8. The unit of electrical resistance, now universally adopted, is called the international ohm. One international ohm is the resistance offered by a column of pure mercury 106.3 centimeters in length and 1 square millimeter in sectional area at 32° F., or the temperature of melting ice. The dimensions of the column expressed in inches are as follows: length 41.85 inches, sectional area .00155 square inch. Hereafter, the word "international" will be omitted, and simply the word "ohm" used; the international ohm, however, as defined above, will always be implied unless otherwise stated.

9. In Table 1 are given various data respecting the copper wire used in electrical installations. In the first column is the gauge number by American Wire Gauge; in the second column is the diameter as measured in mils (one mil = one one-thousandth of an inch); the third column shows the area of cross-section in circular mils. It is usual to adopt this method for a round wire, instead of the old way of expressing the area in fractions of a square inch, in which case the diameter is squared and the product multiplied by .7854, as explained in Geometry and Mensuration. If the second operation be omitted, and the diameter, as measured in thousandths of an inch, be only squared (or multiplied by itself), the result is expressed in circular thousandths, or circular mils.

Example

What is the area in circular mils of a wire 2 1/2 inches in diameter ?

Solution

2 1/2 in. = 2,500 mils. 2,5002 = 2,500x2,500 = 6,250,000 circular mils. Ans.

The resistance of copper wire being low, a unit length of 1,000 feet is usually taken in tables of resistance, and this unit is considered in the eighth column.

10. The resistance of a given conductor increases as the length of the conductor increases; that is, the resistance is directly proportional to the length of the conductor. For example, if the length of a conductor be doubled, its resistance will be doubled.

The resistance of any length of conductor may be found by the following formula:

R = LR1

1,000'

(3.) where R = required resistance; L = length of conductor; R1 = resistance per 1,000 feet of conductor.

Table 1. American, Or B. & S., Gauge