US1725773A - Electric conductor - Google Patents

Description

v Aug. 27, 1929. J. H. WHITE ELECTRIC CONDUCTOR Filed NOV 19, 1924 2 Sheets-Sheet Aug. 27, 1929.v H, wHlTE 1,725,773

ELECTRIC CONDUCTOR Filed Nov. 19, 1924 2 Sheets-$heet 2 IlllllllllllllillllllllllllllllIIIIIIIIIIIIIIIIIIIIIIIlllllllllllllIl lPatented Aug. 27, 1929.

UNITED STATES .l PATENT OFFICE.

JOHN H. WHITE, OF CRANFORD, NEW JERSEY, ASSIGNOB TO WESTERN ELECTRIC COMPANY, INCORPORATED, OIF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

ELECTRIC .CONDUCTOR.A

Application filed. November 19, 1924. Serial No. Y750,718.

This invention relates to electric conductors, and particularly to flexible conductors and cables. l

The invention is especially applicable to telephone switchboard cable and distributing frame wire which are used extensively in telephone exchanges.

It is the object of this invention to produce an economical and eilicient insulatedconductor particularly suitable for indoor use such as is encountered in and around telephone switchboards and associated distributing equipment of a telephone exchange.

The mechanical and electrical requirements of telephone switchboard cable are very severe. FromY a mechanical standpoint, the cable must be flexible enough for use in a telephone exchange, for example to allow frequent bending and twisting, as well as to permit a group of cables to ybe wedged apart for the purpose of making necessaryV electrical connections.

From an electrical standpoint, the individualv conductors must be far enough apart.

to keep down the capacity, and the pairs of conductors constituting Y a circuit must be twisted in order to avoid cross-talk. Most important of all, perhaps, the insulation resistance of the individual conductors must be high.

It is well known that different insulating materials vary widely in their electrical 'insulation resistance properties. Fibrous materials such as cotton and tussah floss silk, possess good insulation resistance properties when dry, but it is found that when these materials are exposed to moist air their insulation resistance is considerably lowered. Regardless of their relative insulating properties, the insulation resistance of all such ma-l terials falls otll rapidly under rising humidity conditions.

There is a distinction between the requirel ments of a moisture-proof switchboard cable and a water-proof cable such as is used for under-water signaling. The atmospheric conditions of a telephone exchange mfay approach, but 'never reach the dew point. The

switchboard cable in a telephone exchange;

while never submerged in water, is at all times subject to the deleterious action of moisture in the air which is always present in varying amounts, dependingupon climatic and humidity conditions.

Furthermore, since telephone switchboard the various conductors, thus destroying their elliciency for purposes of telephonie communication, and also sets up an electrolytic action which decomposes the conductors and ultimately renders the cable unfit for service.

Since it is inexpedient for mechanical reasons to cover a switchboard cable with a hermetically impervious sheath, it will be seen that the ability of the cable to resist the action of moist .air must be high.

A switchboard cable which has been used successfully under medium humidity conditions consists of a plurality of black enamel covered conductors each having one, two or even three wrappings of liossisilk, these conductors being twisted in pairs'and assembled Y in layers with interleaving hard paper strips, ormin tors. uch a group of conductors is then overwound with tapes of cloth, tinfoil and a braided covering which is painted, preferably with ireproof paint. f

Although dry oss silk poesses excellent properties of insulation resistance, 1t will be seen from the above that elaborate precautions are taken to minimize the etlect of moisture which would tend to reduce its insulation resistance. The enamel coating 1s employed to assist in excluding moisture, but does not of itself render the conductor molsture-proof since commercial enamel wire frequently contains pinholes'or gaps which are lformed either during the coating process or by cracking the enamel by bending or twistp ing the wire.

Such cables, however, arev open to several important objections, such as the diii'culty of soldering the enameled wire and the hlgh cost and limited supply of loss silk. Furthermore, even with all the above precautions a compact pliable group of conduc' taken, under extreme humidity conditions, l

such as are encountered in the tropics, the

ilo

I c o action of molsture has been found to 1n]ure the cable in a comparatively short time.

' insulation.

`This invention overcomes the diliiculties,

outlined above by providing a cheap and efiicient switchboard cable, the insulation resistance of which remains substantially unaffected by all humidity conditions below the dew point. Y

ln vaccordance with one-of its features, the invention provides a new and improved method of treating the individual conductors of a switchboard cable, in which the use of an' and hygroscopic materials.

The result of the process is to produce a conductor provided with a glossy tube-like-y cover inI which the cotton fibres are enclosed in a solid matrix of cellulose acetate. This insulating cover retains a high insulation resistance under substantially all humidity conditions up to the dew point.

Although the cotton insulation is somewhat repellent to thecellulose acetate solution, the impregnating process is hastenedv and rendered more rthorough by first treating the insulation with a priming agent designed`to eliminate all air pockets and to fa-4 cilitate the entrance of the cellulose acetate solutioninto the pores and interstices of the In the preferred form of the invention, the riming agent consists of a solvent of celluose acetate, such as acetone. The insulated I conductor sotreated is next .dried 0E to such a way that the amount of acetone vapor left a point that while insufficient liquid acetoile is left in the fibres to ldisadvantageously dilute' the cellulose acetate solution which is subsequently applied, still suiiicient acetone vapor is left in the cotton insulation to displace the air. l

This operation may be performed in such in the insulation is such that the unavoidable evaporation of the solvent, such `as acetone, from the cellulose acetate solution is made up by the acetone carried thereto in the insulation, so that the cellulose acetate solution grows neither thinner or thicker.

The impregnating bath consists of a solu-f "tion ofchemically pure cellulose acetate dissolved in highly rened acetone, both of which are entirely free from, hygroscopic 'and electrolytic'materials. ough'impregnation, the insulated conductor is passed through a current of hot air, thus After a thorhastening the evaporation of the acetone and leavingY the conductor when dry with a smooth and glossy cover in which the bres of insulation are enclosed in a matrix of cellulose acetate. The depth to whichthe insulation is impregnated may vary as desired.

v It has previously been proposed to enamel" bare conductors with cellulose acetate lacin the electrical arts, it has heretofore proved l lof negligible value except as avarnish` Conductors coated with cellulose acetate lacquers are entirely unsuited for use in telphone switchboard cable because such a coating is easlly chipped oif when the conductor 1s bent or twisted. Furthermore, the cellu- .lose acetate heretofore employed has always contained `various deleterious substances which tend to attract moisture, and previously known methods of treating electrical apparatus have not been such as to increase the moisture resisting qualities of the insulation. r

rlhe various features and advantages of the invention 'are described in detail with reference t0 the accompanying drawing, in which:

Figure l is a sectional view illustrating the method of treating an insulated conductor in accordance with the invention.

Figure 2 is a plan view illustrating two portions of an insulated` conductor, one of which 'is untreated and the other of which is treated in accordance with the invention.

Figure 3r is a sectional view of a switchboard cable constructed in accordance with theinvention.

Figure 4 shows' a series of curves illus'- trating the eiiicien'cy of telephone conductors insulatedin accordance with the invention.

As shown in Fig. 1, a conductor insulated with cotton or other vsuitable insulating material is drawn2 from a supplyy -reel 5 to an impregnating tank 6 containing acetone. From fthe tank 6, the conductor passes through a channel or chamber 7 to an impregnating tank 8 containing the cellulose vacetate bathf The conductor is then drawn from the tank 8 through a drying chamber 9 to a suitable take-up reel 10.

The insulated conductor on the reel 5 is heated to remove all moisture before the impregnating process. Any number of layers of fibre insulation may be employed. For example, the conductor may be insulated with a single layer of cotton yarn of sufiicient coarseness to secure the desired separation between the conductors ofthe cable to keep v down the capacity therebetween. Itis also preferable to use cotton yarn that is naturally as little hygroscopic as possible, avoiding the use of cotton that has been bleached or otherwise treated in such a wa as to increase its hygroscopic properties. ither before or after being spun into yarn, the cotton may be tested and if found to contain soluble hygroscopic and saline materials, it may be put through V,an exhaustive treatment of pure hot Water to remove such materials.

The tank 6 is provided-with a tightly litting cover 11 and the tank 8 is provided with a similar cover 12 to prevent the entrance of moisture thereto. The chamber 7 is tightly fastened at one end to the tank 6 and at the other end to the tank 8 to prevent the entrance of moisture thereto. The drying chamber 9 issimilarly iixed to the cover 12 en l of the tank 8, thereby providing a continuous air-free passage for the insulated conductor during the entire process.

The insulated conductor is drawn from the reel 5 through a small hole or vent 18 in the tank (i. The vent 13 is preferably just large enough to permit the passage of the conductor, and if desired may be filled with a suitable stufhng material as an added precaution against the entrance of moisture to thetank. The 'tank 6 contains abath ofI highly reiined acetone, free from moisture and electrolytic and hygroscopic materials. rlhe conductor is passed through the acetone over suitable guide rollers at such a rate as to allow the acetone to 'thoroughly impregnato the libere of the insulation. l

The insulated conductor impregnated with acetone is passed from the tank 6 through the chamber 7 to the impregn ting tank 8. A strong current ot hot dry air trom an inlet supply pipe 1li is circulated through the chamber 7- to an outlet pipe v15 to hasten the evaporation of the liquid acetone contained in the insulation. The hot dry air is preferably maintained at about the temperature oi the boiling point of acetone.

The passage of the conchictor through the chamber 7 is preferably so regulated that theinsulation will be dried to such a point that While insuliicient liquid acetone is lettin the insulation to disadvantageously dilute the cellulose acetate solution contained inthe tank 8, still sucient acetone vapor is lett in the pores and interstices of the insulation to displace the air, and also to compensate for theunavoid'able evaporation of the solvent contained in the tank 8. The degree of evaporation may be controlled in several Ways, such as by properly gauging the length'oi chamber 7, or by regulating the temperature and velocity of the hot dry air, and by regulating the speed at Whichthe conductor is drawn through the chamber 7.

rllhe hot Vair containing the acetone vapor liberated in the chamber 7 is drawn od through the outletp'ipe 15 to a suitable condensing apparatus (not shownl Where the acetone is reclaimed, and returned to the tank 6 for .further use. The eirclusionof damp or other air from the tanks G and 8 prevents a film or skin from forming on the top of the solutions contained therein, since the atmosphere above the free surface of the solutionk in these tanks is saturated with pure acetone vapor.

rlhe insulated, conductor treated in the chamber 7 in the manner described above is drawn through the impregnating tank 8 containing the cellulose acetate bath. Great care is taken in the preparation of this solution. A Water Washed cellulose acetate pulp is lirst prepared, which is so thoroughly tree from any residual hygroscopic and electrolytic saline or other mattei', such as sulphuric acid and Zinc chloride which may have been used as catalytic agents in preparing the cellulose acetatethat when a sample is exhaustively extracted with Water in an extraction apparatus, the Water shows no reaction with barium chloride or silver nitrate. The pulp is then'dried, preferably at not more than a few degrees above the temperature of boiling Water until samples taken show constant Weight. The cellulose acetate solution is then prepared by dissolving about 150 grams to 300 grams of the dry pulp the United States gallon of highly refined acetone ci speciiic gravity ot .797 at 60 Fahrenheit. This solution is allowed to settle and the clear supernatant liquor is drawn oli for use in the impregnating tank 8, While the gray undissolved sludge, it such remains, is rejected.

rfhe insulated conductor is passed through the tank 8 containing the cellulose acetate solution prepared in the manner described above, over suitable guide rollers, and becomes thoroughly impregnated with the solution.

The impregnated conductor emerges i'r-oi the tank 8 through a small vent 1G in the airtight cover l2 into the drying chamber E). lt desired, the vent 16 may contain a Wiper or scraper for removing the excess solution which clings to the insulation. A strong current oii hot dry air is circulated through the chamber 9 trom inlet pipe 17 which may be associated with the same source 'of supply as the inlet pipe la in the chamber 7, to an outlet pipe 18 which may be associated With the saine condensin' apparatus (not shown) to which the outlet pipe lxi is connected. 'lie hot dry air is preferably maintained at the temperature or" the boiling point of acetone, and the temperature may be observed at times by means of a thern'iometer i9 associated with the inlet pipe 17.

The acetone con ained in the insulation is rapidly evaporated in the chamber i), leaving a glossy coa-.ting upon the insulation. conducto is drawn over a suitable guide roller through. vent 2O in the chamber 9 to Athe take-upreel 10. la scraper or smoothing .die may be mounted on the inner Wall of chamber 9, or example at the vent 2O for the purpose oremoving uneven particles ot cel lulose acetate which may tend to cling to the surface of the insulation, 'thus giving a smooth -lulose acetate solution does not readily displace the air in the lower layers of the fibrous material. 1t has been found, however, that f when an insulated conductor is first impregnated with a solvent such as acetone used in the preparation of the cellulose acetate solution, andthe conductor so treated is then freed of thejliquid solvent to such an extent that only a portion of the vapor remains, and is then immersed in a pure cellulose acetate solution, the insulated conductor is thoroughly impregnated down to the wire and the aws heretofore encountered are entirely obviated.

The evaporation of the acetone in the chamber@ leaves a moisture resisting insulated cover upon the wire in which the fibres of insulation are enclosed in a matrix of solid cellulose acetate. p The cellulose acetate, in drying, causes a shrinking down of thel fibres and produces a solid tube-like insulation ywhich is considerably smaller in diameter lthan the original unimpregnated loose insulation. This is illustrated more clearly in Fig. 2 which is a copyof an actualphotograph showing a portion 21 of a cotton covered wire which is unimpregnated and a portion 22 ofthe same conductor impregnatedinaccordance with the invention.

1t will be seen that the impregnating process has materially reduced the space occupied by the insulation inthe conduction 22, thereby permitting a greater bulk of conductors to be enclosed in a given space in a switchboard cable, when capacity considerations admit of it.

maarre Fig. 3 shows a switchboard cable prepared in accordance with the invention. This cable consists of a'plurality of copper conductors 23, which may if desired be tinned to facilitate soldering operations. rlhe conductors 23 are each covered with one or more layers of cotton insulation 24' impregnated with cellulose acetate in the manner described above. The insulated conductors are twisted in pairs and the bundle grouped together by means of a servinglof cotton yarn 25 .and a spirally served paper. strip 26, and are overwound with a fabric tape 27` and a braided fabric cover 28. 1f desired, the outer cover 28 may be coveredwith a coat of fire-proof paint.

@ther and different covering materials may also be'employed.'

The outer cover 28 may also beimpregnated with cellulose acetate in the manner described above to further increase themoisture resisting quality ofthe cable. rlFhis recaution may be desirable where the cable 1s to be' used in localities where extremely high humidity conditions prevail.

Exhaustive tests have been made which illustrate the efiiciency of an insulated conductor prepared in the manner described above. The tablev given below illustrates the average results of prolonged tests. made in connection With cellulose acetate impreg-- nated cotton covered wire, untreated cotton covered Wire andv untreated tussah silk fioss covered wire under varying humidity conditions. )These tests were conducted in a hu-l midity room maintained at a constant tem- `perature of Fahrenheit. The-humidity, y

Insulation resstance-Megohms per 50 feet of conductor.

From the above table it will' beseen .that the insulation lresistance of the conductorinsulation .E s es its Se se se .as @as sa se* se :2ci *s Es s i Cellulose acetate C0tt0n-. 118.5 23.6 2.02 .35258.8 14.4 1.25 518 .856 4.73 19.8 31-5 .546 `325 .279 .231.111 `Untreated c0tt0n-.- .373 .093 .009 .004 .,200 .068 0 .006 .011 .038 .112 .136 .010 .007 .006 .004.002

Untreated TllSSahSllkilOSS 119.3 l 13.2 .246 .02114.4 1.80 .018 .024 .181 1.39 2.37 .011 .003 .004 .001.0003

resistance of the silk floss covered conductor is slightly higher than the insulation resistance of'the cellulose acetate cotton covered conductor, afterv being subjected to a relative yhumidity of 65% on the first day, it will be noted that this condition is reversed at the end of the second day when both conductors have been subjected to a relative humidity of 75%. 0n the third and fourth days, after being subjected tohumidity conditions of 85% and 90%, the insulation resistance of the conductor prepared in accordance with the invention is much higher than that of either the untreated cotton covered conductor or the untreated silk iloss conductor. On the fifth day, the humidity was dropped from 90% to to determine the manner in which the Several conductors would recover from the effects of the extremely high humidity to which they were subjected on the fourth day. The results, as may be seen from the table, indicate the vastlyv superior qualities of the cellulose acetate impregnated cotton insulation. The varying humidity conditions to which the several conductors were subjected throughoutthe test, while severe, nevertheless indicate the conditions to which the switchboard cable may be subjected under actual operating conditions.

The eliiciency of switchboard cable made of conductors prepared in accordance with the invention for purposes of telephonie communication is illustrated graphically in Fig. 4. Such cables must have a low leakage from wire to wire, which may be expressed as a conductance, for the eicient transmission of speech, otherwise the voice currents would be greatly attenuated by such short circuiting type of wire-to-wire leakage during transmission. Fig. 4 represents such leakage measure as a. conductance in micromhos per fty feet of a pair of No. 22 B. & S. gauge conductors at 1,000 cycles per second, which frequency represents practical operating conditions. The curve H represents the humidit to which the conductors were subjected from day to day over a period of several weeks. The curve 1 represents the leakage between the untreated cotton insulatedl conductors, while curve 2 represents the leakage between the untreated tussah silk floss insulated conductors, and the curve 3 represents leakage between the cellulose acetate impregnated cotton covered conductors constructed in accordance with the invention. It will be seen that the leakage of the cotton insulated conductors increases rapidly with the relative humidity, rendering the conductors highly ineilicient for purposes of telephonie communication under humidity conditions which are encountered in ordinary telephone practice. The leakage of the silk floss covered conductors likewise increases rapidly under rising humidity conditions, although to a somewhat less extent than the cotton covered conductors. The curve 3 indicates, however,

A articular advanta e of the invention is the conversion of the cotton, which is cheap but which possesses a poor insulation resistance, in moist air into a condition such that it has a. higher insulation resistance in moist air than the more expensive silk which has heretofore been employed in the manufacture of switchboard cables.

While the invention has been described with particular reference to cotton covered conductors, it is to be understood-that it is equally applicable to conductors insulated with other materials.

The invention is also capable of other modiications and adaptations not speciiically referred to but included within the scope of the appended claims.

lhat is claimed is:

l. An electrical conductor having a nonwater-proof insulating covering which retains its insulating qualities under conditions where 1t 1s always warmer than the dewpoint of the surrounding air, characterized by thel 3. An electrical conductor having a noni water-proof insulating covering which. retains its insulating qualities under conditions where it is always warmer than the dewpoint of the surrounding air, characterized by the provision of a cotton serving impregnated with cellulose acetate free from hydroscopic and electrolytic materials.

In witnessxwhereof, I hereunto subscribe my name this 14th day of November A. D.,

JOHN H. WHITE.

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