Connecting Resistors in Series

This figure provided here illustrates connection of two resistors in series. The word “series” means that the current leaving the battery, at first, goes through one of the resistors, and then goes through the other one, and then, finally, returns to the cathode side of the battery.

When resistors are connected in parallel the net effect is additive.

Equivalent resistance = R1 + R2

Therefore if two resistors - one of 10 ohms and the other one of 15 ohms - are connected in series, the equivalent resistance will be 25 ohms.