US2581212A - Electrically heated fabric - Google Patents

Description

1952 D. c. SPOQNER, JR., ET AL 2,581,212

ELECTRICALLY HEATED FABRIC Filed May 4, 1949 2 SHEETS-SHEET 1 75 90 m m A67 As: /50 A90 2/0 220' 2403172 TEMPERATURE "F m OHMS PER CM 3 I Inventors:

David C, Spoonerxdr Milton 5.6reenhalgh,

TEMPERATURE 0 Their Att e Patented Jan. 1, 1952 2,581,212 ELECTRICALLY HEATED FABRIC David C. Spooner, Jr., Westport, and Milton S.

Greenhalgh, Bridgeport, Conn., assignors to General Electric Company, a corporation of New York Application May 4, 1949, Serial No. 91,396

Claims. 1

This invention relates to electrically heated fabrics and the like, such as a bed cover, and it contemplates an improved flexible fabric of this character having a flexible control element therefor which combines into one flexible unit a heater wire for the fabric and also a temperature sensitive control device therefor.

It is an object of this invention to provide an improved electrically heated fabric having such a control element which has a flexibility comparable to that of the fabric, which is of relatively small diameter, which may be made of any desired length, and which will produce or contribute to a. significant, predictable control effect upon exposure to a predetermined change in temperature anywhere along its length; the temperature sensitive device, that is, the temperature responsive portion of the control element, is reversible, that is, self-restoring to original condition after exposure to the control level of temperature and subsequent return therefrom so as to exercise the control effect as many times as the thermosensitive device reaches the control temperature.

It is another object of our invention to provide such a heated fabric which lends itself to be manufactured by mass production methods at low cost.

The control portion of the control element of the present invention utilizes changes in electrical characteristics such as conduction, reactance, and impedance exhibited by a certain group of organic solids upon change in temperature thereof, to create a useful, consistent, control effect. A temperature sensitive device utilizing a flexible, temperature sensitive organic solid pursuant to the present invention is non-destructive and selfresetting within the temperature range of its application, and is commercially producable, at relatively low cost, as a flexible, wire-like structure having great length and a diameter of the order, for example, of 1 6 of an inch.

A combined heater wire and temperature sensitive control element embodying'our invention comprises a. pair of electrical conductors or electrodes, advantageously wire-like, separated by a continuous, integral film or layer of a flexible solid organic insulator which is physically stable throughout a desired temperature range and exhibits within said range predictable and repeatable changes in electrical characteristics such as resistance, impedance, and the like, of which advantage may be taken for conditioning a control circuit. We have found that cellulose esters, vinyl halide resins, and polyamides, for example,

have at room temperature a resistance value of a very high order, but at elevated temperatures will conduct suflicient current at conventional domestic potential to effect the operation of a work device such as a control relay, signal or the like. In a presently preferred andcommercially successful form of the invention, an inner conductor or electrode-which may be the heater wire of an electric blanket-is wound upon an insulating core as flexible as is a blanket and a thin layer of a polyamide resin having a flexibility comparable to that of the core is extruded or otherwise formed thereover. A control electrode is then wrapped tightly about the polyamide layer and the whole suitably encased in a similarly flexible insulation material. When installed in a suitable circuit, the current conduction throughout the polyamide layer at elevated temperature may be used to condition a control circuit to cause a relay or the like to open the load circuit.

The invention has proved of substantial benefit for overheat protection of electric bed blankets, in which a system of heater wires extends over a large portion of the blanket. It is possible to overheat an electric blanket by careless usage-for example, tucking a heated portion of the blanket beneath the mattress, folding the blanket on itself, and in other ways, whereby the normal heat dissipation is prevented. It has been conventional to equip blankets with a number of thermostats distributed throughout the heater system. This system has the disadvantage that due to the thermal insulation of the blanket material and resulting lag in heat transfer from an overheated zone to a thermostat, an impractically large number of thermostat units would be required even to approximate complete protection. Our thermosensitive device, however, may be intimately associated with the heater wire throughout the entire heated area, and will respond to an abnormal temperature rise anywhere within the blanket to open the blanket load circuit before damage can occur. As desired, the open circuit may require manual reclosing, or may automatically restore as the undesirable temperature condition is alleviated, according to the control system used. And as the thermosensitive element itself is self-restoring for future operations the blanket may again be safely used upon return to normal temperature. Also, the small diameter of our control element is in advantageous contrast to the relatively bulky thermostat cartridges previously used, and its high order of flexibility enables it permanently 3 to withstand the frequent flexures and folding incident to use in blankets, electrically heated clothing, and the like.

In the accompanying drawings, Fig. 1 is an enlarged elevation, partially in section, showin one form of temperature sensitive element embodying this invention; Fig. 2 is similar to Fig. 1, showing a second structural formthereof; Fig. 3 is a control circuit diagram associated with the construction of Fig. 1, said circuit utilizing the inner conductor of the thermosensitive element as the load-for examplathe heater conductor of an electric blanket; Fig. 4 is another control circuit, in which the load is independent of the temperature sensitive element; "Fig. 5 is a composite curve showing the D.-C. resistancetemperature characteristics, and the relationship of specific inductive capacitance (dielectric constant) to temperature, for five, polyarni'de resin materials which in accordance with this invention may be used as'the aforementioned 'organic layer of the temperaturesensitive element; Fig. 6 comprises a group of curves predicated upon a 200 foot length of thermosensitive device embodying the invention'and in which the organic material is apolyamide resin, and showing the direct current and alternating current resistance, capacitiveweactance, and impedance over a temperature-range representative of use in ablanket; Fig. '7 contains a D.-C. resistancetemperature curve -and dielectric constant curve for a vinyl halide resin and Fig. 8 comprises similar curves fora cellulose ester, both of which may be used as the flexible organic layer .ofthe temperature sensitive element; Fig. 9 is a representation of a typical electric blanket, showing the same folded; and Fig. 10 is a somewhat schematic view showing an area distribution of heater wires and therewith associated thermosensitive device, such as in an electric blanket or the like.

"Figs. 1 and 2 show typical constructions of combined flexible heater and vthermosensitive elements embodying our invention. In Fig. l, the structure H] includes a preferably ribbonlike bare conductor ll wound upon a flexible strand E2 of fiberglass, stranded cellulose acetate, or other suitable flexible insulation. Over the conductor in intimate contact therewith there is provided, as by extrusion, a layer orfilm E3 of a .flexible organic insulation material having the desired physical characteristics and temperature responsive electrical characteristics to produce a control effect as later described. Wound tightly on said layer i3 is a bare conductor I l, also advantageously ribbon-like. Aftersuitable drying steps to reduce the moisture content of the layer I3, we prefer to apply a water-inhibiting coating l5 of polyethylene, and then the outer insulation layer I511. The outer layer is selected from materials having good qualities of insulation, abrasion resistance and, for use in blankets and the like, ability to withstand laundering and dry cleaning. -We have found polyvinyl chlorides suitable. The combination of the ribbon-like conductor H and flexible core I2 is advantageous where the device is to be subject to frequent flexing, as in electric blankets or clothing. When used where there is no repeated flexing, a conventional solid or stranded conductor ma be used and the core I2 eliminated.

In Fig. 2, the combined heater and thermosensitive element Illa has bare wires l6, l'l, correspondingin function .to the ribbon-like.conductors i i and M of Fig. 1, and tightly wound in l-embddiment the active thermosensitive "materialof the layer l"3"is represented by its radial thickness, whereas in Fig. 2 the effective resistance material comprises an inner layer ex- "tendingibetween the adjacent turns of wires l5,

ii. We prefer to cover the paired wires It, i'!,

"with the "common "layer of organic material, prather thanitoihave one wire coated therewith and an adjacen'tkbare wire laid tightly thereagainst, because of the surer physical contact of each wirewithathe thermosensitive layer. In

Fig..2,.the wall thickness of layer is is not critical except insofar as it interposes thermal insulation between the atmosphere and the said inner thermoresponsive layer' between the spaced con ductorspand 'it'isentirely conceivable that the layer -19 may 'be made suitably thick to act as a :protective cover for the structure, in which event the "additional insulation layer 2t may "be dispensed-with. "The-activebodyof the thermosensitive material may conveniently be desighated the control layer.

The material'of the control layer must exhibitwith change in temperature, a substantial and predictable changein one or more electrical characteristics determining the electrical c0n 'duction ofthe' layer and being capable of" trans 'lation intoa useful control effect. These electricalcharacteristics, for example, include significant "change of direct *or alternatin current resistance, capacitive reactance, and impedance. Gne'or moreof-such'changes may beused in a control-circuit to achieve the desired objective. 'The change should occur sharply within the temperature rangefor which the'material is to function and be of a substantially. greater order of --magnitude'than changes resulting from atmosphericconditions, surface cleanliness and the J like. The extent of change should provide a substantial distinction between the total value of 'thechosenelectrical characteristic of the whole element at its normal temperature and the *total value of this characteristic when a relatively'smalllength of the element is subjected tdthecoritrbl-lev'e'lofitemperature. Further, the materials must be physically stable within the temperature-range to be undamaged thereby and to "resume "their f initial electrical characteristics upon return to" normal temperature levels.

.At 'least :three classes .of organic materials, namely,'cellul ose esters, vinyl halide resins, and polyamides, "Which remain solids to relatively high temperatures and are popularly considered insulators because of their great resistance at room temperature ranges, exhibit a sumciently abrupt change .ofone .or more .of the aforesaid electrical characteristics at temperatures below any.substantial .-softening of the material to be usefulas the control layerbetween the-electrodes of the thermosensitive device, and. canbeapplied as a .filma few mils .in thickness. One of the most satisfactory materials, according to our present knowledge, is the longchain synthetic polymeric amide which has recurring amide groups as. an integral part vof the main polymer chain; said material is known by the generic name nylon (1948 Modern Plastics Encyclop,edia,. p. 177.8) The ,nylons .have certain physical properties which are advantageous for use as the control layer. They remain solid to about 260 C., i. e., 500 F., and maintain their mechanical strength throughout a considerable tempera- -ture range, and may be extruded or applied by suitable means to produce a tough, flexible, integral film in tight contact with the inner electrode. Such hygroscopic qualities that they have may be overcome by the application of a moisture inhibiting coating as previously described.

The curves of Figs. 5, 6, '7, and 8 are plotted against logarithmic ordinates, and they have been prepared under our direction pursuant to the methods of test promulgated by the American Societ for Testing Materials and specifically ASTM designation 257-46 for insulation resistance and Dl50-47T for alternating current meas urement, the latter being at a frequency of. cycles per second.

The curves of Fig. 5 show the changes of D.-C. resistance (curves designated R) and specific inductive capacitance or dielectric constant (curves designated S. I. C.), with respect to temperature, of five nylon compositions which are designated by the manufacturer E. I. du Pont de Nemours and Company as JRN8942, JRNQMA/ JRN9443, FM3604, and FM 3003. Fig. '7 represents direct current resistance change (Rnc) and dielectric constant (S. I. C.) change of plasticized polyvinyl chloride, a vinyl halide resin, over a range of from 30 to 110 C.; and the curve of Fig. 8 represents a similar change of a cellulose ester, specifically cellulose acetate, from 30to approximately 110 C.

It is evident that all three types of control layer materials have sharp temperature resistance gradients, and adequate dielectric constanttemperature changes, of which advantage may be taken to condition a control circuit. It should be mentioned, however, that where the polyamides remain solid up to approximately 260 C., the polyvinyl chloride and cellulose acetate show some softening at about 90 C. The construction of Fig. 2 is perhaps better suited for the last two materials for the reason that the control wires are Wrapped tightly about the center strand and there is less tendency of one wire to move rela-- tive to the other than in the Fig. 1 embodiment, in which the outer electrode is wound under tension. Therefore, although the control layer may soften somewhat, the spaced relationship of the wires will not be materially affected.

We consider that a great advantage of our invention as compared with prior art devices in which the control factor comprises the resistance change with temperature of a signal wire, such as a metallic wire having a temperature-resistance coefiicient, is the sharp control effect which derives from heating even a portion of the thermosensitive device to the control temperature. This will be better understood when it is considered that heating any portion of our thermosensitive device above the temperature of the remainder reduces the resistance or impedance of the control layer to provide in effect many individual resistance or impedance elements in parallel across the outer and inner electrodes. Their effeet on the control circuit is in relation to the sum of the reciprocals of their individual resist ance or impedance values with respect to the reciprocal of the resistance or impedance value of the total length of the device at the initial temperature, whereas in prior art devices, such as referred to, heating a portion of the signal wire having a. known temperature-resistance coefficient merely establishes a new series-resistance relationship, with less overall effect.

So long as the operating characteristics of the control circuit are sumciently known to fix the chosen control factor of the control layer at which the control is to function, the length of our device necessary to be raised to the control temperature to change the control factor of the control circuit sufiiciently to cause a control effect is indicated by the curves of Fig. 6. These curves portray the changes with respect to temperature "of direct current resistance (RDc) 60 cycle alternating current resistance (RAG), capacitive reactance (X0) and impedance (Z) of a 200 foot sample of thermostatic device taken as a chance selection from regular production stock and constructed according to Fig. 1 with the exception that the outer insulation did not include the water inhibiting layer IS. The control layer was polyamide of the nylon FM3604 type, and the outer insulation consisted of polyvinyl chloride. Two hundred feet is the approximate maximum length used in a full-sized bed blanket. Assuming that the impedance of the control layer is chosen as the control factor, it is apparent from the temperature-impedance values of Fig. 6 that at relatively low control encountered, for example, in drying rooms or the like, a greater length of the device must reach the control temperature than where the control temperature is relatively high, since at the lower temperatures the change in impedance with temperature change is less than at the higher temperatures.

In other words, it will be seen from Fig. 6 that since the impedance falls with temperature rise the total impedance of the thermostatic device falls with increase in temperature above the nonmal of any portion of the length thereof, and therefore the length of the device that must be heated above normal to change the total impedance to the control point varies inversely with its temperature rise above normal. In a blanket, for example, having a thermostatic device two hundred feet long, the heater circuit should be open when the temperature of the control layer throughout its length is about F. Because of the impedance characteristic of the control layer, a shorter length of the thermostatic device heated to a higher temperature will produce the control effect promptly; for instance, if a length of ten feet of the two hundred foot thermostatic device is heated so that the control layer has a temperature of 215 F. (with the remainder of its length substantially at room temperature) the control effect will be produced. As a further example of response at even higher temperatures, a seven inch flatiron soleplate heated to about 350 F. will promptly produce the control effect when placed in close proximity to the thermosensitive device. In this case about two feet of the thermostatic device is subjected to the high temperature (the remainder being at room temperature), and the temperature of the two-foot length of the heater wire will not rise above about 260 F.

It will be apparent that structural dimensions of the several component parts of the heater and thermosensitive elements of Figs. 1 and 2 will be governed largely by usage factors incident to a particular end purpose. Used in electrically heated blankets, the core I2 may have a diameter of .020 inches; the inner conductor may be .008

.inches in width by .002 inches thickness, wound v35 turns per inch of core length; and the outer aconductormay be. .015 inches in width by .002

casein 1.12

inches inthickness, wound 351i turns per inch o'f length. The greater wi'dth of the external conductor -is advantageous :where both :conductors are woundin'the same direc'tion in; providing-(that it will always'be in overlapping. relation wlth;the inner conductor. Either conductor, :but I preferably the inner one, may be the1blanket heater wire. In the Fig. 2 embodiment wiresrlfis l'lmay be 36AWG,-and. core 18, 020 vinchizindiameter. In Fig. 1, the control layer. TQTmaY bBIESLthiIIflS 1 consistent with its: function 2asasa tenaciousgillexible, normally insulating film idGYOldiLOf; breaks or cavities. In the typical usagezexemplifiedzahove, in which line voltagesiwill be oftthe' order-.of .115 volts A.-C. and wherein oneof therpolyamides .of Fig. 5 1s 'used, .a "radial zwall thickness ;of.::about .006 to .01 inch f the "thermosensitiveinsulation provides adequate electrical :and structural lobar- 'acteristics, and quick temperature :response.

Serial No. 91,402, entitled TemperatureZResponsive Control Circuits, assigned to theassignee of this application and nowPatent No..2,565,478, granted August 28, 1951. The subjectrmatter of this application is also related .to 'co pending applications of J. W. McNairy,Serial:No...134,002, filed December 20, 1949, and-oi Holmes, Serial No. 208,965,.fi1edFebruarysl, 1951 both of :which applications are assigned totheiassigneeof this application.

1 In "Fig.3,theinner ;conductor ti ,ofithe thermosensitive element is usedas.the'blanket heater,

vandthe outer :conductor I i-as:thesignabwire.

The temperature of the blankethere shown is normally controlled in relation to roomtemperature by a cycling controldevice- 2 l'describedand claimed in the United States Patent to William K. Kear'sley, No.;2,l-95;958, April 2;:1-940. Itwill be understood that in awblanket the-element l0 preferably will be distributed in .a series'otaconvolutions over the blanket area to .be. heated, and may be runinv passagesprbv-ided fiorit as described in Patent No. 2,203,918 issued June .11,

1940, to I. O .Moberg. As shown -in=E$ig..-;3, .the electrical system within the blanket -22 zis-connectible by the multiterminal plug and:socket=2-3 to the controlsystem, which is housed withinza control box 2 (see also Fig. 9). Plug -P afiords connection to the power source, for example. the conventional 115 volt,=60 cycle, A.-C. domestic circuit. In Fig. 3, the load circuit includes .theconductor I! of the element ill arranged-inttwo sections, as shown, to be connected in parallel .to the power source; the resistance-of heater! I will be of the order of 65 ohms. The outer conductor H! is theoverheat protective signal -wire,-ancl the control layer'i3 maybe a .006 inchfilm of one of the organic materials previously noted, nylon being preferable. overheat :protection -,is affordedby utilizing the change in impedance of the organic material astheelectrical-responsive characteristicat thecontrol temperature level, :and is provided throughthevmedium of a'work device suchas a'lockout-relay ;;andthecycling :.control, responsivexto :room temperature; .oiithe 'nal adjustmentknob 21. the relay 25 is connected 'across en-impedance aforementioned :.Kears1ey. :control..is 4 provided :by

the rbimetallic switch 226 having a suitableexter- The operating coihof bridge consisting of 0.1 .mfd. capacitor "zilxina resonance circuit :with .a 7 5 henry chokei 29, :the

sother: two legs of the bridge being'theresistorstil, .3l of about 6800 ohms 'each. The 1 coil "impedanceof 'relay25 should be :of theorder of. 90,001)

:ohms. .The induced voltage of thetresonant'cir- :cuit, available1at:relay.25, is approximately 13.0

volts; the; relay will pull in: somewhat below that level, butwill drop out at vaboutf'i5 volts. eAssuming that the-cycling control 2| isfcalling or heat and its-contacts are closed,:power canjbe applied to the blanket heater wire I l'by momentarily closing the normally open. switch 32 which will complete a circuit through'power conductor .33,:bothbranches of heater wire I I, conductor34,

12,000 ohm resistor 35, switch 32' and power conductor 36. The signal wire M, which in this instance preferably has a resistance of about-400 ohms, is in series with the center of the resonance circuit of choke 29 and capacitor 28, being vconnected thereto by the conductors 3?, 38, which terminate suitably atplug'23. Therelay25twill pull in to bridge its contacts 3% and complete the operating circuit for the heater wire. The -resistors 3!), 3: provide a voltage divider circuit such that if a dead short 'occurred'across'the extremities of conductors H, M while relay 12 5 was in, the voltage in the relay coilwould drop to about one half line voltage-whereupon therelay would drop out and open the load circuit. In

normal operation, both the resonant .circuit. and the power circuit are completed through -conductor M), relay contacts 39, andzcond-uctors ill,

36. Voltage limiting resistor 35 :is-now in the resonance circuit, to' establish the relay holding voltage. Neon lamp 4-2 .is-energized through a circuit including a 200,000 ohm resistor-M conductors A4, 40, relay contacts 39, and. conductors -41, 36,.and indicates that the blanket is in operation. IIheblanket will .nowremainin operation,

subject .only to the periodic cycling oiv bimetallic element 26 so long as the temperatureof the controllayer l3 remains belowthe cut-o'fi'temperature. The resonance .of the control circuit .is...not..afiected.by the-operation of the cycling .theswitch 32, providing, howeventhat the temperature of the thermostatic device'has ,fallen sufiiciently below the control level.

To purposely open the load circuit-there is provided a normally open switch l5, shunted-across the relay 25. When switch'lifi is closed-'the'rela-y coil is deenergized, and relay contacts 39 "will open.

It will be noted that failure ofany'of'the com ponent parts of the control'circuit will de-energize the relay coil '25 by destroying the circuit resonance. The resistor 35 whichisaplaced in series with the load wire :I I xwhen5sr'vitchrf32zis manually closed. is reflective. to reduceuthe; current flow 1 through the. loadconductor 'ftO :suchwant-extent that even if the: switch .32 werezheld iclosed control circuit, there. would be no 9i continuously in'an attempt to circumvent the appreciable heating of the blanket. 1

The circuit of Fig. 4 operates to shunt-out the control relay by conduction between the respective electrodes of the thermosensitive device when the control layer reaches the control temperature. The aforementioned Kearsley control may, of course, be added for normal blanket temperature control. In contradistinction to the circuit of Fig. 3, the Fig. 4 circuit provides a cycling control in that return from the elevated temperature level to the normal operating level will re-energize the relay and reinstate the load. 'I'hethermosensitive device is independent of the load and although its use in a blanket 22a (Figs. 4 and 10) has again been selected for purpose of description it is obvious that said circuit is applicable to many othertypes of installation in which it is desired to distribute a thermoresponsive device over a large area or throughout a coal pile or grain bin. The blanket Me, which may have the construction described in the aforementioned patent to I. O. Moberg, is provided-with a heater wire 50. The thermosensitive element In of Fig. 1, or Ilia of Fig. 2, is arranged in the heater wire pockets so as to be thermally responsive over the entire heated area. The load 50 is connected to the power source when the energized relay 5! bridges its contacts 52, as will be obvious. The respective conductors I l and I4 ofthe thermosensitive device I (or the conductors I6, I! of the device 10a.) are in series with each other and with the coil 53 of the relay. A limiting resistor 54 is included in said series circuit. The system may be placed in operation by momentarily closing the switch 55 and may be taken out of opera tion by closing the switch 56 which shunts out the relay coil 53.

Under usual temperature conditions experienced in normal blanket operation, that is temperatures of the order of 105 F., which corresponds to a medium position setting of the control knob 21 in Fig. 3, the resistance value of the control layer or film is extremely high and the circuit path is through the relay coil 53 in series with the respective conductors of the thermosensitive device. Upon increase of a portion of the thermosensitive device to the cut-off temperature-for example, a portion in a fold of the blanket of Fig. 9, assuming the same to have been left connected to power when in the folded condition-the drop in resistance or impedance of the control layer along a suitable length will create a shunt path between the control conductors and deenergize the relay The increase in temperature may be gradualbut the load circuit will be cut out before the blanket can scorch.

In the Fig. 4 circuit the following values are typical: load 50 from 65 to 70 ohms; thermosensitive conductors less than 500 ohms each; limiting resistor 54, 12.0000 ohms; and relay coil 53, 90,000 ohms impedance.

It will be appreciated that pursuant to the present invention we have provided means whereby a thermosensitive device may be disposed co-extensive with a load wire or distributed over a large area to respond to any abnormal temperture increase therein, said device being capable of repeated flexures without failure and of inclusion in circuits in which a load conductor of very great length is wound in tightly packed coils. In particular, We consider that we have provided a new and useful electrically heated .iii)

body covering such as a bed blanket or the like, in which overheat protection throughout the heated area is provided by means intimately associated with the heater wire, said means having a flexibility substantially equal to that of the blanket and, unlike conventional thermostat cartridges previously used, causing no enlarge" ments or protuberances within the blanket.

While we have shown particular embodiments of our invention, it will be understood, of course, that we do not wish to be limited thereto since many modifications may be made; and we therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of our invention.

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

1. As a new article of manufacture, an e1ectrically heated fabric normally subjected in use to freguent flexure and folding, and a combined heating means and thermosensitive device distributed over a predetermined area of said fabric having an order of flexibility comparable to that of said fabric and comprising, a flexible heater conductor, a substantially uniform continuous, tenacious control layer of flexible solid organic material on said conductor in intimate surface contact therewith, a flexible signal conductor coextensive with said heater conductor fixedly spaced therefrom by saidlayer of organic material and in intimate surface contact with said layer, said material having a temperature impedance coefficient effective to insulate electrically said heater and signal conductors one from the other at the normal operating temperature of the fabric of around F. and to provide a conductive path therebetween at predetermined elevated temperatures of the fabric to produce a control effect characterized by current flow between said conductors, which elevated temperatures are materially in excess of said normal operating temperature and are inversely proportional to the portion of the total length of the said combined device which is heated above said operating temperature but materially below a temperature of around 500 F., terminals for connecting said heating conductor across apower source, and a work circuit connected to said signal conductor and said power source includin work device means responsive to said current flow to disconnect said heater terminals from said power source at said elevated temperatures.

2. As a new article of manufacture, an electrically heated bed cover and the like, and a combined heating means and thermosensitive device distributed over a perdetermined area of said coverhaving an order of flexibility comparable to that of said cover and comprising, a pair of helically wound heating and signal conductors disposed in spaced relationship by a substantially uniform body of solid flexible organic material in continuous electrical contact therewith, said material having a temperature impedance coefficient to insulate electrically said heater and signal conductors one from the other at normal operating temperature of the bed cover of around 105 F. and to provide a conductive path there between at predetermined elevated temperatures of the bed cover to produce a control effect characterized by current flow between said conductors, which elevated temperatures are materially in excess of said normal operating temperature and are inversely proportional to the portion of the total length of said combined device which is heated above said operating temperature but materially below a temperature of 500 F., externally accessible terminals for connecting said heater conductor across a power source, and a work circuit connected to said signal conductor and power source including work device'means responsive tosaid current flow to disconnect said heater conductor terminals from said ower source.

3. As a new article of manufacture, an electric bed cover and the like, and a combined heating means and thermosensitive device distributed over a predetermined area of said cover having an order of flexibility comparable to that of the bed cover and comprising, a flexible heating conductor, a flexible signal conductor, a substantial? 1y uniform, continuous solid layer of a flexible synthetic polyamide resin disposed between said heating and signal conductors in intimate surface contact therewith to hold them in spaced relation, said resin having a temperature impedance coefficient to insulate electrically said conductors one from the other at the normal operating temperature of the bed cover of around 105 F. and to provide a conductive path therebetween at predetermined elevated temperatures in the bed cover to produce a control effect characterized by current flow between said conductors, which elevated temperatures are materially in excess of said normal operating temperature but materially below a temperature of around 500 F., terminals for connecting said heater conductor across a power source, and a work circuit connected to said signal conductor and said power source including work device means responsive to said current flow to disconnect said heater terminal from said power source.

4. As a new article of manufacture, the combination with a flexible fabric of a, heater wire and control element therefor, said heater wire and control element comprising a flexible electrically insulating core, an electric heat generating conductor helically wound thereon, a substantially uniform, relatively thin layer of a flexible synthetic polyarnide resin disposed about said conductor in contact therewith throughout its length, a signal electric conductor helically wound'thro'ughout its length upon said layer in surface contact therewith, said heat generating and signal conductors and layer being substantially coextensive, and said resin having a temperature impedance coeflicient to insulate electrically said heat and signal conductors one from the other at a normal temperature of around 105 F., and to provide a conductive path between said conductors to pass electrical current of controlling magnitude at elevated control temperatures materially in excess of 105 F. but materially be- 12 low a temperature ofaround 500 F. and which temperatures vary inversely: with the lengthy of said resin which is heated to said elevated temperatures.

5. As a new article of manufacture, the combination with a flexible fabric of a temperature sensitive heater wire and control element, said heater wire and control element comprising a flexible strand of insulation, a pair of closely spaced bare conductors spirally wound thereon, and a layer of synthetic polyamide resin at least partially enveloping said conductors in intimate surface contact to maintain the said fixed spaced relationship, said resin having a temperature impedance coefficient to insulate electrically said bare conductors one from the other at a tempera ture of around F., and to provide a conductive path between said bare conductors to pass an electrical current of controlling magnitude at elevated control temperatures materially in excess of about 105 F. but materially less than 500 F. and which temperatures vary inversely with the length of said resin which is heated to said elevated temperatures.

DAVID C. SPOONER, JR. MILTON S. GREENI-IALGI-I.

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

UNITED STATES PATENTS Number Name Date 2,313,234 Gavitt Mar. 9, 1943 2,316,872 Kernen- Apr. 20, 1943 2,413,125 Walbridge Dec. 24, 1946 2,477,348 Postal July 26, 1949 FOREIGN PATENTS Number Country Date 338,880 Great Britain Nov. 18, 1930 453,929 Great Britain (Not accepted but opened to public inspection on June 24, 1935.)

OTHER REFERENCES Hartshorn et al.: Journal of the Institution of Electrical Engineers; vol. 83; pages 315-332.

Baker and Yager: The Relation of Dielectric Properties to Structure of Crystalline-Polymers II Linear Polyamides, Jour. Am. Chem. 800., September 1942, page 2175.

Fowles: Geon in the Wire and Cable Industry, Rubber Age, March 1946, pages 706, 707.

Polyethylene Tubing in the Greenhouse, Modern Plastics,-August 1946, pages 108-109.

Cook: Electrical Engineering, July 1949, pages 623-627.

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