Hopkinson's Law describes the relationship between the reluctance of a magnetic circuit, the magnetomotive force and the magnetic flux. It is similar to Ohm's Law
Ohm's Law defines the relationship between voltage, current and resistance
It is described as
Voltage = Current x Resistance
Similarly, Hopkinson's Law can be written as
Magnetomotive Force = Flux x Reluctance
Hopkinson's Law is also known as Rowland's Law