This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.

To find the resistance of any length of wire, divide that length in feet by 1,000, and multiply by the figure giving the resistance of that wire per thousand feet, as found in Table 1.

What is the resistance of a No. 4 wire, 1,530 feet in length ?

The resistance R1 per 1,000 feet of No. 4 wire is, from the table, .259 ohm. The length L is 1,530 feet. Then, by formula 3,

R = 1.530 X .259 = .396 ohm. Ans. 1,000

11. The resistance of a given conductor diminishes as its sectional area increases; that is, the resistance varies inversely as the sectional area of a conductor. For example, if the sectional area of a conductor be doubled, its resistance will be halved.

No. 8 wire has a resistance of .652 ohm per 1,000 feet. What will be the resistance of 1,000 feet of a wire having three times the sectional area of No. 8 wire ?

A wire three times the area of No. 8 will have one-third its resistance, or .652/3 = .217 ohm. Ans.

The resistance of two or more conductors connected in series (Art. 3) is equal to the sum of their separate resistances. For example, if four conductors, having separate resistances of 8, 12, 22, and 34 ohms, respectively, are connected in series, their total or joint resistance would be 8 + 12 + 22 + 34 = 76 ohms.

12. The microhm is a unit of resistance devised to facilitate calculations and measurements of exceedingly small resistances, and is equal to one millionth (1/1000000) of an ohm. Hence, to express the resistance in microhms, multiply the resistance in ohms by 1,000,000; and, conversely, to express the resistance in ohms, divide the resistance in microhms by 1,000,000. For example, .75 ohm = .75 X 1,000,000 = 750,000 microhms; or, 750,000 microhms = 750,000 ÷ 1,000,000 = .75 ohm.

The megohm is a unit of resistance intended for the measurement of very high resistances, such as the insulation of cables, and is equal to one million (1,000,000) ohms. Hence, to express the resistance in megohms, divide the resistance in ohms by 1,000,000; on the other hand, when the resistance in megohms is given, multiply by 1,000,000 in order to express the same in ohms. For example, 12,500,000ohms = 12,500,000 ÷ 1,000,000 = 12.5 megohms; or 12.5 megohms = 12.5 X 1,000,000 = 12,500,000 ohms.

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