Mutual Inductance

# When two circuits are so placed that a current in one causes magnetic flux to link the other, a change of current in one necessarily induces an E.M.F. in the other, and the magnitude of this induced E.M.F. when the current is changing at unit rate is called the coefficient of mutual induction, or more briefly, the mutual inductance, between the two circuits [17].

# By simply moving one circuit relatively to the other it is possible to change the direction of the induced E.M.F., so that the mutual inductance reverses its sign, and it becomes necessary in writing down the circuit equations to adopt some convention as to what is to be called a positive and what a negative mutual inductance [17].