A carboard tube is wrapped with two winding of insulated wire wound in opposite directions as shown in Fig. 3.24. Terminal `a and b` of winding `A` may be connected to a battery through a reversing switch. State whether the induced current in the resistor `R` from left to right or from right to left in the following cicumstances.

(a) The current in winding `A` is from `a to b` and is increasing.
(b) The current in winding `A` is from `b to a` and is decreasing.
( c) The current in winding `A` is from `b to a` and is increasing.

Explanation

When current is passing from `a` to `b` and is increasing, the magneitc field becomes stronger to the left, so the induced field points towards right, and the induced current must flow from right to left through the resistor.
( b) If the current passes from `b to a`, and is decreasing, then there is less magnetic field pointing right, so the induced field points towards right, and the induced current must flow from right to left through the resistor.
( c) If the current passes from `b to a`, and is increasing, then there is more magnetic field pointing right, so the induced field points towards left, and the induced current must flow from left to right through the resistor.