Page:Electricity (1912) Kapp.djvu/135

Rh proportional to a particular physical property which, he called the "resistance" of the wire. Expressed mathematically, Ohm's law is

where $$I$$ stands for current strength, $$E$$ for electromotive force, and $$R$$ for resistance. He also found that in a double length of wire the same e.m.f. will only produce half the current strength, whilst by increasing the cross section of the wire (which can conveniently be done by using two or more wires side by side), the current strength is proportionately increased. He thus found that the resistance of the conductor is directly proportional to its length and inversely proportional to its section. This, again expressed mathematically, is

where $$L$$ is the length and $$q$$ the section. The coefficient $$\varrho$$ depends on the material, and is called "the specific resistance." The two formulæ here given are generally valid, whatever may be the system of units chosen. They may, therefore, also be used with the practical units of the "ampere" for current