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Ohm’s Law.

Mention has been made above of certain electrical magnitudes, namely, voltage or electromotive force and amperage or strength of current. These bear an important relation in determining a property of an electric circuit called resistance.

No conducting body possesses perfect electrical conductivity, but presents a certain amount of obstruction or resistance to the passage of electricity. The practical unit of resistance is the Ohm. It is represented by the resistance offered to an unvarying electric current by a column of mercury at the temperature of melting ice, 14.4521 grams in mass, of a constant cross sectional area and of the length of 106.3 centimetres.

The resistance of a conductor is proportional to its length, that is, provided two conductors are made of the same material and of the same diameter and one is twice as long as the other, the resistance of the longer will be twice that of the shorter conductor. The resistance is inversely proportional to the cross sectional area, which is to say that a conductor of smaller cross section has a greater resistance than one of larger section.