The resistance of a conductor depends upon four things: length, cross section, material, and temperature. Experiment has determined the following laws:
- The resistance of a conductor is directly proportional to its length.
- The resistance of a conductor is inversely proportional to its area of cross section. For a wire a more convenient form of the law is that the resistance is inversely proportional to the square of the diameter of the wire.
- The resistance of a conductor depends upon its material.
- The resistance of a metallic conductor increases as its temperature rises. The resistance of carbon and electrolytes, on the other hand, decreases.
temperature rises. The resistance of carbon and electrolytes, on the other hand, decreases.
The first three laws, for a wire, may be expressed by the formula
in which I is the length in feet, d the diameter in thousandths of an inch, or mils, and K is the resistance of 1 mil foot of the wire; i.e., of a wire 1 foot long and 1 mil in diameter.
It is well to notice certain facts concerning the above table as follows: The diameter of wire for a certain number is one half as great as it is for the sixth number before it. The resistance and the number of feet per pound doubles every third number. If one remembers, for example, that copper wire No. 10 is practically 100 mils in diameter, has a resistance of 1 ohm per thousand feet, and contains 31.4 feet per pound, he can make a very good estimate for wire of any length and size.
