In 1827, when he first published his famous law, Georg Ohm wrote: "the electromotive force acting between the extremities of any part of a circuit is the product of the strength of the current, and the resistance of that part of the circuit."
The electromotive force acting between two points is equivalent to the voltage in that part of the circuit.
The law as stated above can be expressed mathematically as V = IR – where V is the voltage, I is the current, and R is the resistance.
(The use of I to represent current originates from the French phrase intensité de courant, meaning current intensity.)
The law is usually stated today as "the current through a conductor between two points is directly proportional to the voltage across the two points." In other words, the current is equal to the voltage multiplied by a constant – or divided by a different constant. When he expressed the law as he did in 1827, Ohm defined the resistance as the constant by which we have to divide the voltage to get the current: I = V/R.
The other constant – the one by which we have to multiply the voltage to get the current – is known as the conductance. For a given conductor, the resistance and the conductance are reciprocal to each other – in other words, if you multiply them by each other the result is always 1.
The conductance is represented mathematically as G. (Don't ask me why.) Just as V = IR and I = V/R (and R = V/I), the equivalent equations for the conductance are: I = VG, V = I/G, G = I/V.
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