Comparison of Electrical Resistances and Electromotive Forces

We have seen that if two points on a conductor be at different potentials, a current of electricity flows through the conductor. As yet we have said nothing about the relation between the difference of potential and the current produced. This is expressed by Ohm's law, which states that the current flowing between any two points of a conductor is directly proportional to the difference of potential between those points so long as the conductor joining them remains the same and in the same physical state. Thus, if C be the current, and E the electromotive force, C is proportional to E, and we may write

where R is a quantity which is known as the resistance of the conductor. It depends solely on the shape and temperature of the conductor, and the nature of the material of which it is composed, being constant so long as these remain unaltered.