US2052533A - Sesistance unit - Google Patents

Description

Aug. 25, 1936. l Hl FENDER v 2,052,533

RESISTANCE UNIT Filed Dec; 8, 1933 4 Sheets-Sheet 1 L i 3 INVENTGR.

A TTORNEY.

RESISTANCE UNIT Aug. 25? i936' 4 sheets-Sheffc 2 Filed De0- 8 1933 INVENTOR' fm @M` BY 'MXM m.\ mL.

ATTORNEY' Aug. 25, 1936. H. PE'NDER RESISTANCE UNIT 4 Sheets-Sheet 3 INVENTOR.

Filed Dec. 8, 1953 Aug. 25, 1936. H. FENDER RESISTANCE UNIT 4 Sheets-Sheet 4 Filed Dec. 8, 1933 INVENTOR.

#ma @Wm A TTORNEY.

Patented Aug. "25, 1936.

UNITED sucresl PATENT i OFFICE 2,052,533 I RESISTANCE UNIT Harold Pender, Mei-ion Township, Montgomery County, Pa.

Application December 8, 1933, Serial No. 701,471

' 9 claims. (ci. aol-1s) My invention relates to resistance units, of general application, and more particularly to those finding their applicationin radio receiving Viilamentary or rod-like element of insulating' material suchas glass, porcelain or other vitreous product coated Iwith a resistance material, the

specific resistance of which may vary within wide limits in accordance with the uses to which the unit is to be adapted, or the resistance element may comprise a molded body of any oi several well-known mixtures of conductive and insulating materials. Such resistance elements in genl eral have a 'relatively high speciilc resistance which gives rise alsoto a relatively high contact resistance between the element and its associated terminal structures. This contact resistance may be not only relatively high, but may tend to fluctuate substantially and rapidly, particularly when the resistance elem'ent has a hard and rough surface, giving rise to noises or disturbances'in the apparatus of which the resistance unit forms a part. My invention is directed primarily to resistance units of the above type in which provision is made for reducing to a minimum and maintaining substantially constant the contact resistance 'between the. terminal structure of the unit and the resistance element.

My invention resides also in new and improved methods of forming resistance .unitscf the type described above and to new and'iin- 40 proved methods forensuring axsatisfactory contact between a resistance element and its termnal structure and to new and improved methods for pressure molding an insulating compound@ about a frangible resistance element.

My invention resides also in new and improved methods of forming resistanceunits of the type described above and to new and improved methods for ensuring a satisfactory contact between a resistance element and its terminal structure 50 and to n'ew`andimprpved methods for pressure molding-an insulating compound about a fran' gible resistance element.

It is an object of my invention, therefore', to l provide ra new and improved resistance unit which 55 is simple, missed and economical to manufacture.

It is a still further object of my It is another object of my invention to provide a new and improved resistance unit in which a frangible resistance element is completely molded in and supported by a mass of insulating material. l

It is another object of my invention to provide anew and improved resistance unit comprising a resistance element molded in a mass of insulating material in-which the parts projecting from the. molded insulation are reduced to a minimum.

Itis a further object of my invention to provide a new and improved resistance unit which will maintain the value of itsresistance substantially constant over a long period of use.

It is a further object of my invention to provide a new and improved resistance unit in which the contact resistance between the resistance element and its terminal structure is maintained at a constant and xed minimum.

It is a further object of my invention to provide a new and improved resistance unit including a relatively high resistance frangible resistance element in which the contact resistance between the frangible resistance element and its terminal structure is maintained at a constant and fixed f minimum.

It is a still further object of my invention to' provide anew and improved resistance unit which may be applied in communication or ksignalling circuits .without the production of noises or other electrical or audio disturbances.

invention to provide a new and improved resistance element suitable for use in my improved resistance units which may be readily manufactured in large quantities with a small and uniform tolerance as to resistance value.

It is` another object of my invention to provide a new and improved method of forming a resistance unit including a frangible resistance ele.- ment. l

`It is another object ofmy invention to provide a new and improved method of forming a resistance unit by means ci which, the .contactresi'stancev between the resistance element and its terminal structure is maintained at a constant and iixed minimum. -It is a still further object oi my invention to provide a new and improved method vof forming a resistance unit including a frangible resistance element which will minimize the breakage of the vide a new and improved method of forming resistance elements suitable for use in my improved resistance unit. v

This application is in part a-` continuation of my application Serial No. 655,890, filed February 9, 1933.

For a better understanding Vof my invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings and its .scope will be pointed out in the appended claims. y

Figs. 1, 1A and 1B are. respectively, a longitudinal section, a cross section and top plan view yof ya resistance unit embodying my invention;

Figs. 1C and 1D areyrespectively, side and end 'elevational views of a resistor or resistance' elei ment with a lead applied thereto;

Figs. 2 and 2B are, respectively, a longitudinal` 20 section anda cross-section of a. modication in which the conducting leads are in the form of metal lugs; E

Figs. 3, 3A andB are, respectively, a longitudinalsection of a unit of modied structure,`

a plan View when completed, and a cross-sectional View on the line B -B; Fig. 3, on enlarged scale,

' of a modification in which anv external helical connecting lead is .temporarily held in the molded mass and thereafter more or less completely uncoiled;

Fig. a is a top plan view of the lower member of a mold by means of which my improved method of forming resistance units may be practiced, while Fig. 5 is a longitudinal cross-sectional view along the lines D-'D of Fig. 4. Fig. 4, on the other hand, is a view along the lines E-E of -Fig. 5. Fig. 6 is a cross-sectional view along the line C-C of Fig. 5;

Figs. 7 and 8, are, respectively, a cross-sectional o view and a longitudinal sectional view of a modi:

ed form of a mold particularly suitable for forming resistance units including frangible resistance elements;

Figs. 9, 10, 11, 11a, 12 and 13 illustrate the 45 several steps in the development of a complete f resistance unit as produced in the mold of Figs.

7 and 8;

Figs. 14, 14a and 14h are, respectively, a topv plan View, an elevation and a cross-sectional view 50 along the lines G-G of Fig. 14 of a jig particularly suitable for forming resistance elements for use in my improved resistance unit, while Fig. 15 shows a detail of an assembly of. my improved resistance elements.

55 Referring to Figs. 1, 1A and 1B, the resistor ruggedness, fully mechanically to protect the contained resistance element and associated parts.

The insulating material constituting the casing 70 or envelope B may be anything suitable for the purposes in view. There may be employedl moldable condensation products, such asvphenol-` formaldehyde resins, common among which -is bakelite; phenol-furfural resinoids; or -plastic 75 compounds of rubber or equivalent, all moldable.

undejxi pressureat elevated temperatures. Or, for

cold molding, such material as asphaltic base compounds or equivalents may be utilized.

f The resistor R of the severalA figures of the draw-1 ings may comprise a core of insulating material 5 of cross-section of any suitable shape, commonly circular, of any suitable diameter and material. It may be in the form of a illament, rod or tube, coated, either exteriorly or, when in the form of a tube, interiorly with/any suitable conductive material, including nely divided metal, carbon and the like, to constitute conductively coated iilament or core of glass, ceramic and the like. The resistance element R may, for example, be of the character described in my prior Letters Patent No.

1,832,419, dated November 17, 1931. When the element R comprises a tube interiorly conductively coated, the terminals t will be of a form to enter the end of the tube and contact with its interior conductive coating; or the element R. may comprise a molded body of any material having suitable resistance characteristics.

The resistance elements of the types mentioned above may vary in resistance from values as low as a few ohms to as high as hundreds of meghorns.

As stated above, the contact resistance of such a resistance unit is ailected bythe specific resistance of the resistance element. Furthermore,

this contact resistance is subject to substantial and. troublesome variations. my invention, this diiculty is overcome by dis- -posing a plastic coating of c'onductive or rela-ll tively low resistance material upon the portions of the resistance element to which the terminal structures are attached. This coating is illustrated in detail in Figs. 1, 1A, 1C and 1D of the drawings in which the coating material'is indi cated by the reference d. The coatings d. formed on the resistance R maybe of any suitable mate:

rial having a relatively Alow resistance such as a 40 composition comprising `a nely divided metal or other conductor and a suitable plastic binder, although I prefer to use as this coating material a composition of varnish or similar binder and graphitic or vamorphous carbon. The preferred 4,5

coating material has a specic resistance` of the order of 0.03 ohms per centimeter cube. The coatings d may be applied in any suitable way, as by spraying, dipping,l painting, etc., although I prefer to spray the coating on the resistance'ele- 50 ment R., as described more fully' hereinafter.

The leads L .are providedwith aterminal portion t which may be, for example, of iiexible or readily malleable metal Wire or ribbon. For example, the terminal portion may be formed by attening one end of the lead'L and applying it under suitable pressure, by the ngers or atool, into at least sumciently rm engagement with `the end of element R to hold it in relative position suitable. for placing in'` a mold such as hereinafter 6o referred to. The pressure applied in molding the insulating material upon the element R exerts,

upon each terminal t, pressure which is eiectiveto compress the terminal portion t about the coated section of the resistor R so that the coating material d iills the interstices of the resistance element R and the terminal portion t, establishing `a good electrical contact between these two ele-4 ments. This pressure is more or less completely maintained upon the solidication'of the molding 70 material, thereby permanently establishing arrV electrical contact-between the resistance element and its terminal structure having a constant and xed minimum value. The pressure of the molding operation is also eiective to bringl the mold- 7.5

In accordance with 30 ing material into direct and intimate contact with all portions of the resistance unit establishing a rugged, protective and supporting envelope. It is preferable that the molding insulating material B should be of such material asA will wet during the molding operation, the leads or conductor elements L thereby effecting a close moisture-proof connection or attachment consisting oi or comparable with a hermetic seal.

Whilethe resistance element R has been described as comprising an insulating filament or rod covered with a 'conductive coating, this element may, if desired, be additionally coated with y a suitable film-like insulating coating oi any conventional type.

In Figs. 2, 2A and 2B the structure is generically the same as hereinbei'ore described. differing only as to the modification and the structure of the leads. In this instance the external leads L2, of any suitable conducting material, such, for example, as the leads L of Fig. 1, have any suitable terminalstructure t2 embracing and contacting with the resistor R. The leads L2 are in the form of lugs or more or less rigid plates projecting beyond the exterior of the envelope B, and having any'suitable structure such as end hooks h, h by which they may be hooked on to or otherwise attached to other conductors in the form of posts, wires or the like.

In Figs. 3, 3A and 3B, the terminals t3, contacting with the resistor R, are in the form oi a conductor or wire wrappedvaround or coiled upon the ends of the element R in electrical contact therewith. The wire may extend beyond the exterior of the'block B proper in the form of a straight lead. r,'as illustrated, the wire continues to the exterior of the block B properfin the form of a coil or helix c, or otherwise substantially compactly arranged, and molded in a temporary lug a integral with the envelope B proper. 'I'he material of the lug a illls the interior of the coil or compactly arranged conductor c, and preferably extends between neighboring turns or convolutions thereof, and in some lnstances, as indicated in Figs. 3 and 3B, may extend somewhat outwardly beyond the conductor c. After molding the structure into the form shown in Fig. 3, the material of the-lug a' being of relatively small dimensions and more or less readily frangible, is broken oi! from the conductor c and envelope B, whereupon the wire c may then be drawn out substantially axially yielding a more or less straight or undulating ilnal lead Ll, Fig. 3A.

My invention also comprises improved methods of forming resistance units of the type described above. In the conventional methods of molding inserts, suchas the resistance elements R in an insulating medium, the insert to be molded is rigidly mounted in a chamber or cavity ofthe mold, together with a quantity of the molding compound. These elements are then subjected to heat and pressure. In the molding process there are ordinarily developed substantial unbalanced pressures due to the fact that the change in volume oi the molding material is ordinarily oi the order o! 3 or 4 to l, and due to the impossibility of exactly distributing the mass ot molding materlal uniformly with respect to the insert. Also, the heating' is progressive Irom the parts of the molding material directly in contact with the hot mold to' those portionsl in intimate contact withv the insert, causing wide variations in the viscosity oi the molding material. These unbalanced pressures tend to cause a breakage. of the inserts which may become a serious problem particularly in case the inserts are of a irangible nature, as in the preferred form of my resistance element. This tendency to breakageis further aggravated by the fact that the molding pressure is generally 'applied at right angles to the length 'of the" insert. If the ratio oi' length to diameter of the insert is great. as is often the case, particularly with the type of resistance element described above, a relatively small unbalanced pressure is sufllcient to cause' a breakage oi the insert. As an example, my invention has been utilized to mold a resistance element comprising a glass nbre-0.04 `inches in diameter and.2 inches long.

My present invention embraces an improved method o1' molding resistance units which overcome the above-described diillculties and makes the molding operation of very irangible elements a simple, economical and practical procedure.

In Figs. 4, 5 and 6 is illustrated one of various mold structures by means of which my improved method of forming resistance units may be practiced. In the lower mold member i is formed a pocket or cavity 2 of the length and breadth oi! the unit to be formed, and of a depth which is a fraction, practically one-half, of the depth of the unit to be molded. Extending from one side of the cavity 2 are channels or grooves 3 to receive the terminals or leads L of the resistance element. Adjacent the comers of the member i are upstanding posts or pins 4, held by screws,

rods or equivalent velements 5; the middle mold element 8 is provided with holes, receiving the posts 4, and whose inner wall forms part of the end and side wall structure registering with the like walls of the cavity 2. The plunger or upper mold member l, also provided with holes to receive posts 4, has a projecting lower portion entering into the opening into the middle member 6,- and whose bottom face I forms the top wall of the upper halt of the mold cavity, lforming with the cavity 2in the member i a mold oi' the form and dimensions of the unit.

With the mold members 6 and l removed, a resistor R, having its leads Lattached thereto by terminal elements t which engage the -resistor R about the coatings d, isI placed in the position indicated, with the leadsL in the channels or grooves 3. Due to the thinness of the terminal portions t of the leads L, they are ilexible or easily bendable, so that the resistor R is accordingly flexibly held in place', within or adjacent the top surface of the bakelite or other molding powder in .the cavity 2. In certain instances, it may be desirable also to support the resistance element R upon an auxiliary strip or block 9 of molding material whichhas b een previously molded under heat and pressure. 'I'his block 9 is preferably provided with slots or perforations ill which are effective to key it to the main mass oi molding material and also to give it a certain may be used to support the resistanoe'element the dimensions or volume of the finished casing B. The pressure developed in the molding operation'causes the ilexible terminals t of the resistor R. to firmly grip or engage the resistor. about the coated portions compressing the plastic coating d into the interstices 'of the resistance element and the terminals forming a good electrical contact of a substantially ixed resistance. As the molding material B sets, upon treatment with heat and pressure, this relatively low resistance contact ls maintained at a substantially fixed value.

The resistance elements illustrated as subject to the molding operation of Figs. 4, 5 and 6 are similar to those illustrated in Figs. 1 and 2 of the drawings. As stated above. the thinness of the terminal portions t of the leads L gives the resistance element R a certain flexibility of support which permits the element a limited freedom of motion so that it may adjust its position in response to unbalanced pressures developed in the molding operation, as described above, thus reducing breakage to a negligible factor. In Fig. 3, described above, is illustrated'another terminal structure for resistance elements effective to give the resistance element a similar flexible support for the molding operation.

In. certain cases where the resistance element to be molded is particularly frangible or where the nature of thel molding operation demands it. an auxiliary perforated block 9, as described,

intermediate the terminal supports, as illustrated l in detail in Fig. 5'of the drawings. Thisauxiliary supporting block is preferably of the same material as the main mass of molding material,

thus forming a substantiallyintegral envelope of ther reduced by forcing 'the molding Vmaterial axially into the mold cavities containing resistance elements. The moldi apparatus of Figs. f v

ng -sents the deformed 'turn which ac tsvas a stop against the end of the resistor R.A`

7 and 8 is somewhat similar to that of Figs. 4, 5 and 6 and comprises a lower mold member la having a plurality of mold cavities 2a,l an intermediate mold section la provided with mold cavities 2b registering with and complementary to the cavities 2a. The mold is also provided with an upper mold element or plunger la having a pressure surface la. -for forcing the molding material B into the mold cavities. In this arrangement, however, the molding material B is not placed in the mold cavities with the resistance elements to be molded. but the intermediate and communicating therewith at one end of the.

mold through ports i2. Inv these gures there is illustrated the molding of resistance elements R provided with exible terminal structures LI' coaxial with the resistance elements and extending throughv grooves 3a in the end of the mold element ia. In the operation of this molding apparatus-when heat and pressure are applied to ,the upper mold element or plunger la, the moldstreams through the ports I2 into the mold cavities 2a-,2b in a direction parallel to the length of the frangible resistance element R. The direction of the iiow of the molding material is as indicated in the small arrows in Figs. 7 and 8. 5

The molding material may be introduced into each of the mold cavities through a. separate port, or two or more ports may supply the molding compound in parallel, as shown in Fig. '1 of the drawings. In this method of molding, the unbalanced pressures transverse tothe length of the frangible resistance elements `are substantially eliminated.

InFigs. 9 and l0 are illustrated the developmentof the preferred form of the resistance element for embodiment in my improved resistance unit. In Fig. 9 is shown the resistance element R with the coatings d disposed upon its end portion. As stated above, these coatings arepreferably of a mixture of varnish or other plastic binder and iinely divided carbon. After the coating, the elements are preferably treated at elevated temperatures for a period to partially harden the coating material. However, this heat treatment is stopped at such a point that it is still quite soft compared to ordinaryl metallic conductors. The leads L4 are then `formed with terminal portions t4 comprising substantially helical coils, and these coils are threaded on the relatively soft coatings of the element R. The 3o helical coil cuts its own thread-on the coated end making an intimate contact of very low rev sistance, irrespective of the resistance of the element itself. This contact resistance is still further reduced in the molding operation which is effective to still further embed the, turns of the helical coil into the coating material, and this low resistance contact is permanently maintained by the setting of the molding insulating material.

In the preferred form of the terminals t4, the turns .immediately adjacent the leads L4 are deformedso that the helical coils begin at approximately the axis ofthe resistor element and the leads L4 are coaxial with the resistor element.' These deformed turns act as automatic stops lto limit the travel of the terminals-t4 as they are. threaded on the resistor element R. This is ilr lustrated more clearly in Figs. 11 and 11a in which the portion e of the terminal structure t4 repre- In the forming of resistance units of the type covered'by my invention, the value of the resistance of the unit is dependent upon the distance between the terminal structures, and 'it is important that this distance should not be varied by variations'encountered in the assembly procedure. This problem is also solved by the improved resistor element of Figs. 9 and 10, provided only that the terminal coatings are accurately spaced on the resistor element. As shown in more detail in Fig. 12, if the terminals t4, due to inaccuracies of manufacture or assembly, extend beyond the coated portions of the resistor element, the projecting portions f of these terminals are separated from the element R by the thickness of the terminal coatings d so that the effective length of the resistor element is maintainedat the value g, as indicated in Fig. l2. In Fig. 13 -0 is shown, in longitudinal section, the completed resistance unit embodying the preferred form of my invention. It will be noted that this unit embodies the auxiliary perforated supporting block 9 described above.

there is shown in Figs. 14, 14a and 14b a simple apparatus by means of which this coating may be applied with great accuracy and facility. This apparatus or jig comprises the complementary .supporting members or racks I3 and Il maintained in alignment by means of pins or bolts I 5.

IThe supporting members I3 and I4 are formed regularly and accurately spaced openings I6, the distance between the openings corresponding to the desired length of the resistor elements and the width of the openings being approximately twice the width of a terminal. One of the snpporting elements, for example, the member I 4, is provided with a plurality of longitudinal slots I'I of the proper dimensions to snugly hold a length of resistance element. One such element R is shown as placed in one of the slots of the apparatus. By filling the several slots Il of this apparatus with resistance elements, the coatings d may be applied by means of a spray gun with great rapidity and ease. If desired, there may be provided severing apparatus .comprising a plurality of cutters, one of which is designated by the reference numeral I8 of Fig. l5. Such an apparatus is effective to accurately sever the several resistance elements at a single operation either by cutting, nicking or chipping, according to the physical properties of the particular --resistance element which is b eing utilized.

What I claim is:

1. In a resistor construction, in combination,

an elongatedresistance element, electrical conductive coatings on theV end portions of said element immediately adjacent the ends thereof, said,

coatings being concentric with said element and I in intimate contact with said element, thereby providing an excellent electrical connection between said element and said coatings, said coatings being of much greater electrical conductivity than said element, wire terminals having portions encircling said coatings and proportioned to fit iirmly against said coatings when pressure is exerted thereagainst, and a mass of solid electrical Y insulating material completely encasing said element and the encircling portions of said terminals and exerting pressure thereagainst so that said terminals are held firmly against said coatings to provide an excellent electrical connection between said terminals and said coatings.

2. In a resistor construction, in combination, an elongated resistance element, electrical conductive coatings on the end portions of said element immediately adjacent the ends thereof, said coatings being concentric with said element and in intimate contact with said element, thereby to provide an excellentV electrical connection between said element and said coatings, said coatings having much greater electrical conductivity than said element, wire terminals having helical shaped portions encircling said coatings andproportioned to fit firmly thereagainst when pressure is exerted thereon, said terminals having sub- "stantially straight portions extending away from said encircling portions, and a solid mass of elecltrical insulating material completely encasing ductive' coatings on the end portions of said element immediately adjacent the ends thereof, said coatings being concentric vwith said element and in intimate contact with said element, thereby to provide an excellent electrical connection between said 'element and said coatings, wire terminals including portions encircling said coatings and proportioned to iit firmly thereagainst, portions of each of said terminals extending inward` ly to engage the ends of said element to act as stops, and a solid mass of electrical insulating material completely encasing said element and the encircling portions of said terminals to exert a pressure on said terminals and hold them in firm engagement with said coatings.

4. In a resistor construction, in combination, an elongated resistance element, electrical conductive coatings superimposed on the end portions of said element immediately adjacent the ends thereof, said coatings beingconcentric with said element and in intimate contact with said element, thereby to provide an excellent electrical connection between said -element and said coatings, said coatings having much greater conductivity than said element, wire terminals comprising helical -shaped portions and substantially straight portions, the helical shaped portions encircling said coatings, said terminals being deformed at the junctions of said straight portions and said helical portions to fomstops' in engagement with the ends of said element, and a mass of electrical insulating material completely eircasing said element and the helical portions of said terminals to maintain said terminals in iirm engagement with said coatings thereby providing anv excellent electrical connection between said terminals and said coatings.

5. In a resistor construction, in combination, an elongated resistance element having an irregular surface electrical conductive coatings concentrically superimposed on the end portions of said element and immediately adjacent the ends thereof, said coatings comprising a binder filling the irregularities in the surface of said element and having particles of conducting material suspended therein thereby t-o provide a connection between said coatings and said element having a constantv resistance value, wire terminals encircling said coatings and proportioned to fit firmly against said coatings, and a solid mass of electrical insulating material completely encasing said element and the encircling portionsof said terminals to hold said terminals permanently and iixedly against said coatings whereby a connection havingy a constant resistance value is provided between said terminals and said coatings.

6. In a resistor construction, in combination, an elongated element formed from electrical insulating material, an electrical resistance coating on said element comprising a binder having particles of conducting material suspended therein, the surface of said coating being irregular, electrical conductive layers on the end portions of said element immediately adjacent the ends thereof and Aconcentric with said element and superimposed on said coating, said layers comprng a binder filling the irregularities in the surface of said element and having conducting particles suspended therein, thereby forming an excellent electrical connection between said coating and said layers, said layers having much greater conductivity than said coating, wire terminals having portions encircling said layers and proportioned to rest nrmly thereagainst, and a solidmass of 6 t cocasse l.

electrical insulating material encasing said element and the encircling portionsof said terminals to hold said terminals nrmly and immovably against said layers thereby providing an excellent electrical connection between said terminals and said layers.

7. In a resistor construction, incombination, an elongated resistance element having irreguf larities in the surface thereof, electrical conduc- 10 ,tive coatings onthe end portions oi said element immediately adjacent the ends thereof, said coatings being concentric with said element and illiing said irregularities in s aid element, thereby providing an electrical connection between said 15 element and said coatings having a stable resistance value, terminals having' portions encircling said coatings and proportioned to fit iirmly against said coatings, and a mass of solid electri- 'cal insulating material completely encasing said l element and the encircling portions of said terminals to hold'said terminals permanently and ilxedly against said coatings whereby a stable electrical connection is provided between said terminals and said coatings. v- 8. In aresistor construction, in combination. an

elongated' resistance element, electrical conductive coatings on the end portions of said element' immediately adjacent the ends thereof, said coatings being in intimatepontact with said element therebyproviding excellent electrical connections between said element and saidl coatings, said.

coatings being oi' much greater electrical conductivity than said element, wire terminals having portions in interi'itting and engaging relation to said coatings to form connections with said ,ele-

ment and proportioned to t rmly against said 5 coatings when pressure is exerted against said ings being ot much greater electrical conductivity 'than said element, wire terminals having portions in irteriittting and engaging relationto said ocat- 20 ings -totorm connections with said' element and proportioned to tit firmly against said coatings when pressure is exerted against 'said connections. and a mass oi solid electrical insulating material completely enclosing said element and the 25,

relatively interiitting portions oi' said terminals and said element. said terminals having bendable ends of the unit tor attachment purposes, 30

LD mslm4 nn. l

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