What is Hopkinson's Law

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