US2576405A

US2576405A - Protector for electric circuits

Info

Publication number: US2576405A
Application number: US24647A
Authority: US
Inventors: Craig L Mcalister
Original assignee: McGraw Electric Co
Current assignee: McGraw Electric Co
Priority date: 1948-05-01
Filing date: 1948-05-01
Publication date: 1951-11-27
Anticipated expiration: 1968-11-27
Also published as: GB662660A; NL71719C; US2636850A; FR985230A; CH280489A

Description

Nov. 27, 1951 C, L, MCALISTER l 2,576,405

PROTECTOR FOR ELECTRIC CIRCUITS Filed May l, 1948 .BY n

Patented Nov. 27, 1951 2,576,405 PROTECTOR FOR ELECTRIC CIRCUITS Craig` L. McAlister,

McGraw Electric Overland, Mo.. signor to Company. poration of Delaware Elgin, lll., a cor- Application May 1, 1948, Serial No. 24,647

(Cl. o-131) Claims. 1

This invention relates to improvements in protectors for electric circuits. More particularly, this invention relates to improvements in protectors for electric circuits that are intended to carry very small amounts of electric current.

It is therefore an object of the present invention to provide an improved protector for electric circuits that are intended to carry very small amounts of electric current.

In the construction and operation of a certain type of protector for electric circuits, it is customary to have a fusible link engage, and extend between, the terminals of the protector. That fusible link will conduct electric currents, below a predetermined value, indefinitely; but on overloads, that fusible link can open the circuit by heating itself to its fusing temperature. This action of the fusible link protects the electric circuits against damage which might occur if the overloads were permitted to continue. Fusible links of this type are customarily used in protectors for electric circuits that are rated to carry upwards of one (1) ampere of current; and, if properly designed and constructed, those links work quite well. Fusible links, capable of "earryingone (1) or more amperes of current continuously, are usually strong enough physically to support themselves within the casings of the protectors and to avoid accidental contact with those casings. Where fusible links of this type are used in protectors that are rated to carry currents of less than one (1) ampere. those links may have such small diameters that they are not strong enough physically to support themselves within, and to avoid accidental contact with, the casings of the protectors. The mere heating of those fusible links, consequent upon the passage of electric currents through the protectors, may cause those fusible links to elongate and bend until they contact the casings of the protectors. Any such contact between the fusible links and the casings of the protectors will make the ratings of the protectors unreliable; because the casings can absorb large or small quantities of heat from the fusible links, depending upon the intimacy and area of contact between links and casings.

In recognition of this problem, those skilled in the art have provided bridge structures to support the fusible links in protectors for electric circuits which carry small amounts of electric current. Such bridge structures provide the required support for the fusible links and hold those links against accidental contact with the interiors of the casings of the protectors.

The bridge structures that have been proposed and employed by those skilled in the art are of two general types: one bridge structure being formed by securing two stiff Wires together with a bead of glass or other dielectric material, and the other bridge structure being formed by passing two stiff wires through openings formed in a bar of insulating material. In each instance, the ends of the stiff wires act as sub-terminals to which the fusible link can be secured; and when so secured, the fusible link completes the circuit between the terminals of the protector.

The bridge constructions of these two general types are operative, and they can be used effectively; however, the cost of manufacturing those bridge constructions is quite high because the operations used in making those constructions must be performed with great care by highly skilled personnel. For example, those ends of the stiff wires which support the fusible link should be exactly the same distance apart in each case to assure uniformity of rating of the protectors; but exact spacing of those ends of the wires is hard to attain. Moreover, the fusible link should be soldered to the same sections of the stiff wires in each case so the effective lengths of all links will be the same, but this is not easy to do because the small size of the component parts of the protector makes their handling dimcult. In addition, the stiff wires act as cantilever beams; and thus they introduce the undesirable possibility of bending under conditions of vibration and shock. For these various reasons, prior bridge constructions which are used to support the fusible links of protectors for electric circuits, that carry small electric currents, are objectionable. The present invention obviates these objections by providing a rigid support of insulating material that is unitary and extends between the terminals of the protector and that directly supports the fusible link. This rigid support is provided With a conductive coating to which a fusible element can be soldered, and the conductive coating will be as solid and unyielding as the rigid support; and thus the fusible link will be held rigidly. In addition, the rigid nature and the unitary character of the support x the distance which the fusible link must span, thus making it possible to have all of the links exactly the same in length, and also making it possible to insert the assembled structure of the protector into the casing as a unit. It is therefore an object of the present invention to provide a protector for electric circuits which has a rigid support of insulating material that is of unitary form and that extends between the" terminals of a protector for electric circuits, and which is provided with a conductive coating to which a fusible link can be soldered.

In making the protector of the present invention it is necessary to provide two spaced points on the support of insulating material that are provided with a conductive coating. In addition, those spaced points and the conductive coatings thereon must be capable of withstanding the heat required in soldering operations and must be such as to minimize arcing when the fusible link fuses. Where the areas of contact between the spaced points and the conductive coatings are large, the problem of soldering the fusible links to those points is not overly difficult; but where those areas are small, as they must be to minimize arcing when the fusible links fuse, the problem of soldering the links to the coatings is difcult. Consequently, it is not possible, in making protectors for electric circuits that carry small amounts of electric current, merely to select a support of insulating material, form two spaced coatings on that support, and then solder a fusible link to those spaced coatings. In particular, it is not possible to select just any support of glass, steatite, porcelain, Bakelitef Lucite, or dielectric material, form two spaced coatings of copper, silver, aluminum, or other relatively inexpensive conducting metal on that support, as by painting or printing the support with paints or inks containing those metals, and then solder a fusible link to those spaced coatings; because the heat required in the soldering operation is apt to cause those conductive coatings to melt and form a ball unless the areas of contact between the coatings and support are large, and yet if the area and volume of those coatings are large the metal of those coatings can volatilize and can feed any arc which forms when the fusible link fuses. However it is possible, by employing the principles and teachings of the present invention, to make a support of insulating material which has spaced conductive coatings thereon that can withstand soldering temperatures and that will not unduly prolong electric arcs which may occur when the fusible-link fuses. Such a support will have two narrow projections thereon and will have conductive coatings securely attached to those projections. Those coatings will adhere so tightly to the projections on the support that they will be able to withstand the heat applied in the soldering operation; and the volume of metal in those coatings will be small enough to avoid undue prolongation of any arcs that may form when the fusible link fuses.

In making the two general types of bridge structures previously used in protectors for electric circuits, it was necessry to use great care in dimensioning the overall length of the bridge structures. If such care was not taken, the bridge structures could be bent in placing them in the casings, or they could be ineompletely secured to the terminals of the protector. The great care required in making those bridge structures increased the cost of making those protectors, as by necessitating the employment of highly skilled personnel. The present invention avoids any such needless cost by providing a bridge structure which can be manufactured in large quantities with great precision by personnel having usual and ordinary skill. It is therefore an object of the present invention to provide a protector for electric circuits which has a bridge structure that can be made precisely and in large quantities.

The bridge structure present invention has a conductive coating at each end; and those conductive coatings receive the solder which holds the fusible link in position, and they also receive the solder which secures the bridge structure to the terminals of the protector. With this arrangement, it is easy to solder the fusible link directly to the bridge structure, thus yforming a strong, completelyrigid structure that can be handled freely and can be inserted in the casing as a unit and that can withstand considerable vibration or shock in use. It is therefore an object of the present invention to provide a protector for electric circuits wherein a rigid bridge structure of insulating material is provided with spaced conductive coatings.

Other and further objects and advantages of the present invention should become apparent from an examination of the drawing and accompanying description.

In the drawing and accompanying description a preferred embodiment of the present invention is shown and described but it is to be understood that the drawing and accompanying description are for the purposes of illustration only and do not limit the invention and that the invention will be defined by the appended claims.

In the drawing, Fig. l is an enlarged plan view of a bridge structure which can be used in making the protector for electric circuits that is provided by the present invention,

Fig. 2 is an enlarged side elevational view of the bridge structure shown in Fig. 1,

Fig. 3 is an enlarged and elevational view of the bridge structure shown in Figs. 1 land 2,

Fig. 4 is an enlarged plan view of the bridge structure of Figs. 1-3, as that structure has a fusible link soldered to the spaced conductive coatings thereon,

Fig. 5 is an enlarged plan view of the bridge structure of Figs. 1-3, as that structure has a silver-sheathed wire soldered to the spaced conductive coatings thereon,

Fig. 6 is an enlarged plan view of the bridg' structure of Fig. 4 after the silver sheath of the wire has been removed, and

Fig. '7 is an enlarged cross sectional plan view of a protector for electric circuits which utilizes the bridge structure of the present invention.

Referring to the drawing in detail, the numeral III generally denotes a bridge structure that is made in accordance with the principles and teachings of the present invention; and that bridge structure has a central portion I2 which extends between and connects two end portions I4. The end portions Id are of greater thickness than the central portion I2, and the upper surfaces of end portions I4 project above the upper surface of the central portion I2. The inner ends of end portions I4 take the form of narrow projections I6 that extend along the opposite edges of the bridge structure; and the outer ends of the end portions I4 are beveled or tapered. The bridge structure I0, with its component portions I2, I4 and I6 can easily be made by molding it under pressure. The bridge structure I0 is preferablyniade of a hard, strong, moldable dielectric material; one such material being steatite.

Once the bridge structure I0 has been formed, conductive coatings must be applied, and securely attached, to the upper surfaces of the end porin the protector or tnetions I4 while leaving the central portion I2 bare. These conductive coatings can be made from different metals and they can be applied in different ways; but a preferred way of applying those coatings includes forming an initial coating of metal on each oi.' the portions I4, nring the coating to make it adhere to the surfaces ofthe end .portions I4, and then applying asecond coating of metal over the rst coating. Where this is done, an intimate engagement is provided between the end portions I4 and the conductive coatings, and those coatings will be able to withstand` the temperatures required for soldering. The initial coating of metal can be applied to the end portions by spraying molten metal or molten alloy onto the end portions I4, by forming a colloidal suspension of a metal or alloy and 4brushing or printing it onto end portions I4, by covering up the central portion I2 and then immersing the bridge structure I in a colloidal suspension of a metal or alloy, by chemically or electrochemically depositing a metal or alloy on the end portions I4, or by heating a metal or alloy and causing it to deposit directly on the end portions I4. Where this initial coating is formed of relatively inexpensive metals, it will ordinarily not have the desired adherence to the end ,portions I4; and that coating must be fired to cause it to adhere firmly to end portions I4. The need oi firing the initial coating of metal or alloy prevents the use of certain metals and alloys which would otherwise be of considerable interest. For example, copper which is a relatively inexpensive, readily available, and readily worked metal with a low electrical resistivity and which might therefore seem to be a logical choice for the metal coating to 4be formed on end portions I4 Will oxidize at the temperatures required to re it. When oxidized, copper is not well adapted to receiving solder; and it has a higher resistivity. As a result, copper is not desirable for the initial coating; and many copper alloys are unsuitable because of the action of the copper therein. Aluminum, which is a relatively inexpensive, readily available metal with a lovv electrical resistivity might also seem to be a logical choice; but aluminum is rather diiiicult to solder and it forms an oxide when fired.` For these reasons, aluminum and aluminum alloys are not desirable as initial coatings for the end portions One metal that is suitable for the initial coating is silver; and while silver is more expensive than copper or aluminum, it is readily available,

has low electrical resistivity, can easily be soldered, and is not prohibitively expensive. The silver can be applied by painting, printing, spraying, electroplating, chemical deposition, or other ways; but the silver in the initial coating tends, during the soldering operation, to form metal balls on the surfaces of the end portions I4 or tends to dissolve in the molten solder. In either event, a poor solder joint between the conductive coatings and the fusible link can result. By applying a reinforcing coating to the end portions ili, it is possible to largely overcome the tendency o the rst coating to make a poor solder joint with the fusible link. This second coating may be of copper, silver, or other metal but is preferably of copper. This reinforcing coating can be formed by spraying molten metal onto the rst coating, by chemical or electrochemical deposition, by brushing on a colloidal suspension of metal, by masking the central portion i2 or the bridge structure I0 and dipping the bridge structure in a colloidal suspension of metal, or by condensing or sputtering the metal onto the initial coating. This second coating will increase the thickness of the conductive coating and thus decrease the resistance of that coating to current flow. and it will also make that coating capable of resisting the heat of the soldering operation. This second coating need not be red since it will adhere Well to the initial coating.

As indicated herein, various methods may be used in forming the bridge structure I0, forming the initial coating and forming the secondary coating, but one preferred method that has been found to be satisfactory is as follows. A bridge structure I0 is molded from steatite; and it is formed with an overall length of nine hundred and sixty thousandths (0.960) of an inch and a width of one hundred and thirty thousandths (0.130) of an inch, and with projections I6 that are thirty thousandths (0.030) of an inch wide and twenty-five thousandths (0.025) of an inch high and are spaced apart to provide a gap of one tenth (0.1) of an inch between their ends. A solution, made and sold by the E. I. du Pont de Nemours Company under the designation #4545, and which is understood to contain sixty percent (60%) colloidal silver admixed with a low-melting-point glass and some butyl acetate and some organic lacquer, is brushed onto the end portions I4 of the bridge structure I0 and is permitted to dry in the air. Thereafter, the bridge structure is placed in a heated chamber and held at a temperature between twelve hundred and twenty (1220) to twelve hundred and seventy (1270) degrees Fahrenheit for one hour. This heat treatment makes the silver coating firmly adherent to the end portions i4. Upon cooling, the bridge structure I0 is placed in a plating bath, plating terminals are attached to the end portions i4, and current is passed through the bath. The bath is made by admixing six and one half (6l/2) ounces of sixty-six (66) Baume sulfuric acid, thirty (30) ounces of copper sulfate, and a trace of water-soluble glue; and then adding enough water to make a gallon of solution. With this solution, currents of less than one ampere are sufficient to provide the desired coating within thirty (30) minutes. When thoroughly rinsed with water, the bridge structure i0 has a heat-resistant, composite coating of silver and copper that has low electrical resistance and is firmly adherent to the end portions i4.

In preparing this bridge structure for use in a protector for electric circuits, a fusible link i8 of the required dimension is placed so it rests upon the ends of the projections i6 of end portions I4. While in this position, the fusible link will have its central portion spaced above the central portion I2 of the bridge structure i0. This spacing is desirable since it provides substantially uniform dissipation of heat from the central portions of the fusible links of the various protectors, thus assuring uniform operation and rating o those protectors. The ends oi' the fusible link I8 are secured to the conductive coatings on projections I6 by means of sold-er 20. This solder completes a continuous circuit through the fusible link i3 and the two spaced conductive coatings on the end portions i4.

in securing the bridge structure i0 Within the casing 22 of the protector, the metal coatings on lthe end portions I 4 can be soldered to the metai terminals 24 by solder 26. The bridge structure I can be slipped into the casing 22, a lump of solder can be placed adjacent one end portion I4, a terminal 24 can be slipped onto the end of the casing 22, and heat applied to melt the solder and cause it to adhere to the metal coating on the end portion I4. By setting the casing 22 so its axis is vertical, it is possible to have the solder simultaneously run onto the lower terminal 24 and the lower end portion I4. Inverting the casing 22 and repeating the process securesV the bridge structure I0 in place and electrically con- -nects the terminals 24, the conductive coatings A on end portions I4, and fusible links I8 in series relation.

The bridge structure I0 is made almost as Wide as the interior of the casing 22, and the lower edges of the bridge structure are beveled; the beveled edges seating on the arcuate interior of the casing 22 and holding the upper surface of the bridge structure at the approximate center of the casing 22. In this way, the fusible link I8 is held out of contact with the interior of the casing 22; and thus the operation and rating of the protectors can be made uniform.

It will be noted that the projections I6 of end .portions I4 are quite narrow, being about one quarter (1/4) the width of structure I0. This arrangement is desirable to minimize the amount of conductive coating that will be fused when the fusible link I8 melts and fuses. If the fusible link I8 were to be soldered direct to the large area sections of end portions I4, the metal of those sections might fuse and vaporize under some conditions of circuit breaking and the pressure of the vapor might burst the casing 22. However, by solderingv the fusible link I8 to the narrow projections I6, this danger is minimized since the amount of metal on those projections is too small to cause difficulty even if it'vaporized.

The projections I6 not only space the fusible link above the central portion I2 of the bridge structure I8, but they also make it easy to demark the coated anduncoated portions of the bridge structure. Moreover they increase the surface distance between the coated portions I4. This increase in surface distance increases the resistance of the protector to leakage of current after the fusible link I8 has fused. Thus, once the fusible link I8 has interrupted the circuit through the protector, that circuit will not be f again completed through that protector.

The projections I6 are further desirable because they facilitate rapid soldering of the fusible links I8 to the conductive coatings on the bridge structures The projections I6 extend along the sides of the bridge structure and they are dimensioned so the fusible links I8 extend transversely of the bridge structures; in the form shown in the drawing, the fusible links I8 have their axes inclined at an angle of one hundred and forty-three (143) degrees to the major axes of the bridge structure I0. Consequently, where the bridge structures are placed side by side, and

displaced slightly in inclined position so the fusible links I8 of all the bridge structures lie on the same straight line, a continuous Wire can be soldered to each successive projection I6 to form the fusible links I8 of the various bridge structures. Thereafter, the wire can be severed between adjacent bridge structure to free those structures for insertion in separate casings 22.

The Wire used in making the fusible links I8 is quite small in diameter and it is more easily handled in a long section than in a series of short sections of less than one quarter (V4) of an inch in length. To facilitate the contemporaneous soldering of a number of fusible links I8 to their various bridge structures, a number oi the bridge structures are placed in slots in a jig; the slots in the jig holding the bridge structures so their axes are parallel and so the tips of all projections I6 lie in the same straight line. The free end of the fusible wire is soldered to the first projection I6 of the rst bridge structure in the jig, and then the wire is guided along the tips of succeeding projections I6 and soldered to those projections.

For economy of manufacture the bridge structures can be placed in a jig after they are red, and can remain in that jig during the plating and soldering operations. This enables as many as twenty-ve or more of the bridge structures to be picked up and moved as a unit. This decreases the cost of manufacture, and also provides uniform treatment of each bridge structure.

In those instances where the currents to be carried and interrupted by the protectors of the present invention are so small that ordinary fusible wire cannot be used, a special, small-diameter wire 26 must be used. This wire can be formed by coating a bar of platinum with a sheath of silver and then drawing the sheathed bar through progressively smaller dies until it is quite small in diameter. Thereafter the silver coating 28 can be removed to expose the still smaller platinum wire 26. For convenience in handling, the sheathed wire is soldered to the projections I6 before the silver coating 28 is removed. After the coating 28 is removed, the wire 26 would be almost invisible and too fragile to handle.` While the securement of the sheathed wire to the projections l6 solves the problem of handling the wire, it makes necessary the removal of the silver coating 28 of the wire 26 after the sheathed wire is secured to the conductive coatings. This removal could easily remove the conductive coating of silver and copper; and therefore a protective coating for the conductive coating is provided. Such a coating permits removal of the silver coating 28 of the wire 26 without appreciable removal of the silver and copper conductive coating on the bridge structure. Although various metals could be used in 'making such a coating, one such metal is nickel; and it can be readily electroplated onto the coppercoating. One nickel plating solution that has been used successfully is made by admixing fifty-five (55) ounces of nickel sulfate, twenty-ve (25) ounces of nickel chloride, six (6) ounces of boric acid, and a fraction of an ounce of a wetting agent; and then adding enough water to make a gallon of solution. The nickel coating is formed on the copper coating before the soldering operation; and where this is done, the bridge structures can be held in a jig throughout the complete process.

The silver coating on the sheathed wire can be removed in several ways, as by dissolvingv with chemicals or by deplating electrochemically. One very workable way of removing the silver sheath includes immersing the bridge structures in a mixture of four hundred and fifty (450) parts of sulfuric acid and tv lty-ve (25) parts of nitric acid. The mixture is heated to a temperature range of from seventy-five to eighty-five degrees centigrade, and the bridge structures are left in the mixture until the silver coating has been removed. Visual and electrical tests should be made to ascertain the completeness of the removal of the silver.

75 'I'he coatings on the bridge structures will be quite thin. The initial silver coating will be approximately ve ten thousandths (0.0005) of an inch thick, and the copper coating will be approximately the same thickness. Where bare fusible links are secured to the metal coatings, the total thickness of those coatings may be about one thousandth (0.001) of an inch thick. Where a sheathed wire is used, the thickness of the nickel coating will be added; the nickel coating ranging from one ten thousandth to one thousandth (00001-0001) of an inch.

The foregoing represents an economical way of making protectors which carry and interrupt small electric currents. However, other ways of making those protectors are feasible although more diillcult or expensive. One such way applies a solution of silver and platinum to the end portions I4 of the bridge structures. This solution should be made as thick as possible so it can provide a large quantity of platinum on the end portions I4. When the bridge structures are fired, the platinum will deposit on the end portions I4 and adhere to those portions so well that it will not come off. Unlike silver, the initial coating of platinum will be capable of resisting the heat `oi! the soldering operation. Thus it is possible to form a conductive coating in one step; and furthermore it will be noted that this coating will need no protective coating since i1; will not be,

affected by the dissolving action. However, due to the cost of the platinum, the cost of such protectors may be greater than the cost of protectors using the preferred coatings described above. Another way of forming coatings on the end portions I I of the bridge structures can be used. That method includes painting the end portions I4 and their projections I6 with a paint of metallic palladium and a low melting point glass, firing the bridge structure to cause the glass to fuse and bond the palladium to the bridge structure, immersing the coated structure in a solution of sodium hyposulte and nickel or cobalt or nickel-cobalt alloys, and heating the solution to temperatures above ninety (90) degrees centigrade. The nickel or cobalt or nickel-cobalt alloys will deposit on the palladium coating and form usable conductive coatings. Still another way of forming the coatings is to make a thick paste of colloidal silver and low melting point glass and coat it heavily onto the end portions I4. Such a coating, when red, will be adherent to the end portions Il and will be able to receive solder. However, such coating is not as desirable as the preferred coatings provided by the present invention because solder does not flow readily on that coating and because of the increased cost of material used in making that coating.

Whereas the drawing and accompanying description have shown a preferred embodiment of the present invention it should be obvious to those skilled in the art that various changes may be made in the form of the invention without affecting the scope thereof.

What I'claim is:

l. A protector for electric circuits that comprises a bridge structure of insulating material, raised portions on said bridge structure, said raised portions being spaced apart, said raised portions being narrow, a silver coating adherent to said raised portions, a copper coating adherent to said silver coating, a fusible link spanning the space between said raised portions, said link being soldered to the copper coating on said raised portions, a casing dimensioned to telescope over said bridge structure, and terminals soldered to I the copper coating on said raised portions.

2. A protector for electric circuits that comprises a bridge structure of insulating material,

5 projections on said bridge structure, said projections being spaced apart, said projections being narrow, a heat-resistant electrically conductive coating on each of said projections, a fusible link spanning the space between said projections and being soldered to said coatings, a casing dimensioned to telescope over said bridge structure, and terminals for said casing that are electrically connected to said coatings.

3. A protector for electric circuits that comprises a bridge structure of insulatingimaterial, raised portions on said bridge structure, said raised portions being spaced apart, said raised portions being narrow, a silver coating adherent to said raised portions, a copper coating adherent to said silver coating, a fusible link spanning the space between said raised portions, said link being soldered to the copper coating on said raised portions, a casing dimensioned to telescope over said bridge structure, and terminals soldered to the copper coating on said raised portions, each of said raised portions being approximately one quarter (1A) as wide as said bridge structure.

4. A protector for electric circuits that comprises a casing, a bridge structure o1' insulating material positioned within said casing, a narrow projection extending along one edge of said bridge structure, a second narrow projection extending along the opposite edge of said bridge structure. a metallic coating adherent to each of said projections, and a fusible link soldered to said coatings.

5. A unitary bridge structure for protectors for electric circuits which comprises a piece of insulating material, prominences that are formed on said piece of insulating material but are spaced apart and rise abruptly from one surface of said pieceof insulating material, metal surfaces on said prominences, and a fusible element extending between and soldered to the metal surfaces on those prominences, said fusible element being spaced from and held out of -contact with said one surface of said piece of insulating material, said prominences being formed so a straight line between the ends thereof is angularly disposed relative to the axis of said `bridge structure.

6. A unitary bridge structure for protectors for electric circuits which comprises a piece of insulating material, prominences that are formed on said piece of insulating material but are spaced apart, metal surfaces on said prominences, and a fusible element extending between and soldered to the metal surfaces on those prominences, the inner ends of said prominences being disposed so a straight line between them is transverse to the axis of said bridge structure.

7. A protector for electric circuits that comprises a bridge structure of insulating material, projections on said bridge structure, said projections being spaced apart, said projections being 55 narrow, a conductive coating on each of said projections, a fusible link spanning the space between said projections, said link being soldered to the coatings on said projections, a casing enclosing said bridge structure and said fusible link soldered to the said coatings on the said projections of said bridge structure, and terminals on said casing and electrically connected to said fusible link.

8. A protector for electric circuits that comprises a bridge structure of insulating material with projections thereon, said projections being spaced apart, said projections being narrow, a multi-layer .conductive coating on each of said projections, a fusible link spanning the space between said coatings on said projections and being soldered to said coatings, a casing enclosing said bridge structure and said fusible link soldered to the said coatings on the said projections of said bridge structure, and terminals on said casing and electrically connected to said fusible link.

9. A protector for electric circuits that comprises a bridge structure of insulating material vwith spaced projections thereon, said projections being narrow, a multi-layer conductive coating on each of said projections, one layer of each of said multi-layer coatings being iired onto said projections, a fusible link spanning the space between said coatings on said projections, and being soldered to said coatings, a casing enclosing said bridge structure and said fusible link soldered to the said coatings on the said projections of said bridge structure, and terminals on said casing and electrically connected to said fusible link.

10. A protector for electric circuits that comprises a support of insulating material having spaced projections thereon, spaced conductive coatings intimately adherent to said support, each of said coatings having a narrow link-receiving portion extending therefrom, a fusible link spanning the space between said link-receiving portions of said conductive coatings soldered to each of said coatings, a casing enclosing said support and said fusible link soldered to the said coatings on the said projections of said support, and terminals on said casing and electrically connected to said fusible link.

11. A unitary bridge structure for protectors for electric circuits which comprises a piece of insulating material, prominences which are formed on said piece of insulating material but are spaced apart, metal surfaces on and integral with said prominences, said surfaces having narrow link-receiving portions extending therefrom, and a fusible link extending between and soldered to the link-receiving portions of said metal surfaces on those prominences.

12. In a protector for electric circuits which comprises a casing, a fusible element and a bridge structure, the improvement which comprises a unitary bridge structure of insulating material which is coextensive with said casing and has spaced metallic surfaces thereon, said metallic surfaces having narrow projections to receive a fusible element, and a fusible element secured to said surfaces by solder.

13. A protector for electric circuits that comprises a casing, terminals of electrically conducting material for said casing, a bridge structure of 'insulating materiai positioned within said casing, said bridge structure being coextensive with said casing, spaced electrically conducting coatings on said bridge structure, each of said coatings having a narrow link-receiving portion extending therefrom. and a fusible link spanning :the space between and being soldered to said linkreceiving portions of said spaced coatings, said spaced coatings terminating immediately adjacent and being in electrical engagement with said terminals.

14. A protector for electric circuits that comprises a casing, terminals of electrically conducting materialfor said casing, a bridge structure ing, said bridge structure being coextensive with said casing, spaced electrically conducting coatings on said bridge structure, each of said coatings having a narrow link-receiving portion extending therefrom, and a fusible link spanning the space between and being soldered to said linkreceiving portions of said spaced coatings, said spaced coatings being in electrical engagement with said terminals, said coatings being formed on raised portions of said bridge structure.

15. A protector for electric circuits that comprises a casing, terminals of electrically conducting material for said casing, a bridge structure of insulating material positioned within said casing, said bridge structure being coextensive with said casing, spaced electrically conducting coatings on said bridge structure, each of said coatings having a narrow link-receiving portion extending therefrom, and a fusible link spanning the space between and being soldered to said linkreceiving portions of said spaced coatings, said spaced coatings being in electrical engagement with said terminals, said coatings being formed on raised portions of said bridge structure, said raised portions being approximately one quarter (1A) as wide as said bridge structure.

16. A protector for electric circuits that comprises a support of insulating material having spaced projections thereon, spaced conductive coatings on said support, each of said coatings having a narrow link-receiving portion extending therefrom, a fusible link of greater resistivity than said coatings spanning the space between and being soldered to said link-receiving portions of said conductive coatings, a casing enclosing said support and said fusible link soldered to the said coatings on the said projections of said support, and terminals on said casing and electrically connected to said fusible link, said support being coextensive with said casing.

17. A protector for electric circuits that comprises a support of insulating material, spaced conductive coatings on said support, each of said coatings having a narrow link-receiving portion extending therefrom, a fusible link spanning the space between and being soldered to said linkreceiving portions of said conductive coatings, a casing enclosing said support and said fusible link soldered to the said coatings on said support, and terminals, said coatings being soldered to said terminals.

18. In a protector for electric circuits the improvement Which comprises a unitary bridge structure of insulating material which has spaced metallic surfaces thereon to receive a fusible element, said surfaces being relatively narrow and being comprised of a layer of silver and a layer of a base metal and being adapted to receive and hold solder without balling 19. A protector for electric circuits that comprises a support of insulating material having spaced projections thereon, spaced conductive coatings on said support, each of said coatings having a narrow link-receiving portion extending therefrom, a fusible link spanningv the space between and being soldered to said link-receiving portions of said conductive coatings, a casing enclosing said support and said fusible link soldered to the said coatings on the said projections of said support, and terminaison said casing and electrically connected to said fusible link, said support having the under edges thereof chamfered, said chamfered edges bearing against said casing to hold the upper surface of said of insulating material positioned within said aS- suppQl) adjacent the center 0f Said C2S11g 20. A protector for electric circuits that comprises a support of insulating material having spaced projections thereon, spaced conductive coatings intimately adherent to said support, each of said coatings having a narrow linkreceiving portion extending therefrom, a Ifusible linkrof greater resistivity than said conductive coatings spanning the space between and being soldered to said link-receiving portions of said conductive coatings, a casing enclosing said support and said fusible link soldered to the said fusible link soldered to the said coatings on said projections of said support and terminals on said casing and electrically connected to said fusible link, said support having the under edges thereof charnfered, said chamfered edges bearing against said casing to hold the upper surface of said support adjacent the center of said casing.

21. A protector for electric circuits that comprises a bridge structure with spaced projections thereon, a multi-layer conductive coating on each of said projections, each of said coatings having a narrow link-receiving portion extending therefrom, a fusible link soldered to said link-receiving portions of said coatings on said projections, a casing enclosing said bridge structure and said fusible link soldered to the said coatings on the said projections on said bridge structure, and terminals on said casing and electrically connected to said fusible link, one of said layers being more resistant to acid than another of said layers. 22. A unitary bridge structure for protectors for electric circuits which comprises a piece of insulating material, prominences that are formed on. said piece of insulating material but are spaced apart, metal surfaces intimately adherent to said prominences, said surfaces having narrow linkreceiving portions extending therefrom and a fusible element soldered to and extending between and electrically connecting the link-receiving portions of said metal surfaces on those prominences.

23. A protector for electric circuits that comprises a bridge structure with spaced projections thereon, a multi-layer conducting coating on each of said projections, each of said coatings having a narrow link-receiving portion extending therefrom, a fusible .link soldered to said link-receiving portions of said coatings on said projections, a casing enclosing said bridge structure and said fusible link soldered to the said coatings on the said projections of said bridge structure and terminals on said casing and electrically connected to said fusible link, one of said layers being more resistant to acid than the other layers, said resistant layer being the top layer.

24. In a protector for electric circuits the improvement which comprises an insulating support of ceramic material, a red metallic coating on spaced portions of said support adjacent the ends thereof, an intermediate portion of said support that is bare, reenforcing coatings on said red coatings, said reenforcing coatings having narrow link-receiving portions, a fusible link bridging said intermediate portion of said support, and solder electrically connectingsaid link to Said link-receiving portions of said reenforcing coatings.

25. In a protector for electric circuits the improvement which comprises a bridgestructure of insulation material, spaced metallic coatings firmly adherent to said bridge, each of said coatings having a narrow link-receiving portion extending therefrom, and a fusible link spanning the space between and being soldered to said link receiving portions of said coatings, said coatings being spaced above that portion of the bridge structure between them so expansion of said fusible link, inducedby the passage of current through said protector, cannot cause said fusible link to contact said portion of said bridge structure.

CRAIG L. MCALISTER.

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

UNITED STATES PATENTS Number Name Date 1,234,100 Morgan July17, 1917 1,281,917 Eby Oct. 15, 1918 1,337,949 Petrovics Apr. 20, 1920 1,498,858 Warner June 24, 1924 1,846,66 Rumble Feb. 23, 1932 2,263,752 Babler Nov. 25, 1941