When a number of resistances are connected in head to tail manner one after another in series is called resistances connected in series or simply resistances in series. We sometimes also call it series resistances.

The equivalent resistance of a series combination of resistances is algebraic sum of all the resistances connected in series.

The voltage drop across the series resistances is also the algebraic sum of the voltage drops across individual resistance.

Let us connect three resistances of value R_{1}, R_{2} and R_{3} in series as shown below.

Let us connect one battery of voltage V volt across the series combination of the resistances.

We will also consider that the voltage drop across R_{1} is V_{1}, the voltage drop across R_{2} is V_{2}, and the voltage drop across R_{3} is V_{3}.

As all three resistances are connected in series the same current will flow through the common path. Let us assume the current is I.

Hence, voltage drop across the resistance R_{1} is IR_{1}.

Voltage drop across the resistance R_{2} is IR_{2}.

Voltage drop across the resistance R_{3} is IR_{3}.

So we can write

From that we can ultimately write that

This is the equivalent resistance of the series combination of three resistances. It instead of three resistances there were n number of resistances connected in series, then the equivalent resistance of the series combination would be written as

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