respect to time and with respect to varying current flow through the heater coil. Thus by' suitably shaping the bimetal around the heater coil it is possible to obtain the desired character of movement of the bimetal with respect to time and with respect to varying current fiow. The forming of the bimetal around the heater coil also greatly simplifies the manufacture of the switch, since it permits the use of a machinewound heater coil. Furthermore, such construction permits the use of a higher ratio between the lamp starting and operating wattage input, allowing sufiicient time for initiation of the arc discharge in starting the lamp and still maintaining the switch contact members separated, under the decreased current fiow existing during lamp operation, so as not to interrupt the continuance of the arc discharge.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A bimetallic switch comprising a support member, a heater element mounted on said support member, and a pair of co-operating contact members secured at their base ends to said support member, one of said contact members comprising a bimetallic strip so formed as to provide an intermediate portion at least partly surrounding the said heater element in relatively close relation thereto and having its base portion substantially unaffected initially by the said heater element but subsequently deflected, by the heat generated by said heater element, in a direction adding to the deflection of the free end portion of said bimetallic strip.

2. A bimetallic switch comprising a support member, a heater element mounted on said support member, and a pair or co-operating bimetallic contact members secured at one end to said support member and normally in engagement with each other at their unsupported ends, one of said bimetallic contact members having an intermediate portion so formed as to at least partly surround the said heater element, and the other of said bimetallic contact members being flexed a slight amount from its normal position by the said one of said bimetallic contact members.

3. A bimetallic switch comprising a support member, a pair of heater elements mounted on said support member. one of said heater elements being of relatively high wattage capacity and the other being of relatively lowwattage capacity, and a pair of co-operating bimetallic contact members secured at one end to said support member, one of said bimetallic members having an intermediate portion so formed as to at least partly surround the said high wattage heater element and the other of said bimetallic members likewise having an intermediate portion so formed as to at least partly surround the said low wattage heater element, said bimetallic members being so arranged with respect to one another as to move in the same direction under the influence of like temperature changes therein. 4. A bimetallic switch comprising a container, a heater element mounted within said container, and a pair of co-operating contact members mounted within said container, at least one of said contact members comprising a bimetallic strip having an intermediate portion disposed closely adjacent the said heater element, said container having a filling of a gas of high heat conductivity at a pressure of approximately four millimeters.

5. A bimetallic switch comprising a support member, a heater element mounted on said support member, and a pair of co-operating contact members mounted on said support, one of said contact members comprising a bimetallic strip having at least a portion thereof disposed closely adjacent said heater element, said contact members having interengageable contact points of a silver alloy which tend to stick when brought together in a heated condition, and a condenser of relatively low capacity connected across the said contact members and capable, when said contacts are separated and a'potential is applied across the condenser, of inducing a spark of sufficient intensity between the contacts to cause them to stick together.

6. A bimetallic switch comprising a support member, a heater element mounted on said support member, and a pair of co-operating contact members, one of said contact members comprising a bimetallic strip having an intermediate portion formed to at least partially surround said heater element in relatively close relation thereto and also having a fixed end portion and a free end portion extending in opposite directions a substantial distance away from said intermediate portion, the end of said fixed end portion being secured to said support, the other contact member being arranged to engage with the free end portion of said bimetallic strip.

7. A bimetallic switch comprising a support member, a heater element mounted on said support member, and a pair of co-operating contact members, one of said contact members comprising a bimetallic strip having an intermediate portion formed to at least partially surround said heater element in relatively close relation thereto and also having a fixed end portion and a free end portion extending in opposite directions a substantial distance away from said intermediate portion, the end of said fixed end portion being secured to said support, the other contact member also comprising a bimetallic strip secured at one end to said support and extending in the same general direction as said first-mentioned bimetallic strip with its free end arranged to engage the free end of said firstmentioned bimetallic strip.

8. A bimetallic switch comprising a support member, a heater element mounted on said support member, and a pair of co-operating contact members each secured at one end to said support member, one of said contact members comprising a bimetallic strip having a base portion extending from said support member to a position adjacent said heater and an intermediate portion bent back toward said support in the form of a U partially surrounding said heater element and v a free end portion extending a substantial distance beyond said heater element, the other contact member also comprising a bimetallic strip extending from said support member in the same general direction as said first-mentioned bimetallic strip with its free end arranged to engage the free end of said first-mentioned bimetallic strip. i

9. A bimetallic switch comprising a support member, a heater element mounted on said support member, and a pair of co-operating bimetallic-contact members secured at their base ends to said support member and normally in engagement with each other at their unsupported .ends, one of said bimetallic members 'being so formed as to provide an intermediate contact members being flexed a slight amount from its normal position by the said one of said bimetallic members.

10. A bimetallic switch comprising a container, a heater element mounted within said container. and a pair of co-operating contact members mounted within said container, at least one of said contact members comprising a bimetallic strip having an intermediate portion disposed closely adjacent the said heater element. said container having a filling at a pressure oi approximately 4 mm. of a gas included within the group consisting of hydrogen and helium.

11. A bimetallic switch comprising a support member, a heater element mounted on said support member, and a pair of co-operating contact members mounted on said support, one of said contact members comprising a bimetallic strip having at least a portion thereof disposed closely adjacent said heater element, said contact members having interengageable contact points of a coin silver alloy, and a condenser of relatively low capacity connected across the said contact members and capable, when said contacts are separated and a potential is applied across the condenser, of inducing a spark of sufllcient intensity between the contacts to cause them to stick together.

LEO R. PETERS.

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