Comparison of Wire Gauges, British and American Standards

Radio Record, Volume I, Issue 35, 16 March 1928, Page 10

Comparison of Wire Gauges, British and American Standards

When purchasing wire for constructional werk it is necessary to ensure that the specified gauge is secured. The fact that there is a difference of one or twe sizes between the same. number gauge of Brown and Sharpe {American) and Standard Wire Gauge, (British) is not always taken into account, and quite frequently is ignored altogether. ‘Lhe B. and S. is in every size thinner than the same number in s.w.g. Around 20’s the difference between the two gauges is least, bemg. about one size; thus 19’s B. and S$, exactly equals 20's s.w.g., but in the. thinner sizes the difference lLecomes very great, 37’s B. and S. leing practically equal to 40’s s.w.g. But wire gauges only take the diameter of wire into account, whereas for electrical current-carrying purposes the crosssectional area is what really counts. It is necessary te realise that taking a small amount off the diameter takes a good amount off the sectional area, and that whereas a 26's s.w.g. is just half the diameter of a 20’s, the crosssectional arca of the 26's is only a quarter that of the 20’s, so four 26’s would be required to equal the cur-

eS TAS EL rentcarrving capacity of 20's. And this rule applies right through, that if you halve the diameter the sectionwl area is reduced to one-quarter. Now we will examine the result of reducing the diameter by only a small amount, as in the case of 20’s s.w.g. being ‘sold out,’ and taking the next stock size, 22’s, which is about five-thonu-sandths less in diameter. The actual cross-sectional area of 20's S.w.g. expressed in square inches is .09102 and of 22’s .00061, which shows that the cross-sectional arca of 22’s is only sixtenths, or a little over half, that of 20’s. Whereas 20’s s.w.g. will safely carry 4.1 amps., 22’s will only carry 2.5 ‘amps., and if used in place of #Ys might get unduly heated and, m addition, add a good amount of unnecessary resistance to the circuit. It should be mentioned here that a mil is the thousandth part of an inci:. Yew tables are published giving the same particulars of both gauges of wire, so that comparison is not always an casy matter. Many. tables give the diamcters in mils, but a most useful factor is the cross-sectional area, which provides a much ceadier means of comparing the current-carrying capacity, which is often an_ important consideration. Circular mils are alsa

‘frequently used, but the circular mil ig the square of the diameter in mils, and therefore represents an area much greater than the actual section of the wire, but still forms a haudy means of coinparison. "In calculating the space to be allowed for a given number of turns of wire, care must be taken not to cut the space too fine, or difliculty may be experienced in getting the required number of turns into the space. Therefore a deduction must be made from the theoretical turns per inch given in the table. Where taps have to be made, space is taken up, and fewer turns can be put on, and in high-te1-sion windings a small amount of space must be left at each end of every layer, and this must be taken into .account. Slight irregularity, permissible, and scarcely noticeable, will get away with several turus per inch when wind- | ing fine wire. ~ Wire that has been previously woud for another purpose usually takes more space than new wire, owing to slight irregularities. Such wire may be greatly improved by running out full length

vat-of-doors aud stretching by pulling, Lut mederation is necessary, as stretching tends to harden the wire, reducing its conductivity. The table herewith lias been compiled Wy the writer from several sources, and may be taken as approximately correct, as,there is a slight variation betwees the product of different factories The table is not given with an idea of favouring the use of the LB. and $S. standard, because s.w.g. is the more widely stocked in New Zealand, and it i$ better for constructors to adopt this standard and keep to it as much as possible. American magazines give B. and S. gauges in specifications, and it is handy to be able to find the nearest procurable, as shown in the table. Only the even sizes are giyen, because odd numbers are usuaily not siocked, although in some cases the odd number would supply almost the exact equivalent for a wire of the other series. Portions of the table are published in various catalogues, but the writer feels that their combination into one table will be appreciated by constructors. . rom the "ohms per 1000 feet" column the resistance of any length of (Continued on page 11.) .

Construction Continued

wire may be calculated. Moving the decimal point one place to the left gives the resistance of 100 feet, and two places io the left, the resistance of 10 feet. ‘The resistance per pound in ohms is found by dividing the ohm per 1000 feet by the weight in pounds of 1000 feet. From the foregoing it will be seen that it will be fer safer in most cases to deviate when a change must be made from specifications, in the direction of using a size larger wire rather than any smaller size. The foregoing refers to wire to be used as conductors for battery or similar current, which occupies the whole area of the wire, whereas high-fre-quency currents travel chiefly along the outside surface only, so that a thin metal tube will offer scarcely any more resistance to a high-frequency current than would a solid conductor of the same diameter. Insulation, With regard to insulation, enamel occupies the least space of any, and is useful for carrying comparatively small currents in apparatus where many turns of wit? are necessary whilst bulk must be kept down as much as possible. So long as there is no great difference of potential between adjacent turns, enamelled wire is quite serviceable, and adds —

much to the efficiency of small transformers, choke coils, electro-magnets, etc., by reason of its compactness, But care should be taken that where the potential difference is greater than a few volts, as at the return ends of layers, extra insulation should be interposed so that a breakdown of the enamel is impossible. Double silk-covered wire is used in some apparatus, and has a smart appearatice. It ocenpies rather more space than enamelled wire, but as it is from two to three times the price of the lJatter, finds little favour with constructors, especially as the difference in efficiency, for many purposes, is nil. Double cotton-covered wire is useful where good insulation is required, as in filament windings of elimination transformers and the like, but too much faith should not be put into its insulating properties, and for the class of work mentioned it is not safe unless well shellaced after each layer has been wound, the shellac to be allowed to dry before being covered with be-tween-layer insulating fabric. Double cotton-covering has the property of automatically providing spacing between the turns when closely wound on a tube of insulating material, but this form of tuning coil is giving way to tinned uninsulated wire space-wound and supported only by three or four strips of celluloid, to which the turns are cemented.

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PARTICULARS OF ENAMELLED COPPER WIRE