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The Early History of Insulated Copper WireAllan A. Mills aa Department of Physics and Astronomy , University of Leicester , Leicester, LE1 7RH, UKE-mail:Published online: 05 Nov 2010.

To cite this article: Allan A. Mills (2004) The Early History of Insulated Copper Wire, Annals of Science, 61:4, 453-467,DOI: 10.1080/00033790110117476

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A S, 61 (2004), 453–467

The Early History of Insulated Copper Wire

A A.M

Department of Physics and Astronomy, University of Leicester,Leicester LE1 7RH, UK. e-mail: am41@leicester.ac.uk

Received 15 November 2001. Revised paper accepted 10 January 2002

SummaryIn the early 1800s galvanometers could be constructed with the fine gauges ofsilk-covered copper or silver wires produced for decorative purposes, but whenFaraday was making his classic electrical experiments in 1831 he needed a sturdiergauge of copper wire. Bare copper wire was available in many diameters formechanical applications, but coils for electromagnetic investigations had to beinsulated with string and calico. It was soon realized that the cotton-coveredspringy iron wire then used to hold out the brims of ladies’ bonnets showed howcopper wire might be similarly wrapped to provide a flexible insulation. Thesimple manual machines used by the bonnet-wire makers were readily adaptedand improved, and a six-head version was built by William Henley. This crafts-man’s vision of the growing importance of insulated copper wire was abundantlyjustified, and he built up a large—but poorly organised—empire in the wire andcable trade. Henley’s original multiple-head wrapping machine has been locatedin the Science Museum, London, and the associated silk-covered copper wiresubjected to physical, chemical, and electrical testing. For comparison, the elec-trical conductivity of the ’mechanical grade’ copper wire used by Faraday hasalso been determined.

Contents1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4532. Insulation with resinous compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4543. ‘Silked’ wire and galvanometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4554. ‘Bonnet wire’ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4565. Commercialization by W. T. Henley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4576. Examination of Henley’s insulated wire . . . . . . . . . . . . . . . . . . . . . . . . . . . 459

6.1. Physical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4596.2. Chemical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4626.3. Electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463

6.3.1. General6.3.2. Measurement of low resistance in situ6.3.3. Specific resistance

7. Faraday’s wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4658. Modern insulated wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4669. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466

1. IntroductionOn 17 October 1831 Michael Faraday1 made the important discovery that the

link between magnetism and electricity was motion. He found that inserting a barmagnet into a coil of insulated copper wire caused a current to flow through a

1 Faraday’s Diary, ed. by Thomas Martin, 6 vols (London: Bell, 1932), , 375–76.

Annals of Science ISSN 0003-3790 print/ISSN 1464-505X online © 2002 Taylor & Francis Ltdhttp://www.tandf.co.uk/journals

DOI: 10.1080/00033790110117476

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Figure 1. Working replica of Faraday’s ‘magnet-and-coil’ apparatus of 1831.

galvanometer connected to the ends of the coil, its direction reversing when themagnet was withdrawn. No effect was apparent when the magnet was stationary.2This epoch-making apparatus still exists in the museum of the Royal Institution inLondon, and photographic illustrations are reproduced in a book by Martin.3

Less widely known is the nature of the construction that Faraday was obligedto employ for this coil, and also for those coils wound on opposite sides of thefamous circular iron ring with which he demonstrated induction.4 Drawn copperwire of about 1/16 inch diameter (1.25 mm; corresponding to 16 or 17 BirminghamWire Gauge) was readily available commercially as ‘bell wire’, being employed tolink the mechanical bells of the period to the appropriate bell-pull. It was, of course,bare metal wire. Therefore, to build coils where each turn was insulated from thenext, Faraday wound the wire side by side with thin string of similar diameter,placing a layer of calico between each layer. Each layer was saturated and securedwith shellac varnish before proceeding to the next. A working replica illustratingthis internal construction is shown in Figure 1.

2. Insulation with resinous compoundsFabric impregnated with soft pitch or resin–beeswax mixtures had long been

used as an insulating covering for metallic conductors in the investigation of static

2Michael Faraday, ‘Experimental Researches in Electricity. 1. On the Induction of Electric Currents;2. On the Evolution of Electricity from Magnetism’, Philosophical Transactions of the Royal Society ofLondon, Part I (1832), 125–62. Michael Faraday, Experimental Researches in Electricity, 3 vols (London:Quaritch, 1839–55), , 9–12.3 Thomas Martin, Faraday’s Discovery of Electro-Magnetic Induction (London: Arnold, 1949).4 Ibid.

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The Early History of Insulated Copper Wire 455

electricity,5 as too was the harder shellac-based sealing wax. The latter, applied towarmed copper wire, was utilized in 1831 by Ritchie6 for insulating the coil of historsion galvanometer. An attempt was made to mechanize the process,7 but therequirement for just sufficiently warming the wire to prevent cracking or spalling ofthe coating made it impractical to store the prepared wire for later use. Softercompositions, sufficiently malleable to be coiled at room temperature, would tendto coalesce on storage to form a solid mass.

Another problem with the above methods of insulation was the large percentageof the volume of the coil occupied by superfluous insulation. In Faraday’s ‘stringand calico’ method it could obviously exceed 50%. What was required was a thin,stable, flexible insulating layer that could be applied to various gauges of copperwire, yet still permit indefinite storage on a reel before the product was utilized forelectrical circuits and apparatus.

3. ‘Silked’ wire and galvanometersA clue was provided by the fine silver, brass, or copper wires wrapped with silk

that were used for tapestry and furnishing trimmings, although a textile core coveredwith metal foil was the more usual construction. The collective term for decorativetrimmings of this nature is ‘passementerie’.8 Known in ancient times, it becamemore common from the sixteenth century onwards.9 Wide helical spacing of acoloured silk ribbon produced a two-colour wire, while close packing under slighttension would completely cover the metal core. The inherent elasticity of the silkheld it in place without the need for additional adhesive, allowing the covered wireto be bent without exposing bare metal.

This commercially available ‘silked’ silver or copper wire was therefore ideal forwinding the coils of the earliest galvanometers.10 These coils were constructed toenclose a magnetic needle suspended on a filament of silk, and were used for thedetection of electric currents. The principle was that the induced magnetic fieldacting on the needle would tend to move it away from a rest position parallel to theEarth’s magnetic field. A more sensitive version was the ‘astatic’ galvanometer,where parallel needles of opposed magnetic polarity (one situated outside the coil )would eliminate the effect of the Earth’s field, leaving only the torque of thesuspension to oppose rotation. These instruments are described in a number ofreviews,11 but none gives the source of the silked wire used in their construction.

5 Tiberius Cavallo, Treatise on Electricity (London, 1786).6William Ritchie, ‘Description and Application of a Torsion Galvanometer’, Journal of the Royal

Institution, 1 (1831), 29–38.7 J. C. Nesbit, Untitled letter, Annals of Electricity, 2 (1838), 381–82 and Plate IX.8 Oxford English Dictionary, ed. by J. A. Simpson and E. S. C. Weiner (Oxford: Clarendon Press,

1989), , 304.9 D. Diderot and J. Alembert, Encyclopedie, ou Dictionnaire raisonne des sciences, des arts et des

metiers . . . (Paris, 1751–65). C. Donzel and S. Marchal, L’Art de la Passementerie (Chene, 1992).Margaret Mitchell, Gone With The Wind (New York, 1936, 1964), ch. 12, p. 232 in the first printing,p. 162 in the second. ‘ ‘‘He insulted us all and the Confederacy too’’ said Mrs Merriwether, and her stoutbust heaved violently beneath its glittering passementerie trimmings . . .’.10 J. C. Poggendorff, ‘Physiche-chemische Untersuchungen zur nahern Kenntniss des Magnetismus

der Voltaischen Saule’, Isis von Oken (Jena), 9 (1821), 689–710. J. C. S. Schweigger, ‘Noch einige Worteuber diese neuen elektro-magnetischen Phanomene’, Journal fur Chemie und Physik, 31 (1821), 35–41.Leopoldo Nobili, Memorie ed Osservazioni (Florence: Passigli, 1834). Paolo Brenni, ‘The Instruments ofLeopoldo Nobili’, Bulletin of the Scientific Instrument Society, no. 8 (1986), 4–6.11 R. S. Whipple, ‘The Evolution of the Galvanometer’, Journal of Scientific Instruments, 11 (1934),

37–43. R. A. Chipman, ‘The Earliest Electromagnetic Instruments’, in US National Museum Bulletin 240(Washington, DC, 1964), paper 38. E. K. Lauridsen and N. Abrahmsen, ‘The History of Astatic MagnetSystem and Suspension’, Centaurus, 40 (1948), 135–69.

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Faraday made and used galvanometers12 in his earliest electrical researches, sowould have been familiar with the nature of the insulated fine copper wire used intheir coils. Indeed, he specifically refers to ‘silked copper wire’ in a diary entry dated6 September 1821.13 However, it appears to have been too fine for the inductionapparatus described above, for in some initial experiments he passed currents froma large copper–zinc battery that, with a suitable nitrosulphuric electrolyte, mightwell have exceeded 10 amps if estimated in modern units.14 The consequent heatingwould have damaged both the fine wire and its insulation.

4. ‘Bonnet wire’A heavier gauge of cotton-covered springy iron wire was used to support the

wide brims of ladies’ outdoor bonnets of the period15 (Figure 2). It was listed innineteenth-century haberdashers’ catalogues16 as ‘millinery wire’ of various gauges:a restricted range is still available today.17 Faraday had himself used this ready-made insulated wire in experiments made as early as 5 September 1821,18 butdoubtless sought the greater conductivity associated with copper in his research oninduction. (Copper is nearly six times more conductive, weight for weight, thaniron.19) He was soon also to become aware of the possibility of confusing effectscaused by the presence of iron in electromagnetic apparatus.

Manufacturing ordinary bonnet wire was the trade of the ‘bonnet-wire maker’.By 1838 interest in electrical matters had grown to the extent that would-be experim-enters were being urged to take a few pounds of copper wire to such a tradesmanand pay him to cover it with cotton or silk.20 The simple hand-powered equipmentthat was employed for this purpose is exemplified by Ettrick’s machine of 183721(Figure 3). The bare wire was pulled through a hollow spindle or headstock at asteady rate, and as it emerged was covered with a close-packed helix of cotton orsilk by a rapidly rotating faceplate carrying a reel of the fibre. The inventor claimedhe could cover 400 feet of wire per hour with this machine, but had already thoughtof mounting a second reel on the faceplate to increase this rate, or make a morethickly covered insulated wire.

Philosophical instrument makers22 requiring insulated copper wire to constructelectrical apparatus would similarly need to engage the services of a bonnet-wiremaker, and would doubtless order more than sufficient to meet their own immediateneeds. Therefore by 1838 we find the catalogue of Watkins and Hill23 listing ‘copper

12 See note 3.13 See note 1.14 Allan A. Mills, ‘Early Voltaic Batteries: an Evaluation in Modern Units with Applications to the

Work of Davy and Faraday’, Annals of Science, 60 (October 2003), 373–398.15 Georgine De Courtais, Women’s Headdress and Hairstyles in England from AD 600 to the Present

Day (London: Batsford, 1986), pp. 108–09.16 Anon., The Haberdasher’s Guide: or a Complete Key to All the Intricacies of the Haberdashery

Business (London, 1826), p. 27. BM General Catalogue Vol. 95, column 297, no. T897(8).17 Juliet Bawden, The Hat Book (London: Letts, 1992), p. 129.18 See note 1.19 CRC Handbook of Chemistry and Physics, ed. by David R. Lide, 81st edn (London: CRC Press,

2000).20 G. Francis, Untitled letter, Annals of Electricity, 2 (1838), 396.21W. Ettrick, ‘Machine for Covering Copper Wire with Thread, for Electro-Magnetical Purposes’,

Mechanics Magazine (London), 27, no. 717 (6 May 1837), 65–67.22 Sometimes popularly but incorrectly known as ‘opticians’.23 Catalogue of Watkins & Hill, 5 Charing Cross, London, 1838. (Copy by courtesy of Dr Brian Gee.)

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The Early History of Insulated Copper Wire 457

Figure 2. Travelling dress of the 1830s.

wire of all lengths and diameters, covered in white silk or cotton, for electromagneticexperiments . . .’.

5. Commercialization by W. T. HenleyMichael Faraday never personally attempted to commercialize any of his inven-

tions or developments, and we hear no more of Mr Ettrick, but a very differentattitude was displayed by William Thomas Henley.24 Born in humble circumstances

24 ‘W. T. Henley’, Dictionary of National Biography, 25 (1891), 421–22.

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Figure 3. Ettrick’s wire-covering machine of 1837.

at Midhurst, Sussex, about 1813, he was initially apprenticed to the leather trade.Disliking this, he made his way to London in 1829, and found employment as aday-labourer in the docks. Hiring averaged four days per week, allowing himsufficient time to buy an old lathe and teach himself to make mechanical things. Aspecial interest in electrical apparatus (initially electrostatic) led him to specialize inthat area, and around 1836 some pieces exhibited in the shop window of a friendlychemist brought sufficient orders to encourage him to set up his own backroomworkshop for their commercial manufacture.

Henley obviously needed insulated copper wire for this endeavour, but was astuteenough to foresee the demand for this basic commodity that would inevitably followthe spread of the telegraph (with its cables and solenoids) and the growing applicationof electric lighting and other apparatus. By 1837 he had built the six-head wrappingmachine shown in Figure 4, and was soon able to sell insulated copper wire at halfthe price charged by the philosophical instrument makers.25 Its operating principleswere exactly the same as those employed by the bonnet-wire makers, but specializa-tion in copper wire and electrical instruments was so successful that by 1846 he wasemploying seven men in enlarged premises. The subsequent rise and tragic fall ofthe W. T. Henley Telegraph Works (he had no partners and it was not originally alimited company) is described in his autobiography26 and in subsequent accountsby Slater27 and Anderson.28 This remarkable self-made man died in 1882: his portraitis shown in Figure 5.

Amazingly, Henley’s original six-head wrapping machine still exists in much thesame state as illustrated in Figure 4. It is held in the Blythe Road store of the ScienceMuseum, London, inventory number 1939-139. Figure 6 gives a close-up of one ofthe heads, showing how the tension of the silk thread was controlled by an adjustable

25 F. White, Letter to The Mechanic and Chemist (London), 4 (1839), 136.26W. T. Henley, ‘The Early Life of W. T. Henley’, Henley Telegraph (staff magazine of W. T. Henley’s

Telegraph Works Co. Ltd), 1924 June 4–10 and 1924 September 3–10.27 Ernest Slater, One Hundred Years: The Story of Henley’s 1837–1937 (Gravesend: W. T. Henley

Ltd, 1937).28 A. F. Anderson, ‘William Henley, Pioneer Electrical Instrument Maker and Cable Manufacturer’,

Institute of Electrical Engineers; Proceedings, 132A (1985), 249–61. A. F. Anderson, ‘William Henley:Imagination Without Discipline’, Institute of Electrical Engineers; Electronics and Power, 31 (1985),593–97.

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The Early History of Insulated Copper Wire 459

Figure 4. Henley’s six-head wire-covering machine of 1837. This still exists in the ScienceMuseum, London, catalogue number 1939–139.

rubber pad acting on the lower face of the stock reel. There is no trace of a seconddiametrically opposed reel of thread, which would have acted as a counterbalanceas well as increasing production. (It is possible that later versions did incorporatethis refinement, and a treadle drive would have been less fatiguing.) With the kindcooperation of the relevant curator, Dr M. T. Wright, a short length of the silk-covered wire protruding from one head was detached for laboratory examination.Of course, there is no reason why this wire should also date from 1837: it is morelikely that Henley and his employees would have used the machine for a decade ormore until it was superseded.

6. Examination of Henley’s insulated wire6.1. Physical characteristics

(a) Examination with a binocular microscope showed that the silk thread consti-tuting the insulating layer was made up of some 60 filaments twisted together.It had been applied to the copper core as a close-packed and sometimesoverlapping right-handed helix ( like a normal screw thread), so the faceplateand reel of Henley’s machine must have rotated clockwise when viewed alongthe emerging wire. This type of wire came to be known as single-silk covered,

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Figure 5. William Thomas Henley.

abbreviated to SSC. A single-cotton covered equivalent would be SCC. Asecond layer of fibre, applied in the opposite hand, would give double-silkcovered (DSC) or double-cotton covered (DCC) wires.

(b) The copper wire core was exposed by dissolving away the insulation withdilute aqueous sodium hydroxide. No enamel coating lay beneath the silk,as it does in some more recent products. Measurements with a micrometershowed the wire to be slightly non-circular in section, of major axis 0.150 mmand minor axis 0.145 mm. This could have resulted from drawing througha circular die that was not fixed exactly perpendicular to the direction ofmotion. The nearest modern wire gauge is 38 SWG, which is 0.152 mm indiameter.

(c) Assuming it to be an ellipse, the area of Henley’s wire may be calculated tobe 1.708×10−4 cm2 . The mass of an 11.50 cm length was 17.5 mg, fromwhich its relative density comes out as 8.91.29 A recent reference work30gives 8.94 for pure copper.

29 The main source of uncertainty in this figure is the estimation of the area of the wire. Methods forthe determination of density that are independent of volume are total immersion (Archimedes’ method)and float/sink in heavy liquids (as used with mineral grains). Unfortunately, attempts to apply the former,using a sensitive torsion balance, proved unsuccessful owing to surface tension forces being far fromnegligible with this small mass. The density of copper is much too large for float/sink methods, for theheaviest liquid available (saturated sodium polytungstate solution) has a relative density of 3.1.30 See note 19.

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The Early History of Insulated Copper Wire 461

Figure 6. Close-up of one of the heads on Henley’s machine.

(d) A short length of Henley’s wire was snapped off from the main sample,coiled into a tight spiral, and the inner end extended upwards. It was thenplaced on the stage of a scanning electron microscope and the fracturedcross-section examined (Figure 7). The metal could be seen to be very porous.

(e) The same spiral of wire was then positioned within a bakelite ring and embed-ded in epoxy resin. Grinding on successively finer grades of alumina paperexposed a plane cross-section, which was subsequently polished with the finestdiamond powders according to standard metallurgical procedures.31 Scanningelectron microscopy (SEM) examination of the carbon-coated preparation

31 A. R. Bailey, A Textbook of Metallurgy (London: Macmillan, 1964).

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Figure 7. Scanning electron micrograph of fractured sample of the copper wire from Henley’smachine, showing its porous interior. Some silk insulation surrounds it.

gave the results shown in Figures 8 and 9. They confirm that the metal is fullof disconnected cavities, probably from the exsolution of oxygen (see below).These voids have been closed up around the circumference by the wire drawingprocess, so that SEM examination of the surface of the wire showed onlyoccasional faint longitudinal striations superimposed on a smooth surface.These striae would have exerted a negligible deleterious influence on conduc-tivity by comparison with the porous nature of the interior.

6.2. Chemical analysisTrace elements were sought by wavelength-dispersive characterization of the

X-ray emission promoted by SEM examination, but no more than a possible trace

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Figure 8. Scanning electron micrograph of polished cross-section of Henley’s wire.

of tin was detected. Unfortunately, the technique is not sensitive to the presenceof oxygen.

6.3. Electrical properties6.3.1. General

Finding the best metal or alloy for their cables, with an optimum balance betweenconductivity, weight, strength, and cost, was a priority for the emerging telegraphand cable companies, yet proved obstinately difficult to establish.32 Eventually it

32W. Thomson, ‘On the Electric Conductivity of Commercial Copper of Various Kinds’, Proceedingsof the Royal Society of London, 8 (1856–57), 550–55. W. Thomson, ‘Analytical and Synthetical Attemptsto Ascertain the Cause of the Differences of Electric Conductivity Discovered in Wires of Nearly Pure

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Figure 9. Higher-magnification view of Henley’s wire. Note compression of cavities aroundthe circumference.

was agreed that silver was the best conductor weight for weight, but its cost madepure copper the best choice for practical electric wires and cables. Purity was thevital factor, mere traces of other elements always depressing its conductivity. Inparticular, it was not at first realized that oxygen could be an important contaminant,being readily absorbed from the air by molten copper but exsolving on cooling to

Copper’, Proceedings of the Royal Society of London, 10 (1859–60), 300–09. A. Matthiessen, ‘On theElectric Conducting Power of Copper and its Alloys’, Proceedings of the Royal Society of London, 11(1860–61), 126–30. A. Matthiessen and M. Holzmann, ‘On the Effect of the Presence of Metals andMetalloids upon the Electric Conducting Power of Pure Copper’, Philosophical Transactions of the RoyalSociety of London, 150 (1860), 85–92.

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The Early History of Insulated Copper Wire 465

produce porosity.33 We see this phenomenon in Figures 7–9. In fact, even very pureelectrolytically refined copper can be spoiled for demanding electrical purposes bycasting the preforms for wire drawing in air.34

6.3.2. Measurement of low resistance in situTo determine a very low resistance it is important to separate the current and

potential circuits, measuring the current flowing through the wire under test whilesimultaneously noting the potential drop across a precisely known length of thesample. Ohm’s law then enables the resistance to be calculated. Portable apparatushas been designed that may be taken to museums, and operates on an in situ 15 mmlength of wire: it is described elsewhere.35 Using it, the resistance of Henley’s wirewas found to be 10.44 mV cm−1 at 25 °C.

6.3.3. Specific resistanceThe specific (volume) resistance sV of a conductor is defined by:

sV=RA

sV cm−3.

where R is the resistance in ohms, A is the area in square centimetres, and s is thelength in centimetres. These dimensions employ the cgs system, which is in myopinion preferable to modern SI units for historical studies. Substituting the resultsfound above gives the specific volume resistance of the copper of Henley’s wire tobe 1.78×10−6 V cm−3. This is 14% inferior to the figure of 1.67×10−6 V cm−3quoted36 for modern high-purity copper and is in accord with the porosity notedabove. Porosity appears to exert a greater percentage effect on conductivity thanon density.

7. Faraday’s wireBoth Faraday’s ‘ring’ and his ‘magnet-and-coil’ apparatus appear to be wound

with the same bare copper wire. As explained above, this ‘hard pitch’ copper wasintended for mechanical purposes. Unfortunately, all the individual layers of thiscoil have been soldered together in parallel to two extensions of sturdy copper wireabout 1/8 inch in diameter, and then the metal was coated with an insulating layerof black japan (a varnish of shellac in alcohol to which has been added a propor-tion of lampblack). All wires were thereby rendered inaccessible for electricalmeasurements.

However, the famous toroid with which Faraday discovered the phenomenon ofinduction has the end of one winding of bare (although tarnished) copper wireprotruding for a couple of inches. With the kind cooperation of Dr Frank James,the Curator of the Faraday Museum at the Royal Institution, I was able to use

33W. J. Russell and A. Matthiessen, ‘On the Cause of Vesicular Structure in Copper’, PhilosophicalMagazine, 23 (1862), 81–84.34W. M. Tuddenham and P. A. Dougall, ‘Copper’, and R. E. Ricksecker, ‘Wrought Copper and

Wrought Copper Alloys’, both in Kirk–Othmer Encyclopedia of Chemical Technology (New York: Wiley-Interscience, 1979).35 A. A. Mills, ‘Improvements in Electric Wires and Cables: Measuring the in situ Resistance of Short

Lengths of Copper Wire’, Bulletin of the Scientific Instrument Society, 74 (2002), 34–36.36 G. W. C. Kaye and T. H. Laby, Tables of Physical and Chemical Constants (London: Longmans,

1966), p. 92.

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portable apparatus to measure in situ the electrical conductivity of this wire. Aprecisely measured current (never exceeding 3 A) was applied to the final 25 mm bya single 1.5 V dry cell in series with a 10 V rheostat, and the potential differencebetween finely pointed probes fixed 15.00 mm apart determined to a microvolt.Application of Ohm’s law then enabled the resistance between the probes to befound, giving a mean value of 0.394 mV. Measurement of the diameter of the wirewas made difficult by waviness produced by frequent bending, but a mean diameterof 1.25 mm was obtained with an engineering micrometer. Calculation then gave aspecific volume resistance for the metal of 3.22×10−6 V cm−3 at 25 °C. This isnearly twice the figure of 1.67×10−6 V cm−3 expected of the best modern electricalgrade copper. It was not possible to examine the interior of the wire for porositybecause this would have required detaching a sample. Blake-Coleman and Yorke37were provided with small samples cut from other coils in the Faraday Collection,and found specific mass resistances between 36% and 68% above that of a purecopper standard. Unfortunately they examined only the surfaces of these wires bySEM, so the nature of the interiors remains unknown.

8. Modern insulated wireThe main disadvantage of textile-covered copper wire is its susceptibilty to both

water vapour (humidity) and liquid water (capillary penetration). For this reason itwas usual to impregnate coils etc. with a varnish of shellac dissolved in alcohol.This also anchored the windings.

Nowadays, braided textile covers are used only for the outermost sleeves of arestricted range of decorative flexible cables for domestic use. The vast majority ofcopper wires are insulated with a thin layer of a tough synthetic enamel, or withthicker coatings of various elastomeric compositions. These may be rapidly and in-expensively applied by extrusion, and are flexible, elastic, and completely waterproof.

9. ConclusionsThe history of insulated wire begins with the fine ‘silked’ copper and silver wires

long used for decorative purposes before being employed in the earliest galvano-meters. However, Faraday’s seminal discoveries of ‘current’ electricity and electro-magnetic induction soon led to a demand for heavier gauges of copper wire readyinsulated with a thin flexible covering that would permit the direct winding ofcompact coils. The millinery trade showed how this could be done using simplemanual machines rapidly winding a continuous helix of silk or cotton thread upona metal wire core. The process became a speciality of William Henley, and he builtup a flourishing business supplying the emerging electrical industry, particularly withtelegraph and associated equipment. Achieving maximum conductivity in the copperproved difficult because it is extremely susceptible to degradation by trace impurities.Particularly important—and not at first realized—was that oxygen was dissolvedfrom the air by the molten metal and then exsolved on cooling to produce porosity.Henley’s copper (and probably Faraday’s too) suffers from this problem. It was notcompletely solved until electrorefining followed by melting of the billets for wire

37 B. C. Blake-Coleman and R. Yorke, ‘Faraday and Electrical Conductors: an Examination of theCopper Wire Used by Michael Faraday between 1821 and 1831’, Proceedings of the Institution of ElectricalEngineers, 128A (1981), 463–71.

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drawing under a blanket of inert gas were introduced in the twentieth century.Enormous worldwide production and utilization38 justify William Henley’s faith inthe future of the electrical industry and its basic need for insulated copper wire.

AcknowledgementsIt is a pleasure to acknowledge the invaluable assistance given by staff of the

Royal Institution, Science Museum, and the Victoria and Albert Museum in thisinvestigation.

38 R. M. Black, The History of Electric Wires and Cables (London: Institution of Electrical Engineers,1983). B. C. Blake-Coleman, Copper Wire and Electrical Conductors: the Shaping of a Technology(Harwood Academic Publishers, 1992).

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